• Erlang

hexChar(Num) when Num < 10 andalso Num >= 0->
    $0 + Num;
hexChar(Num) when Num < 16 ->
    $a + Num - 10.

toHex([Byte | Rest]) ->
    [hexChar(Byte div 16), hexChar(Byte rem 16)] ++ toHex(Rest);
toHex([]) -> [].

• D

s = a.toHexString;

• Perl

$s = unpack('H*', pack('c*', @a));

• C

char *s = calloc(n * 2 + 1, 1);
for(int i = 0; i < n; ++i){
  char temp[3];
  sprintf(temp, "%02x", a[i]);
  s[2 * i] = temp[0];
  s[2 * i + 1] = temp[1];
}

• C#

var s = Convert.ToHexString(a);

• C#

Int32 s = BitConverter.ToInt32(sampleBuffer, 0);
s = s.Replace("-", string.Empty);

• Dart

var s = a.buffer
    .asUint8List()
    .map((e) => e.toRadixString(16).padLeft(2, '0'))
    .join();

• Fortran

  character(len=:), allocatable :: s
  allocate (character(len=2*size(a)) :: s)
  write(unit=s,fmt='(*(Z2.2))') a

• Go

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

• Go

s := hex.EncodeToString(a)

• Groovy

def s = a.encodeHex().toString()

• JS

const s = Buffer.from(a).toString('hex')

• JS

const s = a.map(n => n.toString(16).padStart(2, '0')).join('')

• Java

String s = "";
for (byte b : a)
    s = s + "%02x".formatted(b);

• Java

String s = range(0, n)
    .mapToObj(x -> a[x])
    .map("%02x"::formatted)
    .reduce(String::concat)
    .get();

• Java

var s = HexFormat.of().formatHex(a);

• Lisp

(lambda (bytes &aux (size (* 2 (length bytes))))
  (let ((hex (make-array size :element-type 'character :fill-pointer 0)))
    (prog1 hex
      (with-output-to-string (o hex)
        (map () (lambda (byte) (format o "~2,'0x" byte)) bytes)))))

• Lua

for i,v in ipairs(a) do
 a[i]=string.format("%02x",v)
end
local s=table.concat(a)

• Pascal

s := '';
for b in a do s := s + IntToHex(b,2);

• Python

s = a.hex()

• Ruby

s = a.pack("c*").unpack("H*").first

• Ruby

s = a.unpack("H*")

• Rust

fn byte_to_hex(byte: u8) -> (u8, u8) {
    static HEX_LUT: [u8; 16] = [b'0', b'1', b'2', b'3', b'4', b'5', b'6', b'7', b'8', b'9', b'a', b'b', b'c', b'd', b'e', b'f'];

    let upper = HEX_LUT[(byte >> 4) as usize];
    let lower = HEX_LUT[(byte & 0xF) as usize];
    (lower, upper)
}

let utf8_bytes: Vec<u8> = a.iter().copied().flat_map(|byte| {
        let (lower, upper) = byte_to_hex(byte);
        [upper, lower]
    }).collect();
let s = unsafe { String::from_utf8_unchecked(utf8_bytes) };

• Rust

let s = HEXLOWER.encode(&a);

• Rust

let mut s = String::with_capacity(2 * n);
for byte in a {
    write!(s, "{:02X}", byte)?;
}

• Rust

let s = a.encode_hex::<String>();