for

for.zig

  1. const expect = @import("std").testing.expect;
  3. test "for basics" {
  4.     const items = [_]i32 { 4, 5, 3, 4, 0 };
  5.     var sum: i32 = 0;
  7.     // For loops iterate over slices and arrays.
  8.     for (items) |value| {
  9.         // Break and continue are supported.
  10.         if (value == 0) {
  11.             continue;
  12.         }
  13.         sum += value;
  14.     }
  15.     try expect(sum == 16);
  17.     // To iterate over a portion of a slice, reslice.
  18.     for (items[0..1]) |value| {
  19.         sum += value;
  20.     }
  21.     try expect(sum == 20);
  23.     // To access the index of iteration, specify a second capture value.
  24.     // This is zero-indexed.
  25.     var sum2: i32 = 0;
  26.     for (items) |_, i| {
  27.         try expect(@TypeOf(i) == usize);
  28.         sum2 += @intCast(i32, i);
  29.     }
  30.     try expect(sum2 == 10);
  31. }
  33. test "for reference" {
  34.     var items = [_]i32 { 3, 4, 2 };
  36.     // Iterate over the slice by reference by
  37.     // specifying that the capture value is a pointer.
  38.     for (items) |*value| {
  39.         value.* += 1;
  40.     }
  42.     try expect(items[0] == 4);
  43.     try expect(items[1] == 5);
  44.     try expect(items[2] == 3);
  45. }
  47. test "for else" {
  48.     // For allows an else attached to it, the same as a while loop.
  49.     var items = [_]?i32 { 3, 4, null, 5 };
  51.     // For loops can also be used as expressions.
  52.     // Similar to while loops, when you break from a for loop, the else branch is not evaluated.
  53.     var sum: i32 = 0;
  54.     const result = for (items) |value| {
  55.         if (value != null) {
  56.             sum += value.?;
  57.         }
  58.     } else blk: {
  59.         try expect(sum == 12);
  60.         break :blk sum;
  61.     };
  62.     try expect(result == 12);
  63. }

Shell

  1. $ zig test for.zig
  2. 1/3 test.for basics... OK
  3. 2/3 test.for reference... OK
  4. 3/3 test.for else... OK
  5. All 3 tests passed.

Labeled for

When a for loop is labeled, it can be referenced from a break or continue from within a nested loop:

test_nested_break.zig

  1. const std = @import("std");
  2. const expect = std.testing.expect;
  4. test "nested break" {
  5.     var count: usize = 0;
  6.     outer: for ([_]i32{ 1, 2, 3, 4, 5 }) |_| {
  7.         for ([_]i32{ 1, 2, 3, 4, 5 }) |_| {
  8.             count += 1;
  9.             break :outer;
  10.         }
  11.     }
  12.     try expect(count == 1);
  13. }
  15. test "nested continue" {
  16.     var count: usize = 0;
  17.     outer: for ([_]i32{ 1, 2, 3, 4, 5, 6, 7, 8 }) |_| {
  18.         for ([_]i32{ 1, 2, 3, 4, 5 }) |_| {
  19.             count += 1;
  20.             continue :outer;
  21.         }
  22.     }
  24.     try expect(count == 8);
  25. }

Shell

  1. $ zig test test_nested_break.zig
  2. 1/2 test.nested break... OK
  3. 2/2 test.nested continue... OK
  4. All 2 tests passed.

inline for

For loops can be inlined. This causes the loop to be unrolled, which allows the code to do some things which only work at compile time, such as use types as first class values. The capture value and iterator value of inlined for loops are compile-time known.

test_inline_loop.zig

  1. const expect = @import("std").testing.expect;
  3. test "inline for loop" {
  4.     const nums = [_]i32{2, 4, 6};
  5.     var sum: usize = 0;
  6.     inline for (nums) |i| {
  7.         const T = switch (i) {
  8.             2 => f32,
  9.             4 => i8,
  10.             6 => bool,
  11.             else => unreachable,
  12.         };
  13.         sum += typeNameLength(T);
  14.     }
  15.     try expect(sum == 9);
  16. }
  18. fn typeNameLength(comptime T: type) usize {
  19.     return @typeName(T).len;
  20. }

Shell

  1. $ zig test test_inline_loop.zig
  2. 1/1 test.inline for loop... OK
  3. All 1 tests passed.

It is recommended to use inline loops only for one of these reasons:

  • You need the loop to execute at comptime for the semantics to work.
  • You have a benchmark to prove that forcibly unrolling the loop in this way is measurably faster.

See also: