# \`inline for\` behaves unexpectedly when calling a function **URL:** \ **Category:** Help\ **Tags:** comptime\ **Created:** [June 24, 2026, 3:59pm UTC](https://ziggit.dev/t/inline-for-behaves-unexpectedly-when-calling-a-function/16326 "2026-06-24T15:59:09Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Shrimo](https://ziggit.dev/letter_avatar_proxy/v4/letter/s/6a8cbe/32.png) [@Shrimo](https://ziggit.dev/u/Shrimo)\ **Post date:** [June 24, 2026, 3:59pm UTC](https://ziggit.dev/t/inline-for-behaves-unexpectedly-when-calling-a-function/16326/1 "2026-06-24T15:59:09Z")
```zig fn is_even(n: usize) bool { return (n % 2 == 0); } pub fn main() void { const numbers = .{ 1, 2, 3, 4, 5, 6 }; inline for (numbers) |n| if (n % 2 == 0) @compileLog(n); // 2, 4, 6 inline for (numbers) |n| if (is_even(n)) @compileLog(n); // 1, 2, 3, 4, 5, 6 } ``` The first loop logs `2, 4, 6`, which is expected. The second logs `1, 2, 3, 4, 5, 6`. Why does wrapping the predicate in a function change the behaviour? I would’ve expected it to be evaluated given `n` is comptime known. Version: `0.17.0-dev.956+2dca73595` ---
**Author:** ![SillyProgramming](https://ziggit.dev/user_avatar/ziggit.dev/sillyprogramming/32/8444_2.png) [@SillyProgramming](https://ziggit.dev/u/SillyProgramming)\ **Post date:** [June 24, 2026, 4:01pm UTC](https://ziggit.dev/t/inline-for-behaves-unexpectedly-when-calling-a-function/16326/2 "2026-06-24T16:01:38Z")
You need to call the function at comptime ---
**Author:** ![rpkak](https://ziggit.dev/letter_avatar_proxy/v4/letter/r/a4c791/32.png) [@rpkak](https://ziggit.dev/u/rpkak)\ **Post date:** [June 24, 2026, 4:04pm UTC](https://ziggit.dev/t/inline-for-behaves-unexpectedly-when-calling-a-function/16326/3 "2026-06-24T16:04:16Z")
The `is_even()` function call is evaluated at runtime. Because of this, the compiler evaluates the code, for both possibilities of the return value of `is_even()`. You get 2, 4, 6 if you do `if (comptime is_even(n)) @compileLog(n);`. ---
**Author:** ![ericlang](https://ziggit.dev/user_avatar/ziggit.dev/ericlang/32/8515_2.png) [@ericlang](https://ziggit.dev/u/ericlang)\ **Post date:** [June 24, 2026, 5:51pm UTC](https://ziggit.dev/t/inline-for-behaves-unexpectedly-when-calling-a-function/16326/4 "2026-06-24T17:51:09Z")
huh??? i don’t get this. ---
**Author:** ![Shrimo](https://ziggit.dev/letter_avatar_proxy/v4/letter/s/6a8cbe/32.png) [@Shrimo](https://ziggit.dev/u/Shrimo)\ **Post date:** [June 24, 2026, 6:15pm UTC](https://ziggit.dev/t/inline-for-behaves-unexpectedly-when-calling-a-function/16326/5 "2026-06-24T18:15:22Z")
The question or the solution? ---
**Author:** ![ericlang](https://ziggit.dev/user_avatar/ziggit.dev/ericlang/32/8515_2.png) [@ericlang](https://ziggit.dev/u/ericlang)\ **Post date:** [June 24, 2026, 10:24pm UTC](https://ziggit.dev/t/inline-for-behaves-unexpectedly-when-calling-a-function/16326/6 "2026-06-24T22:24:40Z")
the solution ---
**Author:** ![LucasSantos91](https://ziggit.dev/user_avatar/ziggit.dev/lucassantos91/32/440_2.png) [@LucasSantos91](https://ziggit.dev/u/LucasSantos91)\ **Post date:** [June 25, 2026, 2:18am UTC](https://ziggit.dev/t/inline-for-behaves-unexpectedly-when-calling-a-function/16326/7 "2026-06-25T02:18:12Z")
Just manually unroll the loop and it will make sense. ```zig pub fn main() void { const numbers = .{ 1, 2, 3, 4, 5, 6 }; // First loop if (1 % 2 == 0) { // Known at compile time to be false, // so the compiler doesn't even look // at what's inside here. // The compiler never reaches this // `compileLog`, so the 1 doesn't get logged. @compileLog(1); } if (2 % 2 == 0) { //Known at compile time to be true, // so the block gets compiled, and the compiler // sees the `compileLog` and logs 2 @compileLog(2); //... // Second loop if (is_even(1)) { // Not known at compile time, so the compiler // must compile what's inside, and it // reaches the `compileLog`. // Even if this branch gets removed // by optimizations, the logging has // already been done. @compileLog(1); } // Same for all other elements, so all get logged } ``` ---
**Author:** ![ericlang](https://ziggit.dev/user_avatar/ziggit.dev/ericlang/32/8515_2.png) [@ericlang](https://ziggit.dev/u/ericlang)\ **Post date:** [June 25, 2026, 6:12am UTC](https://ziggit.dev/t/inline-for-behaves-unexpectedly-when-calling-a-function/16326/8 "2026-06-25T06:12:53Z")
ok… it is some `@compilelog` / compiletime thingy. not a logical thing. if we replace @compilelog with print we get the same output for both loops. and… but… still don’t get it! my brain refuses to understand what is happening. ---
**Author:** ![ForeverZer0](https://ziggit.dev/user_avatar/ziggit.dev/foreverzer0/32/2443_2.png) [@ForeverZer0](https://ziggit.dev/u/ForeverZer0)\ **Post date:** [June 25, 2026, 6:22am UTC](https://ziggit.dev/t/inline-for-behaves-unexpectedly-when-calling-a-function/16326/9 "2026-06-25T06:22:21Z")
Dead code elimination. ```zig inline for (numbers) |n| if (n % 2 == 0) @compileLog(n); // 2, 4, 6 ``` This code is being _evaluated_ at comptime. It can detect the dead branches, and they are eliminated. The inner block is never even seen by the compiler. The `@compileLog` is never even reached by the compiler. ```zig inline for (numbers) |n| if (is_even(n)) @compileLog(n); // 1, 2, 3, 4, 5, 6 ``` This is not being evaluated at comptime, therefore the compiler goes through the normal processing of the branches. It is not dead code that gets eliminated at the `if`, so the `@compileLog` is reached for each occurrence due to the loop being unrolled with `inline for`. ---
**Author:** ![ericlang](https://ziggit.dev/user_avatar/ziggit.dev/ericlang/32/8515_2.png) [@ericlang](https://ziggit.dev/u/ericlang)\ **Post date:** [June 25, 2026, 6:25am UTC](https://ziggit.dev/t/inline-for-behaves-unexpectedly-when-calling-a-function/16326/10 "2026-06-25T06:25:19Z")
i will play around with this and watch what is happening. then maybe some neurons will click 🙂