CHAPTER 4

Loops

Sometimes - in fact, very often, you'll want to repeat the same operation: for example, to process each element of an array or to increment a variable several times in a row. Up to now, we've mostly written a linear flow of code, where instructions are executed one after another, with no repetition, like in this example:

linear_scores.zig

const std = @import("std");

const print = std.debug.print;

pub fn main() void {

const a_scores = [_]u16{ 9000, 8000, 5000, 4200, 3800 };

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

print(" ╔════════════════════════════╗\n", .{});

print(" ║ HIGH SCORES ║\n", .{});

print(" ║____________________________║\n", .{});

print(" ║ {}º | {} pts ║\n", .{ 1, a_scores[0] });

print(" ║ {}º | {} pts ║\n", .{ 2, a_scores[1] });

print(" ║ {}º | {} pts ║\n", .{ 3, a_scores[2] });

print(" ║ {}º | {} pts ║\n", .{ 4, a_scores[3] });

print(" ║ {}º | {} pts ║\n", .{ 5, a_scores[4] });

print(" ╚════════════════════════════╝\n", .{});

}

$ zig run linear_scores.zig

╔════════════════════════════╗

║ HIGH SCORES ║

║____________________________║

║ 1º | 9000 pts ║

║ 2º | 8000 pts ║

║ 3º | 5000 pts ║

║ 4º | 4200 pts ║

║ 5º | 3800 pts ║

╚════════════════════════════╝

In the example linear_scores.zig we print the values of the array a_scores line by line, which are the game's high scores. It works... though it’s so rigid! We’ve hardcoded the position and the score. Good thing there were only 5 - imagine having to do the same for 20, 30, 100, or 1000 entries in a table. Doesn’t seem like a good idea. A small improvement would be to use a variable to store the index (we’ll call it n_i) and increment it:

linear_scores_2.zig

const std = @import("std");

const print = std.debug.print;

pub fn main() void {

const a_scores = [_]u16{ 9000, 8000, 5000, 4200, 3800 };

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

print(" ╔════════════════════════════╗\n", .{});

print(" ║ HIGH SCORES ║\n", .{});

print(" ║____________________________║\n", .{});

var n_i: u8 = 0;

print(" ║ {}º | {} pts ║\n", .{ n_i + 1, a_scores[n_i] });

n_i = n_i + 1;

print(" ║ {}º | {} pts ║\n", .{ n_i + 1, a_scores[n_i] });

n_i += 1;

print(" ║ {}º | {} pts ║\n", .{ n_i + 1, a_scores[n_i] });

n_i += 1;

print(" ║ {}º | {} pts ║\n", .{ n_i + 1, a_scores[n_i] });

n_i += 1;

print(" ║ {}º | {} pts ║\n", .{ n_i + 1, a_scores[n_i] });

print(" ╚════════════════════════════╝\n", .{});

}

$ zig run linear_scores_2.zig

╔════════════════════════════╗

║ HIGH SCORES ║

║____________________________║

║ 1º | 9000 pts ║

║ 2º | 8000 pts ║

║ 3º | 5000 pts ║

║ 4º | 4200 pts ║

║ 5º | 3800 pts ║

╚════════════════════════════╝

By the way, to increment or decrement a variable like n_i, we can write it in two ways. The longer form looks like this:

// increment n_i by 1

n_i = n_i + 1;

// we'll decrement variable n_a by 2

n_a = n_a - 2;

Or, shorter:

// increment n_i by 1

n_i += 1;

// we'll decrement variable n_a by 2

n_a -= 2;

Going back to our score table program, the situation has improved a bit by having the index in a variable, but we’re still repeating the same code over and over. The lines:

print(" ║ {}º | {} pts ║\n", .{ n_i + 1, a_scores[n_i] });

n_i += 1;

, are repeated one after another.

You're probably getting the sense that something's missing. And yes, you're right - it can be made much more dynamic and elegant, using recursion and iteration.

Recursion

Recursion here is a technical word meaning “executing a function that calls itself again.”

Iteration

Iteration means “repeating a block of code without writing it again”:

Jumping to a label,
Using for and while loops