• Perl

$i = -1;
$index = 0;
foreach (@items) {
    $i = $index, last if $x eq $items[$index];
    $index++;
}

• Clojure

(defn find-index [x items]
  (or (->> items
           (map-indexed vector)
           (filter #(= x (peek %)))
           ffirst)
      -1))

• Clojure

(defn find-index [x items]
  (reduce-kv
    #(if (= x %3) (reduced %2) %1) -1 items))

• C++

auto it = std::find(items.begin(), items.end(), x);
i = it == items.end() ? std::distance(items.begin(), it) : -1;

• C#

var i = items.IndexOf(x)

• Dart

i=items.indexOf(x)

• Elixir

i = Enum.find_index(items, & &1 == x)

• Fortran

i = findloc(items, x)
if (i == 0) i = -1

• Go

i := slices.Index(items, x)

• Go

i := -1
for j, e := range items {
	if e == x {
		i = j
		break
	}
}

• Haskell

findIndex x items = fst <$> (find ((==x) . snd) . zip [0..]) items

i = fromMaybe -1 (findIndex x items)

• JS

let i = items.indexOf(x);

• Java

int i = -1;
for(int j=0;j<items.length;j++){
	if(items[j].equals(x)){
		i = j;
		break;
	}
}

• Java

int i = items.indexOf​(x);

• Pascal

i := Items.IndexOf(x);

• Python

i = items.index(x) if x in items else -1

• Ruby

i = items.index(x) || -1

• Rust

let i = items.iter().position(|y| *y == x).map_or(-1, |n| n as i32);

• Rust

let opt_i = items.iter().position(|y| *y == x);


let i = match opt_i {
   Some(index) => index as i32,
   None => -1
};