Count positions and pair inputs

Use enumerate(iterable) to get an i64 index starting at zero beside each element. Supply a second argument to choose the first index. Use zip(left, right) to pair elements until the shorter input ends.

def main() -> Result[(), AllocError | IoError | OverflowError[i64]]:
    for index, letter in enumerate("ab", 10):
        print((index, letter))?
    for number, letter in zip(range(3), "xy"):
        print((number, letter))?
    print([index + value for index, value in enumerate(range(3))]?)?
    Ok(())

(10, "a")
(11, "b")
(0, "x")
(1, "y")
[0, 2, 4]

An owned list transfers its elements. Borrow a list to preserve it; its elements retain their reference types. Borrow mutably to edit them.

def main() -> Result[(), AllocError | IoError | OverflowError[i64]]:
    mut values = [10, 20]?
    for index, value in enumerate(&mut values):
        *value += index
    for value, offset in zip(&values, range(2)):
        print(*value + offset)?
    print(values)?
    Ok(())

10
22
[10, 21]

Both helpers work directly in loop and comprehension sources. They add no allocation and evaluate their input expressions once, from left to right. They do not yet produce a value you can store or pass to next. Borrowed inputs require two unpacked bindings; references cannot be placed into an item tuple. Nested helpers are supported when their inner items are owned values. To pair a dictionary’s keys, pass the dictionary itself; items() remains a separate loop form.

zip tests the left input before the right. A left generator may advance once more when the right input ends; that unmatched item is dropped. Early exit drops remaining owned inputs. An enumeration index overflows only when another element requires an index beyond i64.MAX.