Bounds check

Learn bounds checks and how to avoid them.

Bounds checks

Every v[i] access includes a bounds check; it's what prevents undefined behaviour in favour of a panic. Its cost is usually too small to measure, and the compiler removes the check when it can prove the index is in bounds. Ways to make the proof easier:

// 1) use an iterator: no check
for x in v.iter() { .. }

// 2) walk two slices together with zip
a.iter().zip(b).map(|(x, y)| x * y).sum::<f64>()

// 3) validate the length up front: the checks in the loop go away
assert_eq!(a.len(), b.len());
for i in 0..a.len() { t += a[i] * b[i]; }

Hoisting the check with an assert

A single check before the loop can replace the n checks inside it: from i < a.len() and a.len() == b.len() the compiler infers that b[i] is safe too. It's also a good way to document the function's contract:

fn nokta(a: &[f64], b: &[f64]) -> f64 {
    assert_eq!(a.len(), b.len(), "uzunluklar farklı");
    a.iter().zip(b).map(|(x, y)| x * y).sum()
}

Exercise Dot product

Write the nokta function above. Print the result of nokta(&[1.0, 2.0], &[3.0, 4.0]).

Show example solution

fn nokta(a: &[f64], b: &[f64]) -> f64 {
    assert_eq!(a.len(), b.len(), "uzunluklar farklı");
    a.iter().zip(b).map(|(x, y)| x * y).sum()
}

fn main() {
    println!("{}", nokta(&[1.0, 2.0], &[3.0, 4.0]));
}

Test yourself

When is <code>get_unchecked</code> legitimate?
- When a profile shows the check matters and the index's validity is proven another way
- Always
- Never
- In debug builds
Show answer

When a profile shows the check matters and the index's validity is proven another way — Safe options (iterators, zip, assert) should be tried first; unsafe is the last resort.

Walk together

a.iter().____(b).map(|(x, y)| x * y).sum::<f64>()

Show answer

a.iter().zip(b).map(|(x, y)| x * y).sum::<f64>()

Access and bounds check

v[i]

Checked, panics
v.get(i)

Checked, Option
v.iter()

No check
get_unchecked(i)

unsafe, no check

Next lesson: Summary - Zero-cost abstraction

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