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# Programming-Idioms.org

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Go
1
Print a literal string on standard output
fmt.Println("Hello World")
2
Loop to execute some code a constant number of times
for i := 0; i < 10; i++ {
fmt.Println("Hello")
}
Alternative implementation:
fmt.Println(strings.Repeat("Hello\n", 10))
3
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output)
func finish(name string) {
fmt.Println("My job here is done. Good bye " + name)
}
Alternative implementation:
finish := func(name string) {
fmt.Println("My job here is done. Good bye " + name)
}
4
Create a function which returns the square of an integer
func square(x int) int {
return x*x
}
5
Declare a container type for two floating-point numbers x and y
type Point struct {
x, y float64
}
6
Do something with each item x of an array-like collection items, regardless indexes.
for _, x := range items {
doSomething(x)
}
7
Print each index i with its value x from an array-like collection items
for i, x := range items {
fmt.Printf("Item %d = %v \n", i, x)
}
8
Create a new map object x, and provide some (key, value) pairs as initial content.
x := map[string]int {"one": 1, "two": 2}
9
The structure must be recursive because left child and right child are binary trees too. A node has access to children nodes, but not to its parent.
type BinTree struct {
Value valueType
Left *BinTree
Right *BinTree
}
10
Generate a random permutation of the elements of list x
for i := range x {
j := rand.Intn(i + 1)
x[i], x[j] = x[j], x[i]
}
Alternative implementation:
y := make([]T, len(x))
perm := rand.Perm(len(x))
for i, v := range perm {
y[v] = x[i]
}
Alternative implementation:
rand.Shuffle(len(x), func(i, j int) {
x[i], x[j] = x[j], x[i]
})
Alternative implementation:
for i := len(x) - 1; i > 0; i-- {
j := rand.Intn(i + 1)
x[i], x[j] = x[j], x[i]
}
11
The list x must be non-empty.
x[rand.Intn(len(x))]
Alternative implementation:
func pickT(x []T) T {
return x[rand.Intn(len(x))]
}
12
Check if the list contains the value x.
list is an iterable finite container.
func Contains(list []T, x T) bool {
for _, item := range list {
if item == x {
return true
}
}
return false
}
13
Access each key k with its value x from an associative array mymap, and print them.
for k, x := range mymap {
fmt.Println("Key =", k, ", Value =", x)
}
14
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b.
func pick(a, b  float64)  float64 {
return a + (rand.Float64() * (b-a))
}
15
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b.
func pick(a,b int) int {
return a + rand.Intn(b-a+1)
}
16
Call a function f on every node of binary tree bt, in depth-first infix order
func (bt *BinTree) Dfs(f func(*BinTree)) {
if bt == nil {
return
}
bt.Left.Dfs(f)
f(bt)
bt.Right.Dfs(f)
}
17
The structure must be recursive. A node may have zero or more children. A node has access to its children nodes, but not to its parent.
type Tree struct {
Key keyType
Deco valueType
Children []*Tree
}
18
Call a function f on every node of a tree, in depth-first prefix order
func (t *Tree) Dfs(f func(*Tree)) {
if t == nil {
return
}
f(t)
for _, child := range t.Children {
child.Dfs(f)
}
}
19
Reverse the order of the elements of list x.
This may reverse "in-place" and destroy the original ordering.
for i, j := 0, len(x)-1; i < j; i, j = i+1, j-1 {
x[i], x[j] = x[j], x[i]
}
20
Implement a function search which looks for item x in a 2D matrix m.
Return indices i, j of the matching cell.
Think of the most idiomatic way in the language to return the two values at the same time.
func search(m [][]int, x int) (bool, int, int) {
for i := range m {
for j, v := range m[i] {
if v == x {
return true, i, j
}
}
}
return false, 0, 0
}
21
Swap the values of the variables a and b
a, b = b, a
22
Extract the integer value i from its string representation s (in radix 10)
i, err  := strconv.Atoi(s)
Alternative implementation:
i, err := strconv.ParseInt(s, 10, 0)
23
Given a real number x, create its string representation s with 2 decimal digits following the dot.
s := fmt.Sprintf("%.2f", x)
24
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese)
s := "ネコ"
25
Share the string value "Alan" with an existing running process which will then display "Hello, Alan"
go func() {
v := <-ch
fmt.Printf("Hello, %v\n", v)
}()

ch <- "Alan"
26
Declare and initialize a matrix x having m rows and n columns, containing real numbers.
const m, n = 3, 4
var x [m][n]float64
Alternative implementation:
func make2D(m, n int) [][]float64 {
buf := make([]float64, m*n)

x := make([][]float64, m)
for i := range x {
x[i] = buf[:n:n]
buf = buf[n:]
}
return x
}
27
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers.
const m, n, p = 2, 2, 3
var x [m][n][p]float64
Alternative implementation:
func make3D(m, n, p int) [][][]float64 {
buf := make([]float64, m*n*p)

x := make([][][]float64, m)
for i := range x {
x[i] = make([][]float64, n)
for j := range x[i] {
x[i][j] = buf[:p:p]
buf = buf[p:]
}
}
return x
}
28
Sort the elements of the list (or array-like collection) items in ascending order of x.p, where p is a field of the type Item of the objects in items.
type ItemPSorter []Item
func (s ItemPSorter) Len() int{ return len(s) }
func (s ItemPSorter) Less(i,j int) bool{ return s[i].p<s[j].p }
func (s ItemPSorter) Swap(i,j int) { s[i],s[j] = s[j],s[i] }

func sortItems(items []Item){
sorter := ItemPSorter(items)
sort.Sort(sorter)
}
Alternative implementation:
less := func(i, j int) bool {
return items[i].p < items[j].p
}
sort.Slice(items, less)
29
Remove i-th item from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
Note that in most languages, the smallest valid value for i is 0.
items = append(items[:i], items[i+1:]...)
Alternative implementation:
copy(items[i:], items[i+1:])
items[len(items)-1] = nil
items = items[:len(items)-1]
30
Launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
wg := sync.WaitGroup{}
wg.Add(1000)
for i := 1; i <= 1000; i++ {
go func(j int) {
f(j)
wg.Done()
}(i)
}
wg.Wait()
31
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1)
func f(i int) int {
if i == 0 {
return 1
}
return i * f(i-1)
}
32
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers.
func exp(x, n int) int {
switch {
case n == 0:
return 1
case n == 1:
return x
case n%2 == 0:
return exp(x*x, n/2)
default:
return x * exp(x*x, (n-1)/2)
}
}
33
Assign to the variable x the new value f(x), making sure that no other thread may modify x between the read and the write.
var lock sync.RWMutex

lock.Lock()
x = f(x)
lock.Unlock()
34
Declare and initialize a set x containing unique objects of type T.
x := make(map[T]bool)
Alternative implementation:
x := make(map[T]struct{})
35
Implement a function compose (A -> C) with parameters f (A -> B) and g (B -> C), which returns composition function g ∘ f
func compose(f func(A) B, g func(B) C) func(A) C {
return func(x A) C {
return g(f(x))
}
}
36
Implement a function compose which returns composition function g ∘ f for any functions f and g having exactly 1 parameter.
func composeIntFuncs(f func(int) int, g func(int) int) func(int) int {
return func(x int) int {
return g(f(x))
}
}
37
Transform a function that takes multiple arguments into a function for which some of the arguments are preset.
type PayFactory func(Company, *Employee, *Employee) Payroll

type CustomPayFactory func(*Employee) Payroll

func CurryPayFactory(pf PayFactory,company Company, boss *Employee) CustomPayFactory {
return func(e *Employee) Payroll {
return pf(company, boss, e)
}
}
38
Find substring t consisting in characters i (included) to j (excluded) of string s.
Character indices start at 0 unless specified otherwise.
Make sure that multibyte characters are properly handled.
t := string([]rune(s)[i:j])
39
Set boolean ok to true if string word is contained in string s as a substring, or to false otherwise.
ok := strings.Contains(s, word)
40
Declare a Graph data structure in which each Vertex has a collection of its neighbouring vertices.
type Vertex struct{
Id int
Label string
Neighbours map[*Vertex]bool
}

type Graph []*Vertex
41
Create string t containing the same characters as string s, in reverse order.
Original string s must remain unaltered. Each character must be handled correctly regardless its number of bytes in memory.
runes := []rune(s)
for i, j := 0, len(runes)-1; i < j; i, j = i+1, j-1 {
runes[i], runes[j] = runes[j], runes[i]
}
t := string(runes)
42
Print each item v of list a which is not contained in list b.
For this, write an outer loop to iterate on a and an inner loop to iterate on b.
mainloop:
for _, v := range a {
for _, w := range b {
if v == w {
continue mainloop
}
}
fmt.Println(v)
}
43
Look for a negative value v in 2D integer matrix m. Print it and stop searching.
mainloop:
for i, line := range m {
for _, v := range line {
if v < 0 {
fmt.Println(v)
break mainloop
}
}
}
44
Insert element x at position i in list s. Further elements must be shifted to the right.
s = append(s, 0)
copy(s[i+1:], s[i:])
s[i] = x
45
Sleep for 5 seconds in current thread, before proceeding with the next instructions.
time.Sleep(5 * time.Second)
46
Create string t consisting of the 5 first characters of string s.
Make sure that multibyte characters are properly handled.
t := string([]rune(s)[:5])
47
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled.
t := string([]rune(s)[len([]rune(s))-5:])
48
Assign to variable s a string literal consisting in several lines of text, including newlines.
s := `Huey
Dewey
Louie`
49
Build list chunks consisting in substrings of input string s, separated by one or more space characters.
chunks := strings.Split(s, " ")
Alternative implementation:
chunks := strings.Fields(s)
50
Write a loop that has no end clause.
for {
// Do something
}
51
Determine whether the map m contains an entry for the key k
_, ok := m[k]
52
Determine whether the map m contains an entry with the value v, for some key.
func containsValue(m map[K]T, v T) bool {
for _, x := range m {
if x == v {
return true
}
}
return false
}
53
Concatenate elements of string list x joined by the separator ", " to create a single string y.
y := strings.Join(x, ", ")
54
Calculate the sum s of the integer list or array x.
s := 0
for _, v := range x {
s += v
}
55
Create the string representation s (in radix 10) of the integer value i.
s := strconv.Itoa(i)
Alternative implementation:
s := strconv.FormatInt(i, 10)
Alternative implementation:
s := fmt.Sprintf("%d", i)
56
Fork-join : launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
Wait for the completion of the 1000 tasks and then print "Finished".
var wg sync.WaitGroup
wg.Add(1000)
for i := 1; i <= 1000; i++ {
go func(i int) {
f(i)
wg.Done()
}(i)
}
wg.Wait()
fmt.Println("Finished")
57
Create the list y containing the items from the list x that satisfy the predicate p. Respect the original ordering. Don't modify x in-place.
y := make([]T, 0, len(x))
for _, v := range x{
if p(v){
y = append(y, v)
}
}
Alternative implementation:
n := 0
for _, v := range x {
if p(v) {
n++
}
}
y := make([]T, 0, n)
for _, v := range x {
if p(v) {
y = append(y, v)
}
}
58
Create string lines from the content of the file with filename f.
b, err := os.ReadFile(f)
if err != nil {
// Handle error...
}
lines := string(b)
59
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").
fmt.Fprintln(os.Stderr, x, "is negative")
60
Assign to x the string value of the first command line parameter, after the program name.
x := os.Args[1]
61
Assign to the variable d the current date/time value, in the most standard type.
d := time.Now()
62
Set i to the first position of string y inside string x, if exists.

Specify if i should be regarded as a character index or as a byte index.

Explain the behavior when y is not contained in x.
i := strings.Index(x, y)
63
Assign to x2 the value of string x with all occurrences of y replaced by z.
Assume occurrences of y are not overlapping.
x2 := strings.Replace(x, y, z, -1)
64
Assign to x the value 3^247
x := new(big.Int)
x.Exp(big.NewInt(3), big.NewInt(247), nil)
65
From the real value x in [0,1], create its percentage string representation s with one digit after decimal point. E.g. 0.15625 -> "15.6%"
s := fmt.Sprintf("%.1f%%", 100.0*x)
66
Calculate the result z of x power n, where x is a big integer and n is a positive integer.
nb := big.NewInt(int64(n))
var z big.Int
z.Exp(x, nb, nil)
67
Calculate binom(n, k) = n! / (k! * (n-k)!). Use an integer type able to handle huge numbers.
z := new(big.Int)
z.Binomial(n, k)
68
Create an object x to store n bits (n being potentially large).
var x *big.Int = new(big.Int)
Alternative implementation:
x := make([]bool, n)
Alternative implementation:
x := make([]uint64, (n+63)/64)
69
Use seed s to initialize a random generator.

If s is constant, the generator output will be the same each time the program runs. If s is based on the current value of the system clock, the generator output will be different each time.
rand.Seed(s)
Alternative implementation:
r := rand.New(rand.NewSource(s))
70
Get the current datetime and provide it as a seed to a random generator. The generator sequence will be different at each run.
rand.Seed(time.Now().UnixNano())
Alternative implementation:
r := rand.New(rand.NewSource(time.Now().UnixNano()))
71
Basic implementation of the Echo program: Print all arguments except the program name, separated by space, followed by newline.
The idiom demonstrates how to skip the first argument if necessary, concatenate arguments as strings, append newline and print it to stdout.
func main() {
fmt.Println(strings.Join(os.Args[1:], " "))
}
73
Create a factory named fact for any sub class of Parent and taking exactly one string str as constructor parameter.
type ParentFactory func(string) Parent

var fact ParentFactory = func(str string) Parent {
return Parent{
name: str,
}
}
74
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers.
x.GCD(nil, nil, a, b)
75
Compute the least common multiple x of big integers a and b. Use an integer type able to handle huge numbers.
gcd.GCD(nil, nil, a, b)
x.Div(a, gcd).Mul(x, b)
76
Create the string s of integer x written in base 2.

E.g. 13 -> "1101"
s := strconv.FormatInt(x, 2)
Alternative implementation:
s := fmt.Sprintf("%b", x)
77
Declare a complex x and initialize it with value (3i - 2). Then multiply it by i.
x := 3i - 2
x *= 1i
78
Execute a block once, then execute it again as long as boolean condition c is true.
for{
someThing()
someOtherThing()
if !c {
break
}
}
Alternative implementation:
for done := false; !done; {
someThing()
someOtherThing()
done = !c()
}
79
Declare the floating point number y and initialize it with the value of the integer x .
y := float64(x)
80
Declare integer y and initialize it with the value of floating point number x . Ignore non-integer digits of x .
Make sure to truncate towards zero: a negative x must yield the closest greater integer (not lesser).
y := int(x)
81
Declare integer y and initialize it with the rounded value of floating point number x .
Ties (when the fractional part of x is exactly .5) must be rounded up (to positive infinity).
y := int(math.Floor(x + 0.5))
82
Find how many times string s contains substring t.
Specify if overlapping occurrences are counted.
x := strings.Count(s, t)
83
Declare regular expression r matching strings "http", "htttp", "httttp", etc.
r := regexp.MustCompile("htt+p")
84
Count number c of 1s in the integer i in base 2.

E.g. i=6 → c=2
func PopCountUInt64(i uint64) (c int) {
i -= (i >> 1) & 0x5555555555555555
i = (i>>2)&0x3333333333333333 + i&0x3333333333333333
i += i >> 4
i &= 0x0f0f0f0f0f0f0f0f
i *= 0x0101010101010101
return int(i >> 56)
}

func PopCountUInt32(i uint32) (n int) {
i -= (i >> 1) & 0x55555555
i = (i>>2)&0x33333333 + i&0x33333333
i += i >> 4
i &= 0x0f0f0f0f
i *= 0x01010101
return int(i >> 24)
}
Alternative implementation:
c := bits.OnesCount(i)
85
Write boolean function addingWillOverflow which takes two integers x, y and return true if (x+y) overflows.

An overflow may be above the max positive value, or below the min negative value.
func addingWillOverflow(x int, y int) bool {
if x > 0 {
return y > math.MaxInt-x
}
return y < math.MinInt-x
}
86
Write boolean function multiplyWillOverflow which takes two integers x, y and return true if (x*y) overflows.

An overflow may be above the max positive value, or below the min negative value.
func multiplyWillOverflow(x, y uint64) bool {
if x <= 1 || y <= 1 {
return false
}
d := x * y
return d/y != x
}
87
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it.
os.Exit(0)
88
Create a new bytes buffer buf of size 1,000,000.
buf := make([]byte, 1_000_000)
89
You've detected that the integer value of argument x passed to the current function is invalid. Write the idiomatic way to abort the function execution and signal the problem.
return nil, fmt.Errorf("invalid value for x: %v", x)
90
Expose a read-only integer x to the outside world while being writable inside a structure or a class Foo.
type Foo struct {
x int
}

func (f *Foo) X() int {
return f.x
}
91
Read from the file data.json and write its content into the object x.
Assume the JSON data is suitable for the type of x.
buffer, err := os.ReadFile("data.json")
if err != nil {
return err
}
err = json.Unmarshal(buffer, &x)
if err != nil {
return err
}
Alternative implementation:
r, err := os.Open(filename)
if err != nil {
return err
}
decoder := json.NewDecoder(r)
err = decoder.Decode(&x)
if err != nil {
return err
}
92
Write the contents of the object x into the file data.json.
buffer, err := json.MarshalIndent(x, "", "  ")
if err != nil {
return err
}
err = os.WriteFile("data.json", buffer, 0644)
93
Implement procedure control which receives one parameter f, and runs f.
func control(f func()) {
f()
}
94
Print the name of the type of x. Explain if it is a static type or dynamic type.

This may not make sense in all languages.
fmt.Println(reflect.TypeOf(x))
Alternative implementation:
fmt.Printf("%T", x)
95
Assign to variable x the length (number of bytes) of the local file at path.
info, err := os.Stat(path)
if err != nil {
return err
}
x := info.Size()
96
Set boolean b to true if string s starts with prefix prefix, false otherwise.
b := strings.HasPrefix(s, prefix)
97
Set boolean b to true if string s ends with string suffix, false otherwise.
b := strings.HasSuffix(s, suffix)
98
Convert a timestamp ts (number of seconds in epoch-time) to a date with time d. E.g. 0 -> 1970-01-01 00:00:00
d := time.Unix(ts, 0)
99
Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD.
x := d.Format("2006-01-02")
100
Sort elements of array-like collection items, using a comparator c.
type ItemCSorter []Item
func (s ItemCSorter) Len() int           { return len(s) }
func (s ItemCSorter) Less(i, j int) bool { return c(s[i], s[j]) }
func (s ItemCSorter) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }

func sortItems(items []Item) {
sorter := ItemCSorter(items)
sort.Sort(sorter)
}
Alternative implementation:
type ItemsSorter struct {
items []Item
c     func(x, y Item) bool
}

func (s ItemsSorter) Len() int           { return len(s.items) }
func (s ItemsSorter) Less(i, j int) bool { return s.c(s.items[i], s.items[j]) }
func (s ItemsSorter) Swap(i, j int)      { s.items[i], s.items[j] = s.items[j], s.items[i] }

func sortItems(items []Item, c func(x, y Item) bool) {
sorter := ItemsSorter{
items,
c,
}
sort.Sort(sorter)
}
Alternative implementation:
sort.Slice(items, func(i, j int) bool {
return c(items[i], items[j])
})
101
Make an HTTP request with method GET to the URL u, then store the body of the response in the string s.
res, err := http.Get(u)
if err != nil {
return err
}
buffer, err := io.ReadAll(res.Body)
res.Body.Close()
if err != nil {
return err
}
s := string(buffer)
102
Make an HTTP request with method GET to URL u, then store the body of the response in file result.txt. Try to save the data as it arrives if possible, without having all its content in memory at once.
resp, err := http.Get(u)
if err != nil {
return err
}
defer resp.Body.Close()
if resp.StatusCode != 200 {
return fmt.Errorf("Status: %v", resp.Status)
}
out, err := os.Create("result.txt")
if err != nil {
return err
}
defer out.Close()
_, err = io.Copy(out, resp.Body)
if err != nil {
return err
}
103
Read from the file data.xml and write its contents into the object x.
Assume the XML data is suitable for the type of x.
buffer, err := os.ReadFile("data.xml")
if err != nil {
return err
}
err = xml.Unmarshal(buffer, &x)
if err != nil {
return err
}
104
Write the contents of the object x into the file data.xml.
buffer, err := xml.MarshalIndent(x, "", "  ")
if err != nil {
return err
}
err = os.WriteFile("data.xml", buffer, 0644)
105
Assign to the string s the name of the currently executing program (but not its full path).
path := os.Args[0]
s = filepath.Base(path)
Alternative implementation:
path, err := os.Executable()
if err != nil {
panic(err)
}
s = filepath.Base(path)
106
Assign to string dir the path of the working directory.
(This is not necessarily the folder containing the executable itself)
dir, err := os.Getwd()
107
Assign to string dir the path of the folder containing the currently running executable.
(This is not necessarily the working directory, though.)
programPath := os.Args[0]
absolutePath, err := filepath.Abs(programPath)
if err != nil {
return err
}
dir := filepath.Dir(absolutePath)
109
Set n to the number of bytes of a variable t (of type T).
var t T
tType := reflect.TypeOf(t)
n := tType.Size()
110
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise.
blank := strings.TrimSpace(s) == ""
111
From current process, run program x with command-line parameters "a", "b".
err := exec.Command("x", "a", "b").Run()
112
Print each key k with its value x from an associative array mymap, in ascending order of k.
keys := make([]string, 0, len(mymap))
for k := range mymap {
keys = append(keys, k)
}
sort.Strings(keys)

for _, k := range keys {
x := mymap[k]
fmt.Println("Key =", k, ", Value =", x)
}
113
Print each key k with its value x from an associative array mymap, in ascending order of x.
Note that multiple entries may exist for the same value x.
type entry struct {
key   string
value int
}

type entries []entry
func (list entries) Len() int { return len(list) }
func (list entries) Less(i, j int) bool { return list[i].value < list[j].value }
func (list entries) Swap(i, j int) { list[i], list[j] = list[j], list[i] }

entries := make(entries, 0, len(mymap))
for k, x := range mymap {
entries = append(entries, entry{key: k, values: x})
}
sort.Sort(entries)

for _, e := range entries {
fmt.Println("Key =", e.key, ", Value =", e.value)
}
Alternative implementation:
type entry struct {
key   string
value int
}

entries := make([]entry, 0, len(mymap))
for k, x := range mymap {
entries = append(entries, entry{key: k, value: x})
}
sort.Slice(entries, func(i, j int) bool {
return entries[i].value < entries[j].value
})

for _, e := range entries {
fmt.Println("Key =", e.key, ", Value =", e.value)
}
114
Set boolean b to true if objects x and y contain the same values, recursively comparing all referenced elements in x and y.
Tell if the code correctly handles recursive types.
b := reflect.DeepEqual(x, y)
115
Set boolean b to true if date d1 is strictly before date d2 ; false otherwise.
b := d1.Before(d2)
116
Remove all occurrences of string w from string s1, and store the result in s2.
s2 := strings.Replace(s1, w, "", -1)
Alternative implementation:
s2 := strings.ReplaceAll(s1, w, "")
117
Set n to the number of elements of the list x.
n := len(x)
118
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values.
y := make(map[T]struct{}, len(x))
for _, v := range x {
y[v] = struct{}{}
}
119
Remove duplicates from the list x.
Explain if the original order is preserved.
y := make(map[T]struct{}, len(x))
for _, v := range x {
y[v] = struct{}{}
}
x2 := make([]T, 0, len(y))
for _, v := range x {
if _, ok := y[v]; ok {
x2 = append(x2, v)
delete(y, v)
}
}
x = x2
Alternative implementation:
seen := make(map[T]bool)
j := 0
for _, v := range x {
if !seen[v] {
x[j] = v
j++
seen[v] = true
}
}
x = x[:j]
Alternative implementation:
seen := make(map[T]bool)
j := 0
for _, v := range x {
if !seen[v] {
x[j] = v
j++
seen[v] = true
}
}
for i := j; i < len(x); i++ {
x[i] = nil
}
x = x[:j]
120
Read an integer value from the standard input into the variable n
_, err := fmt.Scan(&n)
Alternative implementation:
_, err := fmt.Scanf("%d", &n)
121
Listen UDP traffic on port p and read 1024 bytes into buffer b.
ServerAddr,err := net.ResolveUDPAddr("udp",p)
if err != nil {
return err
}
ServerConn, err := net.ListenUDP("udp", ServerAddr)
if err != nil {
return err
}
defer ServerConn.Close()
n,addr,err := ServerConn.ReadFromUDP(b[:1024])
if err != nil {
return err
}
if n<1024 {
return fmt.Errorf("Only %d bytes could be read.", n)
}
122
Create an enumerated type Suit with 4 possible values SPADES, HEARTS, DIAMONDS, CLUBS.
type Suit int

const (
Spades Suit = iota
Hearts
Diamonds
Clubs
)
123
Verify that predicate isConsistent returns true, otherwise report assertion violation.
Explain if the assertion is executed even in production environment or not.
if !isConsistent() {
panic("State consistency violated")
}
124
Write the function binarySearch which returns the index of an element having the value x in the sorted array a, or -1 if no such element exists.
func binarySearch(a []T, x T) int {
imin, imax := 0, len(a)-1
for imin <= imax {
imid := imin + (imax-imin) / 2
switch {
case a[imid] == x:
return imid
case a[imid] < x:
imin = imid + 1
default:
imax = imid - 1
}
}
return -1
}
Alternative implementation:
func binarySearch(a []int, x int) int {
i := sort.SearchInts(a, x)
if i < len(a) && a[i] == x {
return i
}
return -1
}
Alternative implementation:
func binarySearch(a []T, x T) int {
f := func(i int) bool { return a[i] >= x }
i := sort.Search(len(a), f)
if i < len(a) && a[i] == x {
return i
}
return -1
}
125
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration.
t1 := time.Now()
foo()
t := time.Since(t1)
ns := int64(t / time.Nanosecond)
fmt.Printf("%dns\n", ns)
Alternative implementation:
t1 := time.Now()
foo()
t := time.Since(t1)
ns := t.Nanoseconds()
fmt.Printf("%dns\n", ns)
126
Write a function foo that returns a string and a boolean value.
func foo() (string, bool) {
return "Too good to be", true
}
128
Call a function f on every node of a tree, in breadth-first prefix order
func (root *Tree) Bfs(f func(*Tree)) {
if root == nil {
return
}
queue := []*Tree{root}
for len(queue) > 0 {
t := queue[0]
queue = queue[1:]
f(t)
queue = append(queue, t.Children...)
}
}
129
Call the function f on every vertex accessible from the vertex start, in breadth-first prefix order
func (start *Vertex) Bfs(f func(*Vertex)) {
queue := []*Vertex{start}
seen := map[*Vertex]bool{start: true}
for len(queue) > 0 {
v := queue[0]
queue = queue[1:]
f(v)
for next, isEdge := range v.Neighbours {
if isEdge && !seen[next] {
queue = append(queue, next)
seen[next] = true
}
}
}
}
130
Call th function f on every vertex accessible from the vertex v, in depth-first prefix order
func (v *Vertex) Dfs(f func(*Vertex), seen map[*Vertex]bool) {
seen[v] = true
f(v)
for next, isEdge := range v.Neighbours {
if isEdge && !seen[next] {
next.Dfs(f, seen)
}
}
}
131
Execute f1 if condition c1 is true, or else f2 if condition c2 is true, or else f3 if condition c3 is true.
Don't evaluate a condition when a previous condition was true.
switch {
case c1:
f1()
case c2:
f2()
case c3:
f3()
}
132
Run procedure f, and return the duration of the execution of f.
func clock(f func()) time.Duration {
t := time.Now()
f()
return time.Since(t)
}
133
Set boolean ok to true if string word is contained in string s as a substring, even if the case doesn't match, or to false otherwise.
lowerS, lowerWord := strings.ToLower(s), strings.ToLower(word)
ok := strings.Contains(lowerS, lowerWord)
134
Declare and initialize a new list items, containing 3 elements a, b, c.
items := []T{a, b, c}
135
Remove at most 1 item from list items, having value x.
This will alter the original list or return a new list, depending on which is more idiomatic.
If there are several occurrences of x in items, remove only one of them. If x is absent, keep items unchanged.
for i, y := range items {
if y == x {
items = append(items[:i], items[i+1:]...)
break
}
}
Alternative implementation:
for i, y := range items {
if y == x {
copy(items[i:], items[i+1:])
items[len(items)-1] = nil
items = items[:len(items)-1]
break
}
}
136
Remove all occurrences of value x from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
items2 := make([]T, 0, len(items))
for _, v := range items {
if v != x {
items2 = append(items2, v)
}
}
Alternative implementation:
j := 0
for i, v := range items {
if v != x {
items[j] = items[i]
j++
}
}
items = items[:j]
Alternative implementation:
j := 0
for i, v := range items {
if v != x {
items[j] = items[i]
j++
}
}
for k := j; k < len(items); k++ {
items[k] = nil
}
items = items[:j]
137
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise.
b := true
for _, c := range s {
if c < '0' || c > '9' {
b = false
break
}
}
Alternative implementation:
isNotDigit := func(c rune) bool { return c < '0' || c > '9' }
b := strings.IndexFunc(s, isNotDigit) == -1
138
Create a new temporary file on the filesystem.
tmpfile, err := os.CreateTemp("", "")
139
Create a new temporary folder on filesystem, for writing.
dir, err := os.MkdirTemp("", "")
140
Delete from map m the entry having key k.

Explain what happens if k is not an existing key in m.
delete(m, k)
141
Iterate in sequence over the elements of the list items1 then items2. For each iteration print the element.
for _, v := range items1 {
fmt.Println(v)
}
for _, v := range items2 {
fmt.Println(v)
}
142
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7"
s := strconv.FormatInt(x, 16)
Alternative implementation:
s := fmt.Sprintf("%x", x)
143
Iterate alternatively over the elements of the list items1 and items2. For each iteration, print the element.

Explain what happens if items1 and items2 have different size.
for i := 0; i < len(items1) || i < len(items2); i++ {
if i < len(items1) {
fmt.Println(items1[i])
}
if i < len(items2) {
fmt.Println(items2[i])
}
}
144
Set boolean b to true if file at path fp exists on filesystem; false otherwise.

Beware that you should never do this and then in the next instruction assume the result is still valid, this is a race condition on any multitasking OS.
_, err := os.Stat(fp)
b := !os.IsNotExist(err)
145
Print message msg, prepended by current date and time.

Explain what behavior is idiomatic: to stdout or stderr, and what the date format is.
log.Println(msg)
146
Extract floating point value f from its string representation s
f, err := strconv.ParseFloat(s, 64)
147
Create string t from string s, keeping only ASCII characters
re := regexp.MustCompile("[[:^ascii:]]")
t := re.ReplaceAllLiteralString(s, "")
Alternative implementation:
t := strings.Map(func(r rune) rune {
if r > unicode.MaxASCII {
return -1
}
return r
}, s)
148
Read a list of integer numbers from the standard input, until EOF.
var ints []int
s := bufio.NewScanner(os.Stdin)
s.Split(bufio.ScanWords)
for s.Scan() {
i, err := strconv.Atoi(s.Text())
if err == nil {
ints = append(ints, i)
}
}
if err := s.Err(); err != nil {
return err
}
149
As an exception, this content is not under license CC BY-SA 3.0 like the rest of this website.

150
Remove last character from string p, if this character is a slash /.
p = strings.TrimSuffix(p, "/")
151
Remove last character from string p, if this character is the file path separator of current platform.

Note that this also transforms unix root path "/" into the empty string!
sep := fmt.Sprintf("%c", os.PathSeparator)
p = strings.TrimSuffix(p, sep)
Alternative implementation:
sep := fmt.Sprintf("%c", filepath.Separator)
p = strings.TrimSuffix(p, sep)
152
Create string s containing only the character c.
s := fmt.Sprintf("%c", c)
153
Create the string t as the concatenation of the string s and the integer i.
t := fmt.Sprintf("%s%d", s, i)
Alternative implementation:
t := s + strconv.Itoa(i)
154
Find color c, the average between colors c1, c2.

c, c1, c2 are strings of hex color codes: 7 chars, beginning with a number sign # .
Assume linear computations, ignore gamma corrections.
r1, _ := strconv.ParseInt(c1[1:3], 16, 0)
r2, _ := strconv.ParseInt(c2[1:3], 16, 0)
r := (r1 + r2) / 2

g1, _ := strconv.ParseInt(c1[3:5], 16, 0)
g2, _ := strconv.ParseInt(c2[3:5], 16, 0)
g := (g1 + g2) / 2

b1, _ := strconv.ParseInt(c1[5:7], 16, 0)
b2, _ := strconv.ParseInt(c2[5:7], 16, 0)
b := (b1 + b2) / 2

c := fmt.Sprintf("#%02X%02X%02X", r, g, b)
Alternative implementation:
var buf [7]byte
buf[0] = '#'
for i := 0; i < 3; i++ {
sub1 := c1[1+2*i : 3+2*i]
sub2 := c2[1+2*i : 3+2*i]
v1, _ := strconv.ParseInt(sub1, 16, 0)
v2, _ := strconv.ParseInt(sub2, 16, 0)
v := (v1 + v2) / 2
sub := fmt.Sprintf("%02X", v)
copy(buf[1+2*i:3+2*i], sub)
}
c := string(buf[:])
155
Delete from filesystem the file having path filepath.
err := os.Remove(filepath)
156
Assign to the string s the value of the integer i in 3 decimal digits. Pad with zeros if i < 100. Keep all digits if i1000.
s := fmt.Sprintf("%03d", i)
157
Initialize a constant planet with string value "Earth".
const planet = "Earth"
158
Create a new list y from randomly picking exactly k elements from list x.

It is assumed that x has at least k elements.
Each element must have same probability to be picked.
Each element from x must be picked at most once.
Explain if the original ordering is preserved or not.
y := make([]T, k)
perm := rand.Perm(len(x))
for i, v := range perm[:k] {
y[i] = x[v]
}
159
Define a Trie data structure, where entries have an associated value.
(Not all nodes are entries)
type Trie struct {
c        rune
children map[rune]*Trie
isEntry  bool
value    V
}
160
Execute f32() if platform is 32-bit, or f64() if platform is 64-bit.
This can be either a compile-time condition (depending on target) or a runtime detection.
if strconv.IntSize==32 {
f32()
}
if strconv.IntSize==64 {
f64()
}
161
Multiply all the elements of the list elements by a constant c
for i := range elements {
elements[i] *= c
}
162
execute bat if b is a program option and fox if f is a program option.
var b = flag.Bool("b", false, "Do bat")
var f = flag.Bool("f", false, "Do fox")

func main() {
flag.Parse()
if *b {
bar()
}
if *f {
fox()
}
}
163
Print all the list elements, two by two, assuming list length is even.
for i := 0; i+1 < len(list); i += 2 {
fmt.Println(list[i], list[i+1])
}
164
Open the URL s in the default browser.
Set boolean b to indicate whether the operation was successful.
b := open.Start(s) == nil
Alternative implementation:
func openbrowser(url string) {
var err error

switch runtime.GOOS {
case "linux":
err = exec.Command("xdg-open", url).Start()
case "windows":
err = exec.Command("rundll32", "url.dll,FileProtocolHandler", url).Start()
case "darwin":
err = exec.Command("open", url).Start()
default:
err = fmt.Errorf("unsupported platform")
}
if err != nil {
log.Fatal(err)
}

}
165
Assign to variable x the last element of list items.
x := items[len(items)-1]
166
Create the list ab containing all the elements of the list a, followed by all the elements of the list b.
ab := append(a, b...)
Alternative implementation:
var ab []T
ab = append(append(ab, a...), b...)
Alternative implementation:
ab := make([]T, len(a)+len(b))
copy(ab, a)
copy(ab[len(a):], b)
167
Create string t consisting of string s with its prefix p removed (if s starts with p).
t := strings.TrimPrefix(s, p)
168
Create string t consisting of string s with its suffix w removed (if s ends with w).
t := strings.TrimSuffix(s, w)
169
Assign to the integer n the number of characters of the string s.
Make sure that multibyte characters are properly handled.
n can be different from the number of bytes of s.
n := utf8.RuneCountInString(s)
170
Set n to the number of elements stored in mymap.

This is not always equal to the map capacity.
n := len(mymap)
171
Append element x to the list s.
s = append(s, x)
172
Insert value v for key k in map m.
m[k] = v
173
Number will be formatted with a comma separator between every group of thousands.
p := message.NewPrinter(language.English)
s := p.Sprintf("%d\n", 1000)
Alternative implementation:
n := strconv.Itoa(23489)
s := thousands.Separate(n, "en")
174
Make a HTTP request with method POST to the URL u
response, err := http.Post(u, contentType, body)
Alternative implementation:
response, err := http.PostForm(u, formValues)
175
From array a of n bytes, build the equivalent hex string s of 2n digits.
Each byte (256 possible values) is encoded as two hexadecimal characters (16 possible values per digit).
s := hex.EncodeToString(a)
Alternative implementation:
s := fmt.Sprintf("%x", a)
176
From hex string s of 2n digits, build the equivalent array a of n bytes.
Each pair of hexadecimal characters (16 possible values per digit) is decoded into one byte (256 possible values).
a, err := hex.DecodeString(s)
if err != nil {
log.Fatal(err)
}
177
Construct a list L that contains all filenames that have the extension ".jpg" , ".jpeg" or ".png" in directory D and all it's subdirectories.
L := []string{}
err := filepath.Walk(D, func(path string, info os.FileInfo, err error) error {
if err != nil {
fmt.Printf("failure accessing a path %q: %v\n", path, err)
return err
}
for _, ext := range []string{".jpg", ".jpeg", ".png"} {
if strings.HasSuffix(path, ext) {
L = append(L, path)
break
}
}
return nil
})
178
Set boolean b to true if if the point with coordinates (x,y) is inside the rectangle with coordinates (x1,y1,x2,y2) , or to false otherwise.
Describe if the edges are considered to be inside the rectangle.
p := image.Pt(x, y)
r := image.Rect(x1, y1, x2, y2)
b := p.In(r)
179
Return the center c of the rectangle with coördinates(x1,y1,x2,y2)
c := image.Pt((x1+x2)/2, (y1+y2)/2)
180
Create list x containing the contents of directory d.

x may contain files and subfolders.
No recursive subfolder listing.
x, err := os.ReadDir(d)
182
Output the source of the program.
package main

import "fmt"

func main() {
fmt.Printf("%s%c%s%c\n", s, 0x60, s, 0x60)
}

var s = `package main

import "fmt"

func main() {
fmt.Printf("%s%c%s%c\n", s, 0x60, s, 0x60)
}

var s = `
183
Make a HTTP request with method PUT to the URL u
req, err := http.NewRequest("PUT", u, body)
if err != nil {
return err
}
req.Header.Set("Content-Type", contentType)
req.ContentLength = contentLength
response, err := http.DefaultClient.Do(req)
184
Assign to variable t a string representing the day, month and year of the day after the current date.
t := time.Now().Add(24 * time.Hour).Format("2006-01-02")
185
Schedule the execution of f(42) in 30 seconds.
timer := time.AfterFunc(
30*time.Second,
func() {
f(42)
})
Alternative implementation:
go func() {
time.Sleep(30 * time.Second)
f(42)
}()
186
Exit a program cleanly indicating no error to OS
os.Exit(0)
Alternative implementation:
defer os.Exit(0)
188
Perform matrix multiplication of a real matrix a with nx rows and ny columns, a real matrix b with ny rows and nz columns and assign the value to a real matrix c with nx rows and nz columns.
c := new(mat.Dense)
c.Mul(a, b)
189
Produce a new list y containing the result of function T applied to all elements e of list x that match the predicate P.
var y []Result
for _, e := range x {
if P(e) {
y = append(y, T(e))
}
}
190
Declare an external C function with the prototype

void foo(double *a, int n);

and call it, passing an array (or a list) of size 10 to a and 10 to n.

Use only standard features of your language.
// void foo(double *a, int n);
// double a[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
import "C"

C.foo(C.a, 10)
191
Given a one-dimensional array a, check if any value is larger than x, and execute the procedure f if that is the case
for _, v := range a {
if v > x {
f()
break
}
}
192
Declare a real variable a with at least 20 digits; if the type does not exist, issue an error at compile time.
a, _, err := big.ParseFloat("123456789.123456789123465789", 10, 200, big.ToZero)
197
Retrieve the contents of file at path into a list of strings lines, in which each element is a line of the file.
func readLines(path string) ([]string, error) {
b, err := os.ReadFile(path)
if err != nil {
return nil, err
}
lines := strings.Split(string(b), "\n")
return lines, nil
}
198
Abort program execution with error condition x (where x is an integer value)
os.Exit(x)
199
Truncate a file F at the given file position.
err := os.Truncate(F, position)
200
Returns the hypotenuse h of the triangle where the sides adjacent to the square angle have lengths x and y.
h := math.Hypot(x, y)
202
Calculate the sum of squares s of data, an array of floating point values.
var s float64
for _, d := range data {
s += math.Pow(d, 2)
}
203
Calculate the mean m and the standard deviation s of the list of floating point values data.
m, s := stat.MeanStdDev(data, nil)
204
Given a real number a, print the fractional part and the exponent of the internal representation of that number. For 3.14, this should print (approximately)

0.785 2
fmt.Println(math.Frexp(a))
205
Read an environment variable with the name "FOO" and assign it to the string variable foo. If it does not exist or if the system does not support environment variables, assign a value of "none".
foo, ok := os.LookupEnv("FOO")
if !ok {
foo = "none"
}
Alternative implementation:
foo := os.Getenv("FOO")
if foo == "" {
foo = "none"
}
206
Execute different procedures foo, bar, baz and barfl if the string str contains the name of the respective procedure. Do it in a way natural to the language.
switch str {
case "foo":
foo()
case "bar":
bar()
case "baz":
baz()
case "barfl":
barfl()
}
207
Allocate a list a containing n elements (n assumed to be too large for a stack) that is automatically deallocated when the program exits the scope it is declared in.
a := make([]T, n)
208
Given the arrays a,b,c,d of equal length and the scalar e, calculate a = e*(a+b*c+cos(d)).
Store the results in a.
func applyFormula(a, b, c, d []float64, e float64) {
for i, v := range a {
a[i] = e * (v + b[i] + c[i] + math.Cos(d[i]))
}
}
209
Declare a type t which contains a string s and an integer array n with variable size, and allocate a variable v of type t. Allocate v.s and v.n and set them to the values "Hello, world!" for s and [1,4,9,16,25], respectively. Deallocate v, automatically deallocating v.s and v.n (no memory leaks).
type t struct {
s string
n []int
}

v := t{
s: "Hello, world!",
n: []int{1, 4, 9, 16, 25},
}
210
Assign, at runtime, the compiler version and the options the program was compilerd with to variables version and options, respectively, and print them. For interpreted languages, substitute the version of the interpreter.

Example output:

GCC version 10.0.0 20190914 (experimental)
-mtune=generic -march=x86-64
version := runtime.Version()
211
Create the folder at path on the filesystem
err := os.Mkdir(path, os.ModeDir)
Alternative implementation:
err := os.MkdirAll(path, os.ModeDir)
212
Set boolean b to true if path exists on the filesystem and is a directory; false otherwise.
info, err := os.Stat(path)
b := !os.IsNotExist(err) && info.IsDir()
214
Append extra character c at the end of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes.
if n := utf8.RuneCountInString(s); n < m {
s += strings.Repeat(c, m-n)
}
215
Prepend extra character c at the beginning of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes.
if n := utf8.RuneCountInString(s); n < m {
s = strings.Repeat(c, m-n) + s
}
217
Create a zip-file with filename name and add the files listed in list to that zip-file.
buf := new(bytes.Buffer)
w := zip.NewWriter(buf)
for _, filename := range list {
input, err := os.Open(filename)
if err != nil {
return err
}
output, err := w.Create(filename)
if err != nil {
return err
}
_, err = io.Copy(output, input)
if err != nil {
return err
}
}

err := w.Close()
if err != nil {
return err
}

err = os.WriteFile(name, buf.Bytes(), 0777)
if err != nil {
return err
}
218
Create the list c containing all unique elements that are contained in both lists a and b.
c should not contain any duplicates, even if a and b do.
The order of c doesn't matter.
seta := make(map[T]bool, len(a))
for _, x := range a {
seta[x] = true
}
setb := make(map[T]bool, len(a))
for _, y := range b {
setb[y] = true
}

var c []T
for x := range seta {
if setb[x] {
c = append(c, x)
}
}
219
Create string t from the value of string s with each sequence of spaces replaced by a single space.

Explain if only the space characters will be replaced, or the other whitespaces as well: tabs, newlines.
whitespaces := regexp.MustCompile(`\s+`)
t := whitespaces.ReplaceAllString(s, " ")
220
Create t consisting of 3 values having different types.

Explain if the elements of t are strongly typed or not.
t := []interface{}{
2.5,
"hello",
make(chan int),
}
Alternative implementation:
a, b, c := 2.5, "hello", make(chan int)
221
Create string t from string s, keeping only digit characters 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.
re := regexp.MustCompile("[^\\d]")
t := re.ReplaceAllLiteralString(s, "")
222
Set i to the first index in list items at which the element x can be found, or -1 if items does not contain x.
i := -1
for j, e := range items {
if e == x {
i = j
break
}
}
223
Loop through list items checking a condition. Do something else if no matches are found.

A typical use case is looping through a series of containers looking for one that matches a condition. If found, an item is inserted; otherwise, a new container is created.

These are mostly used as an inner nested loop, and in a location where refactoring inner logic into a separate function reduces clarity.
items := []string{"foo", "bar", "baz", "qux"}

for _, item := range items {
if item == "baz" {
fmt.Println("found it")
goto forelse
}
}
{
fmt.Println("never found it")
}
forelse:
224
Insert element x at the beginning of list items.
items = append([]T{x}, items...)
Alternative implementation:
items = append(items, x)
copy(items[1:], items)
items[0] = x
225
Declare an optional integer argument x to procedure f, printing out "Present" and its value if it is present, "Not present" otherwise
func f(x ...int) {
if len(x) > 0 {
println("Present", x[0])
} else {
println("Not present")
}
}
226
Remove the last element from list items.
items = items[:len(items)-1]
227
Create the new list y containing the same elements as the list x.

Subsequent modifications of y must not affect x (except for the contents referenced by the elements themselves if they contain pointers).
y := make([]T, len(x))
copy(y, x)
228
Copy the file at path src to dst.
func copy(dst, src string) error {
data, err := os.ReadFile(src)
if err != nil {
return err
}
stat, err := os.Stat(src)
if err != nil {
return err
}
return os.WriteFile(dst, data, stat.Mode())
}
Alternative implementation:
func copy(dst, src string) error {
data, err := os.ReadFile(src)
if err != nil {
return err
}
stat, err := os.Stat(src)
if err != nil {
return err
}
err = os.WriteFile(dst, data, stat.Mode())
if err != nil {
return err
}
return os.Chmod(dst, stat.Mode())
}
Alternative implementation:
func copy(dst, src string) error {
f, err := os.Open(src)
if err != nil {
return err
}
defer f.Close()
stat, err := f.Stat()
if err != nil {
return err
}
g, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, stat.Mode())
if err != nil {
return err
}
defer g.Close()
_, err = io.Copy(g, f)
if err != nil {
return err
}
return os.Chmod(dst, stat.Mode())
}
229
Interrupt an ongoing processing p.
ctx, cancel := context.WithCancel(context.Background())
go p(ctx)

somethingElse()

cancel()
230
Cancel an ongoing processing p if it has not finished after 5s.
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel()
p(ctx)
231
Set b to true if the byte sequence s consists entirely of valid UTF-8 character code points, false otherwise.
b := utf8.Valid(s)
232
Print "verbose is true" if the flag -v was passed to the program command line, "verbose is false" otherwise.
var verbose = flag.Bool("v", false, "verbose")
flag.Parse()
fmt.Println("verbose is", *verbose)
233
Print the value of the flag -country passed to the program command line, or the default value "Canada" if no such flag was passed.
var country = flag.String("country", "Canada", "user home country")
flag.Parse()
fmt.Println("country is", *country)
234
Assign to string s the standard base64 encoding of the byte array data, as specified by RFC 4648.
s := base64.StdEncoding.EncodeToString(data)
235
Assign to byte array data the bytes represented by the base64 string s, as specified by RFC 4648.
data, err := base64.StdEncoding.DecodeString(s)
236
Initialize a quotient q = a/b of arbitrary precision. a and b are large integers.
q := new(big.Rat)
q.SetString(str)
Alternative implementation:
q := new(big.Rat)
q.SetFrac(a, b)
Alternative implementation:
q := big.NewRat(a, b)
237
Assign to c the result of (a xor b)
c := a ^ b
Alternative implementation:
c := new(big.Int)
c.Xor(a, b)
238
Write in a new byte array c the xor result of byte arrays a and b.

a and b have the same size.
c := make([]byte, len(a))
for i := range a {
c[i] = a[i] ^ b[i]
}
Alternative implementation:
var c T
for i := range a {
c[i] = a[i] ^ b[i]
}
239
Assign to string x the first word of string s consisting of exactly 3 digits, or the empty string if no such match exists.

A word containing more digits, or 3 digits as a substring fragment, must not match.
re := regexp.MustCompile(`\b\d\d\d\b`)
x := re.FindString(s)
240
Lists a and b have the same length. Apply the same permutation to a and b to have them sorted based on the values of a.
type sorter struct {
k []K
t []T
}

func (s *sorter) Len() int {
return len(s.k)
}

func (s *sorter) Swap(i, j int) {
s.k[i], s.k[j] = s.k[j], s.k[i]
s.t[i], s.t[j] = s.t[j], s.t[i]
}

func (s *sorter) Less(i, j int) bool {
return s.k[i] < s.k[j]
}

sort.Sort(&sorter{
k: a,
t: b,
})
241
Explicitly decrease the priority of the current process, so that other execution threads have a better chance to execute now. Then resume normal execution and call function busywork.
runtime.Gosched()
busywork()
242
Call a function f on each element e of a set x.
for e := range x {
f(e)
}
243
Print the contents of the list or array a on the standard output.
fmt.Println(a)
244
Print the contents of the map m to the standard output: keys and values.
fmt.Println(m)
Alternative implementation:
fmt.Printf("%q", m)
245
Print the value of object x having custom type T, for log or debug.
fmt.Println(x)
246
Set c to the number of distinct elements in the list items.
distinct := make(map[T]bool)
for _, v := range items {
distinct[v] = true
}
c := len(distinct)
247
Remove all the elements from list x that don't satisfy the predicate p, without allocating a new list.
Keep all the elements that do satisfy p.

For languages that don't have mutable lists, refer to idiom #57 instead.
j := 0
for i, v := range x {
if p(v) {
x[j] = x[i]
j++
}
}
x = x[:j]
Alternative implementation:
for i, v := range x {
if p(v) {
x[j] = x[i]
j++
}
}
for k := j; k < len(x); k++ {
x[k] = nil
}
x = x[:j]
249
Define variables a, b and c in a concise way.
Explain if they need to have the same type.
a, b, c := 42, "hello", 5.0
250
Choose a value x from map m.
m must not be empty. Ignore the keys.
func pick(m map[K]V) V {
k := rand.Intn(len(m))
i := 0
for _, x := range m {
if i == k {
return x
}
i++
}
panic("unreachable")
}
Alternative implementation:
func pick(m map[K]V) V {
k := rand.Intn(len(m))
for _, x := range m {
if k == 0 {
return x
}
k--
}
panic("unreachable")
}
251
Extract integer value i from its binary string representation s (in radix 2)
E.g. "1101" -> 13
i, err := strconv.ParseInt(s, 2, 0)
252
Assign to the variable x the value "a" if calling the function condition returns true, or the value "b" otherwise.
if condition() {
x = "a"
} else {
x = "b"
}
253
Print the stack frames of the current execution thread of the program.
debug.PrintStack()
254
Replace all exact occurrences of "foo" with "bar" in the string list x
for i, s := range x {
if s == "foo" {
x[i] = "bar"
}
}
256
Print the numbers 5, 4, ..., 0 (included), one line per number.
for i := 5; i >= 0; i-- {
fmt.Println(i)
}
257
Print each index i and value x from the list items, from the last down to the first.
for i := len(items) - 1; i >= 0; i-- {
x := items[i]
fmt.Printf("Item %d = %v \n", i, x)
}
258
Convert the string values from list a into a list of integers b.
b := make([]int, len(a))
var err error
for i, s := range a {
b[i], err = strconv.Atoi(s)
if err != nil {
return err
}
}
259
Build the list parts consisting of substrings of the input string s, separated by any of the characters ',' (comma), '-' (dash), '_' (underscore).
re := regexp.MustCompile("[,\\-_]")
parts := re.Split(s, -1)
260
Declare a new list items of string elements, containing zero elements
var items []string
261
Assign to string x the value of fields (hours, minutes, seconds) of date d, in format HH:MM:SS.
x := d.Format("15:04:05")
262
Assign to t the number of trailing 0 bits in the binary representation of the integer n.

E.g. for n=112, n is 1110000 in base 2 ⇒ t=4
t := bits.TrailingZeros(n)
266
Assign to the string s the value of the string v repeated n times, and write it out.

E.g. v="abc", n=5 ⇒ s="abcabcabcabcabc"
s := strings.Repeat(v, n)
fmt.Println(s)
267
Declare an argument x to a procedure foo that can be of any type. If the type of the argument is a string, print it, otherwise print "Nothing."

Test by passing "Hello, world!" and 42 to the procedure.
func foo(x interface{}) {
if s, ok := x.(string); ok {
fmt.Println(s)
} else {
fmt.Println("Nothing.")
}
}

func main() {
foo("Hello, world!")
foo(42)
}
273
Set the boolean b to true if the directory at filepath p is empty (i.e. doesn't contain any other files and directories)
dir, err := os.Open(p)
if err != nil {
panic(err)
}
defer dir.Close()
_, err = dir.Readdirnames(1)
b := err == io.EOF
274
Create the string t from the string s, removing all the spaces, newlines, tabulations, etc.
t := strings.Map(func(r rune) rune {
if unicode.IsSpace(r) {
return -1
}
return r
}, s)
275
From the string s consisting of 8n binary digit characters ('0' or '1'), build the equivalent array a of n bytes.
Each chunk of 8 binary digits (2 possible values per digit) is decoded into one byte (256 possible values).
n := len(s) / 8
a := make([]byte, n)
for i := range a {
b, err := strconv.ParseInt(s[i*8:i*8+8], 2, 0)
if err != nil {
log.Fatal(err)
}
a[i] = byte(b)
}
276
Insert an element e into the set x.
x[e] = struct{}{}
Alternative implementation:
x[e] = true
277
Remove the element e from the set x.

Explains what happens if e was already absent from x.
delete(x, e)
Alternative implementation:
delete(x, e)
278
Read one line into the string line.

Explain what happens if EOF is reached.
s := bufio.NewScanner(os.Stdin)
if ok := s.Scan(); !ok {
log.Fatal(s.Err())
}
line := s.Text()
279
Read all the lines (until EOF) into the list of strings lines.
var lines []string
s := bufio.NewScanner(os.Stdin)
for s.Scan() {
line := s.Text()
lines = append(lines, line)
}
if err := s.Err(); err != nil {
log.Fatal(err)
}
280
Remove all the elements from the map m that don't satisfy the predicate p.
Keep all the elements that do satisfy p.

Explain if the filtering happens in-place, i.e. if m is reused or if a new map is created.
for k, v := range m {
if !p(v) {
delete(m, k)
}
}
281
You have a Point with integer coordinates x and y. Create a map m with key type Point (or equivalent) and value type string. Insert "Hello" at position (42, 5).
m := map[Point]string{}
p := Point{x: 42, y: 5}
m[p] = "Hello"
282
Declare a type Foo, and create a new map with Foo as key type.

Mention the conditions on Foo required to make it a possible map key type.
type Foo struct {
name string
x, y int
}

m := make(map[Foo]string)
283
Build the list parts consisting of substrings of input string s, separated by the string sep.
parts := strings.Split(s, sep)
284
Create a new list a (or array, or slice) of size n, where all elements are integers initialized with the value 0.
a := make([]int, n)
286
Print a line "Char i is c" for each character c of the string s, where i is the character index of c in s (not the byte index).

Make sure that multi-byte characters are properly handled, and count for a single character.
i := 0
for _, c := range s {
fmt.Printf("Char %d is %c\n", i, c)
i++
}
287
Assign to n the number of bytes in the string s.

This can be different from the number of characters. If n includes more bytes than the characters per se (trailing zero, length field, etc.) then explain it. One byte is 8 bits.
n := len(s)
288
Set the boolean b to true if the set x contains the element e, false otherwise.
b := x[e]
Alternative implementation:
_, b := x[e]
289
Create the string s by concatenating the strings a and b.
s := a + b
290
Sort the part of the list items from index i (included) to index j (excluded), in place, using the comparator c.

Elements before i and after j must remain unchanged.
sub := items[i:j]
sort.Slice(sub, func(a, b int) bool {
return c(sub[a], sub[b])
})
291
Delete all the elements from index i (included) to index j (excluded) from the list items.
copy(items[i:], items[j:])
for k, n := len(items)-j+i, len(items); k < n; k++ {
items[k] = nil
}
items = items[:len(items)-j+i]
Alternative implementation:
items = append(items[:i], items[j:]...)
292
Write "Hello World and 你好" to standard output in UTF-8.
fmt.Println("Hello World and 你好")
296
Assign to x2 the value of string x with the last occurrence of y replaced by z.
If y is not contained in x, then x2 has the same value as x.
func replaceLast(x, y, z string) (x2 string) {
i := strings.LastIndex(x, y)
if i == -1 {
return x
}
return x[:i] + z + x[i+len(y):]
}
297
Sort the string list data in a case-insensitive manner.

The sorting must not destroy the original casing of the strings.
func lessCaseInsensitive(s, t string) bool {
for {
if len(t) == 0 {
return false
}
if len(s) == 0 {
return true
}
c, sizec := utf8.DecodeRuneInString(s)
d, sized := utf8.DecodeRuneInString(t)

lowerc := unicode.ToLower(c)
lowerd := unicode.ToLower(d)

if lowerc < lowerd {
return true
}
if lowerc > lowerd {
return false
}

s = s[sizec:]
t = t[sized:]
}
}

sort.Slice(data, func(i, j int) bool { return lessCaseInsensitive(data[i], data[j]) })
Alternative implementation:
sort.Slice(data, func(i, j int) bool {
return strings.ToLower(data[i]) < strings.ToLower(data[j])
})
298
Create the map y by cloning the map x.

y is a shallow copy, not a deep copy.
y := make(map[K]V, len(x))
for k, v := range x {
y[k] = v
}
299
Write a line of comments.

This line will not be compiled or executed.
// This is a comment
302
Give the integer x = 8, assign to the string s the value "Our sun has 8 planets", where the number 8 was evaluated from x.
s := fmt.Sprintf("Our sun has %d planets", x)
304
Create the array of bytes data by encoding the string s in UTF-8.
data := []byte(s)
306
Preallocate memory in the list x for a minimum total capacity of 200 elements.

This is not possible in all languages. It is only meant as a performance optimization, should not change the length of x, and should not have any effect on correctness.
if cap(x) < 200 {
y := make([]T, len(x), 200)
copy(y, x)
x = y
}