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Using chunks windows and split_at

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Using chunks windows and split_at

Use slice view adapters such as chunks, windows, and split_at to express grouped or overlapping access without manual index arithmetic.

What it is

Slice adapters borrow views into an existing sequence. chunks(n) yields non-overlapping slices of up to n elements. chunks_exact(n) yields only full chunks and exposes the remainder. windows(n) yields overlapping slices of exactly n elements. split_at(mid) returns the prefix and suffix around one index. split_at_mut(mid) returns two mutable, non-overlapping regions. These methods work on arrays, slices, and vectors. They avoid off-by-one errors common in manual loops. They do not allocate.

How it works

chunks and windows panic if the size argument is zero. The last chunks item may be shorter than the requested size. chunks_exact skips the final incomplete part from iteration and lets you inspect it with remainder. windows cannot provide mutable overlapping windows because that would create aliased mutable references. For mutable non-overlapping groups, use chunks_mut or chunks_exact_mut. split_at panics if mid > len. split_at_checked returns None instead of panicking for invalid mid. split_at_mut is the safe way to borrow two disjoint mutable parts of one slice. Prefer these adapters when the algorithm’s shape is “groups”, “neighbors”, or “prefix plus rest.”

Example

fn main() {
    let values = [1, 2, 3, 4, 5];
 
    let sums: Vec<i32> = values.windows(2).map(|pair| pair[0] + pair[1]).collect();
    assert_eq!(sums, vec![3, 5, 7, 9]);
 
    let chunks: Vec<Vec<i32>> = values.chunks(2).map(|chunk| chunk.to_vec()).collect();
    assert_eq!(chunks, vec![vec![1, 2], vec![3, 4], vec![5]]);
 
    let mut editable = [10, 20, 30, 40];
    let (left, right) = editable.split_at_mut(2);
    left[1] += 1;
    right[0] += 1;
    assert_eq!(editable, [10, 21, 31, 40]);
}

Best practice

  • ✅ Use windows(2) for adjacent pair logic.
  • ✅ Use chunks(n) when a short final group is valid.
  • ✅ Use chunks_exact(n) when a short final group needs separate handling.
  • ✅ Use chunks_mut for in-place non-overlapping group mutation.
  • ✅ Use split_at_mut to avoid unsafe code when mutating two regions.
  • ✅ Use split_at_checked for user-provided split positions.
  • ✅ Keep chunk sizes non-zero and validate them before calling.
  • ✅ Prefer adapters over for i in 0..len - 1 style loops.

Pitfalls

  • ⚠️ windows(0) and chunks(0) panic.
  • ⚠️ windows(n) yields overlapping immutable slices only.
  • ⚠️ chunks(n) may yield a shorter final slice.
  • ⚠️ split_at(mid) panics when mid is out of bounds.
  • ⚠️ Indexing inside each window can still panic if you use the wrong size.
  • ⚠️ Holding chunk borrows prevents mutation of the original slice until the borrow ends.
  • ⚠️ These adapters borrow data; collecting Vec<&[T]> still depends on the original owner.
  • ⚠️ String text also has split methods, but string indexes are byte boundaries; see String Byte Indexing.

See also

std: Vec, String & Slices · The Slice Type · Slicing and Range Indexing · Vec Methods Reference · Prefer Iterator Pipelines to Manual Indexing · Index Panics vs get · Manual Index Loops for Speed · Iterator Adapters · Borrowing Strings and Slices

Sources

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