Zig While Loops

A while loop repeats a block of code while its condition stays true. The loop stops as soon as the condition becomes false.

A while loop repeats until the condition turns false

The counter starts at 1 and grows by 1 on each run. The loop prints it while it stays at or below 3.

const std = @import("std");

pub fn main() !void {
    const out = std.io.getStdOut().writer();
    var i: i32 = 1;
    while (i <= 3) {
        try out.print("{d}\n", .{i});
        i += 1;
    }
}

The continue expression runs after each run

The step i += 1 sits after the colon, so it runs at the end of every run. The counter itself is printed inside the body, starting from 0.

const std = @import("std");

pub fn main() !void {
    const out = std.io.getStdOut().writer();
    var i: i32 = 0;
    while (i < 3) : (i += 1) {
        try out.print("{d}\n", .{i});
    }
}

A while loop can build up a total

Each run adds the counter to total and then raises the counter. After the loop, total holds the sum of 1, 2, 3, and 4.

const std = @import("std");

pub fn main() !void {
    const out = std.io.getStdOut().writer();
    var n: i32 = 1;
    var total: i32 = 0;
    while (n <= 4) {
        total += n;
        n += 1;
    }
    try out.print("{d}\n", .{total});
}

Example

const std = @import("std");

pub fn main() !void {
    const out = std.io.getStdOut().writer();
    var n: i32 = 5;
    while (n > 0) : (n -= 1) {
        try out.print("{d}\n", .{n});
    }
    try out.print("go\n", .{});
}

Key points

Condition: while tests its bool condition before every run of the block.
Body: the body is the block of code that repeats.
Counter: a var counter is a common way to control how many times the loop runs.
Step: a line inside the loop must move the condition toward false, or the loop never ends.
Continue expression: extra code written after a colon runs at the end of each run of the loop.
Zero runs: if the condition is false at the very first test, the body never runs.

In this example, n starts at 5 and the body prints it on each run. The continue expression lowers n by 1 after each print, so the loop counts down to 1, stops when n reaches 0, and then prints go.

Note

unlike a for loop, a while loop does not count for you, so the counting step is your job.

Try it Yourself

Edit the code, then press Run.

Running is simulated

const std = @import("std");

pub fn main() !void {
    const out = std.io.getStdOut().writer();
    var n: i32 = 5;
    while (n > 0) : (n -= 1) {
        try out.print("{d}\n", .{n});
    }
    try out.print("go\n", .{});
}

Exercise

A loop starts with i at 0 and runs while (i < 4), adding 1 to i inside the body. How many times does the body run?

0 3 4 5