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A Guessing Game

Sobeston
・4 min read

We are going to make a program that randomly picks a number from 1 to 100 and asks us to guess it, telling us if our number is too big or two small.

Getting a Random Number

As Zig does not have a runtime, it does not manage a PRNG (pseudorandom number generator) for us. This means that we'll have to create our own PRNG and initialise it with a source of entropy. Let's start with a file called a_guessing_game.zig.

const std = @import("std");

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

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Let's initialise std.rand.DefaultPrng with a 64 bit unsigned integer (u64). Our rand here allows us to access many useful utilities for our PRNG. Here we're asking our PRNG for a random number from 1 to 100, however if our PRNG is initialised with the same number every time our program will always print out the same number.

    var prng = std.rand.DefaultPrng.init(1625953);
    const rand = &prng.random;

    try stdout.print(
        "not-so random number: {}\n",
        .{rand.intRangeAtMost(u8, 1, 100)},
    );
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For a good source of entropy, it is best to initialise our PRNG with random bytes provided by the OS. Let's ask the OS for some. As Zig doesn't let us declare a variable without a value we've had to give our seed variable the value of undefined, which is a special value that coerces to any type. The function std.os.getrandom takes in a slice of bytes, where a slice is a pointer to a buffer whose length is known at run time. Because of this we've used std.mem.asBytes to turn our pointer to a u64 into a slice of bytes. If getrandom succeeds it will fill our seed variable with a random value which we can then initialise the PRNG with.

    var seed: u64 = undefined;
    try std.os.getrandom(std.mem.asBytes(&seed));

    var prng = std.rand.DefaultPrng.init(seed);
    const rand = &prng.random;
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Taking User Input

Let's start here, where our program already has a random secret value which we must guess.

const std = @import("std");

pub fn main() !void {
    var seed: u64 = undefined;
    try std.os.getrandom(std.mem.asBytes(&seed));

    var prng = std.rand.DefaultPrng.init(seed);
    const rand = &prng.random;

    const target_number = rand.intRangeAtMost(u8, 1, 100);

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As we'll be printing and taking in user input until the correct value is guessed, let's start by making a while loop with stdin and stdout. Note how we've obtained an stdin reader.

    while (true) {
        const stdin = std.io.getStdIn().reader();
        const stdout = std.io.getStdOut().writer();
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To get a line of user's input, we have to read stdin until we encounter a newline character, which is represented by \n. What is read will need to be copied into a buffer, so here we're asking readUntilDelimiterAlloc to allocate a buffer up to 8KiB using std.heap.page_allocator until it reaches the \n character.

        const bare_line = try stdin.readUntilDelimiterAlloc(
            std.heap.page_allocator,
            '\n',
            8192,
        );
        defer std.heap.page_allocator.free(bare_line);
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Because of legacy reasons newlines in many places in Windows are represented by the two-character sequence \r\n, which means that we must strip \r from the line that we've read. Without this our program will behave incorrectly on Windows.

        const line = std.mem.trim(u8, bare_line, "\r");
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Guessing

Let's continue from here. We're expecting the user to input an integer number here, so the next step is to parse a number from line.

const std = @import("std");

pub fn main() !void {
    var seed: u64 = undefined;
    try std.os.getrandom(std.mem.asBytes(&seed));

    var prng = std.rand.DefaultPrng.init(seed);
    const rand = &prng.random;

    const target_number = rand.intRangeAtMost(u8, 1, 100);

    while (true) {
        const stdin = std.io.getStdIn().reader();
        const stdout = std.io.getStdOut().writer();

        const bare_line = try stdin.readUntilDelimiterAlloc(
            std.heap.page_allocator,
            '\n',
            8192,
        );
        defer std.heap.page_allocator.free(bare_line);

        const line = std.mem.trim(u8, bare_line, "\r");

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This can be achieved by passing the buffer to std.fmt.parseInt, where the last parameter is the base of the number in the string. So far we've only handled errors with try, which returns the error if encountered, but here we'll want to catch the error so that we can process it without returning it. If there's an error we'll print a friendly error message and continue, so that the user can re-enter their number.

        const guess = std.fmt.parseInt(u8, line, 10) catch |err| switch (err) {
            error.Overflow => {
                try stdout.writeAll("Please enter a small positive number\n");
                continue;
            },
            error.InvalidCharacter => {
                try stdout.writeAll("Please enter a valid number\n");
                continue;
            },
        };
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Now all we have to do is decide what to do with the user's guess. It's important to leave the loop using break when the user makes a correct guess.

        if (guess < target_number) try stdout.writeAll("Too Small!\n");
        if (guess > target_number) try stdout.writeAll("Too Big!\n");
        if (guess == target_number) {
            try stdout.writeAll("Correct!\n");
            break;
        }
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Let's try playing our game.

$ zig run a_guessing_game.zig 
45
Too Big!
20
Too Small!
25
Too Small!
32
Too Small!
38
Too Small!
41
Too Small!
43
Too Small!
44
Correct!
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