assert! macro

Assert with assert!, assert_eq! and assert_ne!.

assert! macros

There are three basic check macros; all panic on failure and accept an optional message:

assert!(tek_mi(3));                          // the condition must be true
assert_eq!(tek_mi(4), false);                // the two values must be equal
assert_ne!(kare(3), 6, "kare yanlış: {}", kare(3));   // must not be equal

assert_eq! and assert_ne! print both values on failure, which is why the values need Debug and PartialEq.

debug_assert!

The debug_assert! family only runs in debug builds; it's removed entirely in a release build. It's used for expensive internal consistency checks:

fn ortalama(v: &[f64]) -> f64 {
    debug_assert!(!v.is_empty(), "boş dilim");
    v.iter().sum::<f64>() / v.len() as f64
}

Checks required for safety or correctness must stay as assert!; debug_assert! is only for catching "should never happen" cases early.

Exercise Checks

Write fn tek_mi(n: i32) -> bool and fn kare(n: i32) -> i32. In main, run the checks assert!(tek_mi(3)), assert_eq!(tek_mi(4), false) and a assert_ne!(kare(3), 6, ..) with a message, then print denetimler tamam.

Show example solution

fn tek_mi(n: i32) -> bool {
    n % 2 != 0
}

fn kare(n: i32) -> i32 {
    n * n
}

fn main() {
    assert!(tek_mi(3));
    assert_eq!(tek_mi(4), false);
    assert_ne!(kare(3), 6, "kare yanlış: {}", kare(3));
    println!("denetimler tamam");
}

Test yourself

Why does <code>assert_eq!(0.1 + 0.2, 0.3)</code> fail when testing floating-point results?
- Floating-point error: 0.1 + 0.2 = 0.30000000000000004
- assert_eq! doesn't accept f64
- It passes
- It doesn't compile
Show answer

Floating-point error: 0.1 + 0.2 = 0.30000000000000004 — Floats are compared with a tolerance: assert!((a - b).abs() < 1e-9).

Must not be equal

____(kare(3), 6, "kare yanlış");

Show answer

assert_ne!(kare(3), 6, "kare yanlış");

Macro and failure condition

assert!(k)

k is false
assert_eq!(a, b)

a != b
assert_ne!(a, b)

a == b
debug_assert!(k)

Only in debug

Next lesson: Integration test

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