for Loop (Array / Slice / Range Iteration)

// -----------------------------------------------
// for — iterate over an array or slice
// -----------------------------------------------

for (collection) |value| {
    // receives each element as value
}

// -----------------------------------------------
// for — iteration with index
// -----------------------------------------------

for (collection, 0..) |value, i| {
    // value : each element
    // i     : usize index starting from 0
}

// -----------------------------------------------
// for — range (integer range)
// -----------------------------------------------

for (start..end) |i| {
    // receives integers from start (inclusive) to end (exclusive) as i
    // the end value is not included (half-open interval)
}

// -----------------------------------------------
// for — parallel iteration over multiple collections
// -----------------------------------------------

for (collectionA, collectionB) |a, b| {
    // a : each element of collectionA
    // b : each element of collectionB (both must have the same length)
}

// -----------------------------------------------
// for as an expression (break with value)
// -----------------------------------------------

const result = for (collection) |value| {
    if (condition) break value;
} else defaultValue;
// yields the break value if the condition is met, otherwise the else value

steinsgate_for_loop.zig

// steinsgate_for_loop.zig — sample code for the for statement
// Uses Steins;Gate characters to demonstrate
// array, slice, range, parallel iteration, and indexed iteration

const std = @import("std");

pub fn main() !void {
    const stdout = std.io.getStdOut().writer();

    // -----------------------------------------------
    // Basic for — iterates over an array in order
    // -----------------------------------------------

    // Defines lab member names in an array
    // [_] lets the compiler infer the number of elements
    const members = [_][]const u8{
        "Okabe Rintaro",
        "Makise Kurisu",
        "Shiina Mayuri",
        "Hashida Itaru",
        "Kiryu Moeka",
    };

    try stdout.print("=== Future Gadget Lab — Member List ===\n\n", .{});

    // Retrieves each element from the array using for
    for (members) |name| {
        try stdout.print("  Member: {s}\n", .{name});
    }

    try stdout.print("\n", .{});

    // -----------------------------------------------
    // Indexed iteration — retrieves the index with 0..
    // -----------------------------------------------

    // Lab member numbers start from 001 by convention, so we add 1 when printing
    try stdout.print("--- Member List with Numbers ---\n", .{});

    // i in |name, i| is a usize index (starting from 0)
    for (members, 0..) |name, i| {
        try stdout.print("  Member{d:0>3}: {s}\n", .{ i + 1, name });
    }

    try stdout.print("\n", .{});

    // -----------------------------------------------
    // Range — iterates over an integer range
    // -----------------------------------------------

    // Prints D-MAIL send counts from 1 to 5
    // 0..5 is a half-open interval that generates 0, 1, 2, 3, 4
    try stdout.print("--- D-MAIL Send Log (1st to 5th) ---\n", .{});

    for (1..6) |count| {
        try stdout.print("  Send #{d}: Sent to the past\n", .{count});
    }

    try stdout.print("\n", .{});

    // -----------------------------------------------
    // Parallel iteration — iterates over two arrays simultaneously
    // -----------------------------------------------

    // Defines lab member code names in a separate array
    const codenames = [_][]const u8{
        "Hououin Kyouma",
        "Christina",
        "Mayushii",
        "Daru",
        "Part-time Warrior",
    };

    // Traverses members and codenames in parallel
    // A compile error occurs if the lengths differ
    try stdout.print("--- Members and Code Names ---\n", .{});

    for (members, codenames) |name, codename| {
        try stdout.print("  {s} → {s}\n", .{ name, codename });
    }

    try stdout.print("\n", .{});

    // -----------------------------------------------
    // Pointer capture — modifies elements in place
    // -----------------------------------------------

    // Defines worldline divergence values (percentage) in a var array
    // *v receives a pointer to each element; v.* reads or writes the value
    var divergences = [_]f32{ 0.571024, 1.048596, 1.130426, 1.382733, 0.000000 };

    try stdout.print("--- Worldline Divergence (before) ---\n", .{});
    for (divergences) |d| {
        try stdout.print("  {d:.6}\n", .{d});
    }

    // Adds 0.1 to every divergence value
    for (&divergences) |*d| {
        d.* += 0.1;
    }

    try stdout.print("--- Worldline Divergence (after +0.1) ---\n", .{});
    for (divergences) |d| {
        try stdout.print("  {d:.6}\n", .{d});
    }

    try stdout.print("\n", .{});

    // -----------------------------------------------
    // for as an expression — returns a value with break
    // -----------------------------------------------

    // Searches for the index of "Makise Kurisu"
    // break returns the value; if not found, else returns null
    const target = "Makise Kurisu";
    const found_index: ?usize = for (members, 0..) |name, i| {
        if (std.mem.eql(u8, name, target)) break i;
    } else null;

    if (found_index) |idx| {
        try stdout.print("{s} — Member Number: {d:0>3}\n", .{ target, idx + 1 });
    } else {
        try stdout.print("{s} was not found\n", .{target});
    }
}

zig run steinsgate_for_loop.zig
=== Future Gadget Lab — Member List ===

  Member: Okabe Rintaro
  Member: Makise Kurisu
  Member: Shiina Mayuri
  Member: Hashida Itaru
  Member: Kiryu Moeka

--- Member List with Numbers ---
  Member001: Okabe Rintaro
  Member002: Makise Kurisu
  Member003: Shiina Mayuri
  Member004: Hashida Itaru
  Member005: Kiryu Moeka

--- D-MAIL Send Log (1st to 5th) ---
  Send #1: Sent to the past
  Send #2: Sent to the past
  Send #3: Sent to the past
  Send #4: Sent to the past
  Send #5: Sent to the past

--- Members and Code Names ---
  Okabe Rintaro → Hououin Kyouma
  Makise Kurisu → Christina
  Shiina Mayuri → Mayushii
  Hashida Itaru → Daru
  Kiryu Moeka → Part-time Warrior

--- Worldline Divergence (before) ---
  0.571024
  1.048596
  1.130426
  1.382733
  0.000000

--- Worldline Divergence (after +0.1) ---
  0.671024
  1.148596
  1.230426
  1.482733
  0.100000

Makise Kurisu — Member Number: 002

The for statement in Zig is the syntax for iterating over arrays, slices, and integer ranges. The capture syntax |value| receives each element, and the |value, index| form also retrieves the index at the same time. The index is always of type usize, and the starting value is specified as 0... Integer ranges are written as a half-open interval start..end (end is not included), so for (1..6) |i| passes 1, 2, 3, 4, 5 in sequence.

To traverse two collections in parallel, use the form for (a, b) |va, vb| { }. If the two collections have different lengths, a compile error occurs. To modify element values in place, use the pointer capture |*v| and read or write via v.*. Additionally, for can be used as an expression that returns a value — combining break value with an else clause makes it easy to write logic such as "return the first element that satisfies a condition." For related topics, see Array Basics and Slices.