• C

uint32_t c = i;
c = (c & 0x55555555) + ((c & 0xAAAAAAAA) >> 1);
c = (c & 0x33333333) + ((c & 0xCCCCCCCC) >> 2);
c = (c & 0x0F0F0F0F) + ((c & 0xF0F0F0F0) >> 4);
c = (c & 0x00FF00FF) + ((c & 0xFF00FF00) >> 8);
c = (c & 0x0000FFFF) + ((c & 0xFFFF0000) >> 16);

• JS

const c = i.toString(2).replace(/[^1]/g, '').length

• Clojure

(def c (count (re-seq #"1" (Integer/toBinaryString i))))

• C++

uint32_t c = 0;
for (; i; i &= i - 1, ++c);

• C#

public static int BitCount(int i)
{
    var c = 0;
    while (n != 0)
    {
        c++;
        n &= (n - 1); //walking through all the bits which are set to one
    }

    return c;
}

• C#

var c = BitOperations.PopCount((uint)i);

• D

auto c = bitSet(i).length;

• Dart

var c = i.toRadixString(2).replaceAll('0','').length;

• Fortran

c = popcnt(i)

• Go

c := bits.OnesCount(i)

• Go

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)
}

• Haskell

c = Data.Bits.popCount i

• Java

int c = new BigInteger(valueOf(i), 2).bitCount();

• Java

int c = Integer.bitCount(i);

• Lisp

(setf c (logcount i))

• PHP

$i = base_convert($i,10,2);
$x = str_replace(0,'',$i);
$c = strlen($x);

• Pascal

function BitCount(N: Int64): Integer;
var
  Q: QWord;
  i: Integer;
begin
  Result := 0;
  Q := QWord(N);
  for i := 0 to (8 * SizeOf(N) - 1) do
  begin
    if ((Q and 1) = 1) then Inc(Result);
    Q := Q shr 1;
  end;
end;

• Pascal

c := PopCnt(i);

• Perl

# Not best, but simple:
$c=0;
while ($i) {
   $c += $i&1;
   $i /= 2;
}

• Perl

$c = unpack '%b*', pack 'i', $i;

• Python

c = bin(i).count("1")

• Python

c = i.bit_count()

• Ruby

c = i.digits(2).count(1)

• Rust

let c = i.count_ones();