# While Loops

> Conditions, continue expressions, and loops that produce values.

```zig
const std = @import("std");
const expect = std.testing.expect;

test "basic while" {
    var i: u8 = 2;
    while (i < 100) {
        i *= 2;
    }
    try expect(i == 128);
}

test "while with a continue expression" {
    // The continue expression runs after every iteration, including ones
    // ended by `continue`, which is why it is not just the last statement.
    var sum: u8 = 0;
    var i: u8 = 1;
    while (i <= 10) : (i += 1) {
        sum += i;
    }
    try expect(sum == 55);
}

test "break and continue" {
    var sum: u8 = 0;
    var i: u8 = 0;
    while (i <= 3) : (i += 1) {
        if (i == 2) continue;
        sum += i;
    }
    try expect(sum == 4); // 0 + 1 + 3
}

test "while as an expression with else" {
    // `break x` yields a value; `else` runs when the loop ends normally.
    var i: u8 = 0;
    const found = while (i < 10) : (i += 1) {
        if (i * i > 20) break i;
    } else 0;
    try expect(found == 5);
}
```

*Runnable: compiled to WebAssembly and executed by CI against Zig master. (`02-language.while-loops`)*

## The continue expression

```zig
while (i <= 10) : (i += 1) { ... }
```

The `: (i += 1)` part runs after **every** iteration, including ones cut short
by `continue`. That is precisely why it exists rather than just putting the
increment at the end of the body: with `continue` in the mix, the two are not
equivalent, and the bug that results is easy to miss.

## Loops that produce values

`break` can carry a value, and `while` can have an `else` branch that runs when
the loop finishes without breaking:

```zig
const found = while (i < 10) : (i += 1) {
    if (i * i > 20) break i;
} else 0;
```

This is the search-with-fallback pattern as a single expression, with no
sentinel variable and no way to forget the not-found case.