• Ruby

s = x.to_s(16)

• Clojure

• C

char s[32];
snprintf(s, sizeof(s), "%x", i);

• C++

template <typename I>
std::string n2hexstr(I w, size_t hex_len = sizeof(I)<<1) {
    static const char* digits = "0123456789ABCDEF";
	std::string str(hex_len, '-');
    for (size_t i=0, j=(hex_len-1)*4 ; i<hex_len; ++i,j-=4)
       str[i] = digits[(w>>j) & 0x0f];
	return str;
}

• C++

std::ostringstream stream;
stream << std::hex << x;
s = stream.str();

• C#

String s = x.ToString("x")

• D

s = format("%x", x);

• D

s = to!string(x, 16);

• Dart

var s = x.toRadixString(16);

• Elixir

s = Integer.to_string(x, 16)

• Erlang

S = io_lib:fwrite("~.16B",[X]).

• Fortran

write (*,'(Z8.8)') x

• Go

s := strconv.FormatInt(x, 16)

• Go

s := fmt.Sprintf("%x", x)

• Haskell

s :: String
s = printf "%x" 999

• JS

const s = x.toString(16)

• Java

String s = HexFormat.of().toHexDigits(x);

• Java

String s = Integer.toHexString(x);

• Java

String s = "%x".formatted(x);

• Lisp

(setf s (format nil "~X" x))

• Lua

s = string.format("%x",x)

• PHP

$s = dechex($x);

• Pascal

S := Format("%x", [x]);

• Pascal

s := IntToHex(x);

• Perl

my $s = sprintf("%x", $i);

• Python

s = hex(x)

• Python

s = format(x, 'x')

• Python

s = '%x' % x

• Rust

let s = format!("{:X}", x);

• Scala

val s = x.toString(16)