Append an element to an array at comptime

Help
1

i found the following function when googling around:

fn append(a: anytype, e: anytype) @TypeOf(a ++ .{e}) {
    return a ++ .{e};
}

looks right (i think)… but what i can’t figure out is how to declare an array value that i could pass to append… in psuedo code:

comptime {
    var arr = // some zero-length array of (say) u8
    assert(arr.len == 0);
    arr = append(arr, 10);
    assert(arr.len == 1);
    assert(arr[0] == 10);
}

what’s the “right” way to append an element to an array at comptime ???

2

I don’t know how or if it can be done with arrays but I’ve seen it done with slices in a comptime context:

const std = @import("std");

pub fn main() !void {
    comptime var slice: []const u8 = &[0]u8{};
    slice = slice ++ [1]u8{1};
    slice = slice ++ &[1]u8{2};
    slice = slice ++ .{3};
    std.debug.print("{any}\n", .{slice});
}
3

Nice, I like defining comptime constants globally since global context is a comptime one, so there’s no need for the comptime keyword.

const std = @import("std");

const SLICE = blk: {
    var slice: []const u8 = &.{};
    slice = slice ++ .{1};
    slice = slice ++ .{2};
    break :blk slice ++ .{3};
};

pub fn main() void {
    std.debug.print("{any}\n", .{SLICE});
}
4

i tried an “inside-out” variant of your gist:

const LIST = blk: {
    comptime var slice: []const u8 = &[0]u8{};
    const res = struct {
        fn get() []const u8 {
            return slice;
        }
        fn append(e: u8) void {
            slice = slice ++ .{e};
        }
    };
    break :blk res;
};

in its initial state, assert(LIST.get().len == 0) passes…

but after calling LIST.append(10), the underlying slice is still empty…

said another way, i would like my comptime slice to somehow exist OUTSIDE of the scope in which it was originally defined… is this possible in zig???

5

based on this recent post, it looks like i need to keep my comptime var within a function/block scope… i’ll design accordingly…

but then, my question is: can i pass a reference to my “local” comptime var to some (comptime) function that would presumably manipulate the var???

answering my own question, it would appear so after a quick trip to the sandbox…

1 Like