Implementation
C++

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Other implementations
import "strconv"
s := strconv.FormatInt(x, 2)
import "fmt"
import "math/big"
s := fmt.Sprintf("%b", x)
import std.conv;
auto s = to!string(13,2);
var Iter,n:integer;
[...]
  S := '';
  for Iter := 0 to n do
    s:= Char(Ord('0')+(x shr Iter) and 1) + S;   
let s = format!("{:b}", x);
$s = sprintf("%b", $x);
StrUtils;
var
  _x: Integer;
  _s: String;
begin
  _s := IntToBin(_x,8*SizeOf(Integer));
end.
var s = i.toRadixString(2);
s = sprintf "%b", 13;
import Numeric (showIntAtBase)
import Data.Char (intToDigit)
s x = showIntAtBase 2 intToDigit x ""
s = x.to_s(2)
s = Integer.to_string(x, 2)
s = '{:b}'.format(x)
S = io_lib:format("~.2B~n", [X]).
String s = Integer.toBinaryString(x);
var base10number = 123

var base2number = base10number.toString(2)
local s = {}

while x > 0 do
    local tmp = math.fmod(x,2)
    s[#s+1] = tmp
    x=(x-tmp)/2
end

s=table.concat(s)
Convert.ToString(42,2);
// Prints : 101010
Convert.ToString(42, 2).PadLeft(8, '0');
// Prints  00101010
write (unit=s,fmt='(B0)') 13 ! just the string length
write (unit=s,fmt='(B32)') 13 ! Fills it up with zeros