[Zambyte]

The "Debug mode" section does not mention the DebugAllocator[0], which does indeed crash with an error on the "Should be caught!" line. Try this:

    const std = @import("std");
    pub fn main() !void {
        var gpa: std.heap.DebugAllocator(.{}) = .init;
        defer _ = gpa.deinit();
        const allocator = gpa.allocator();
        //const allocator = std.heap.page_allocator;
    
        var buffer = try allocator.alloc(u8, 4);
        defer allocator.free(buffer);
        buffer[0] = 1;
        
        const new_buffer = try allocator.realloc(buffer, 8);
        defer allocator.free(new_buffer);
    
        buffer[0] = 99;
        std.debug.print("{}\n", .{buffer[0]});
    }

With the page_allocator, it will (incorrectly) print 99. With the DebugAllocator, it will crash with:

    Segmentation fault at address 0x7f4a78f80000
    prog.zig:14:11: 0x1158d55 in main (tmp_9CoXPDDeVU3O2FvW.zig)
        buffer[0] = 99;
              ^

[0] https://ziglang.org/documentation/0.15.1/std/#std.heap.debug...

[Zambyte]

Regarding iteration, a simple solution without reaching for while loops is to iterate on a range and operate relative to the length.

    const a = [_]u8{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

    // 2 4 6 8 10
    for (a[0 .. a.len / 2], 0..) |_, i| {
        std.debug.print("{} ", .{a[i * 2 + 1]});
    }

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

    // 1 3 5 7 9 
    for (a[0 .. (a.len + 1) / 2], 0..) |_, i| {
        std.debug.print("{} ", .{a[i * 2]});
    }

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

    // 10 9 8 7 6 5 4 3 2 1
    for (0..a.len) |i| {
        std.debug.print("{} ", .{a[a.len - i - 1]});
    }