Zig unable to evaluate comptime expression

Question

I'm trying to learn Zig using the website https://exercism.org/. I was solving this exercise, which requires us to find the nth prime number.

My idea was to generate all of them during comptime and return as required; this is my current my code:

const std = @import("std");
const mem = std.mem;

const buffer_size = 100000;

fn sieve(comptime buffer: []usize, comptime limit: usize) []usize {
    if (limit < 2)
        return buffer[0..0];
    const sqrt_limit = std.math.sqrt(limit);
    var bitset = std.StaticBitSet(limit + 1).initFull();
    var n: usize = 3;
    var i: usize = 1;
    
    
    buffer[0] = 2;
    while(n <= sqrt_limit): (n += 2) {
        if (!bitset.isSet(n))
            continue;

        buffer[i] = n;
        i += 1;
        var j = 2 * n;
        while (j <= limit): (j += i) {
            bitset.unset(j);
        }
    }
    while (n <= limit) : (n += 2) {
        if (!bitset.isSet(i))
            continue;

        buffer[i] = n;
        i += 1;
    }
    return buffer[0..i];
}


var primes_buffer: [buffer_size]usize = undefined;
const primes = sieve(&primes_buffer, buffer_size);

pub fn prime(allocator: mem.Allocator, number: usize) !usize {
    _ = allocator;
    return primes[number-1];
}

But when I try to compile this, I'm getting the following error:

nth_prime.zig:15:15: error: unable to evaluate comptime expression
nth_prime.zig:15:11: note: operation is runtime due to this operand
nth_prime.zig:39:21: note: called from here

Why am I not able to populate my array at comptime, and why is this line problematic?

buffer[0] = 2;

eggboy · Accepted Answer · 2025-02-16 09:11:28Z

1

I think its because the buffer primes_buffer is actually a runtime value, so you can't get it to work in that way. One way to write it would be like:

const primes = blk: {
    const buffer_size = 1000;
    @setEvalBranchQuota(buffer_size * buffer_size);
    var primes_buffer: [buffer_size]usize = undefined;
    const len = sieve(&primes_buffer, buffer_size);
    break :blk .{ primes_buffer, len };
};

fn sieve(comptime buffer: []usize, comptime limit: usize) usize {
    if (limit < 2)
        return buffer[0..0];
    const sqrt_limit = std.math.sqrt(limit);
    var bitset = std.StaticBitSet(limit + 1).initFull();
    var n: usize = 3;
    var i: usize = 1;

    buffer[0] = 2;
    while (n <= sqrt_limit) : (n += 2) {
        if (!bitset.isSet(n))
            continue;

        buffer[i] = n;
        i += 1;
        var j = 2 * n;
        while (j <= limit) : (j += n) { // should increment j by n
            bitset.unset(j);
        }
    }
    while (n <= limit) : (n += 2) {
        if (!bitset.isSet(i))
            continue;

        buffer[i] = n;
        i += 1;
    }
    return i;
}

pub fn prime(allocator: mem.Allocator, number: usize) !usize {
    _ = allocator;
    if (number >= primes[1]) {
        @panic("no"); // or return an error
    }
    return primes[0][number];
}

This way, the scope we defined as blk will run and evaluate in comptime.

edited Feb 16 at 9:11

answered Feb 13 at 22:15

eggboy

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3 Comments

João Areias Feb 14 at 15:13

For some reason it seems like the code is not skipping the bits that are unset on the bitset. If you check primes for example it will be {2, 3, 5, 7, 9, 11, 13, 15, ...} it does not seem like the bitset is doing anything in the comptime do you have any idea why would that be?

eggboy Feb 16 at 9:02

The inner while loop should increment n by n, not 2. And we could also set j to n * n since smaller numbers are already marked before. So it should be:

while (n <= sqrt_limit) : (n += 2) {         if (!bitset.isSet(n))             continue;          buffer[i] = n;         i += 1;         var j = 2 * n; // change here         while (j <= limit) : (j += n) { // and here             bitset.unset(j);         }     }

eggboy Feb 16 at 9:09

typo: the while should increment j by n, not i.

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