Zig, indexOf vs indexOfPos

(This is referring to zig 0.14.0.)

To find the location of a subslice in a slice, either std.mem.indexOf or std.mem.indexOfPos can be used. If you want to start the search from a subslice of the original slice (e.g. find the first “a” in a string starting at the 5th character), you can either use indexOf with a subslice as the haystack (internally what std.mem calls the slice to search in), or use indexOfPos with the start_index. They return different things:

const std = @import("std");

pub fn main() !void {
    std.debug.print("{any}\n", .{std.mem.indexOf(u8, "abcdef", "d")});
    std.debug.print("{any}\n", .{std.mem.indexOfPos(u8, "abcdef", 2, "d")});
    std.debug.print("{any}\n", .{std.mem.indexOf(u8, "abcdef"[2..], "d")});
}

will yield

3
3
1

In hindsight this is obvious - if the haystack is a subslice, then the returned index is relative to that subslice.

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