Slices and iterators · Coding Class

Advanced · Take control← Read the Learn chapter

Apply the chapter in three steps: an exercise, a program project, and a debugging task. Implement first, then consult the hints and completion criteria. Use a separate file or working directory for each task.

1. Exercise · Compare pipeline order

Use doubled and over_two from the chapter on [1, 2, 3]. Compare map-then-filter with filter-then-map.

Completion: The first order yields 4, 6; the reverse yields 6. Explain which values the callback sees.

Hint: Filter sees values produced by the preceding stage.

2. Build a program · Consume owned boxes

Use into_iter on [box(10), box(20), box(30)], extracting values with into_inner and summing them. Explain why iter and get cannot consume them in the same way.

Completion: The total is 60; do not reuse the original vector after iteration.

Hint: Box is move-only; into_iter takes ownership of the vector.

3. Debug · Fix view lifetime ordering

Fix the code below by reordering statements without copying the slice into a new Vec.

Start with this deliberately broken program.

class_view.xen

fn main() {
    let mut values = [1, 2];
    let view = values.as_slice();
    values.push(3);
    println(view.get(0));
}

xen check class_view.xen

Diagnostic excerpt:

class_view.stderr

cannot mutate 'values' while it is borrowed

Completion: Read 1 through the view, then append 3; checking succeeds.

Hint: Place the view’s last use before changing its source.

Verify and record

Use xen check file.xen followed by xen run file.xen for ordinary programs; use xen test file.xen for test-only files. Record the original error, its cause, and the corrected result, then rerun every input in the completion criteria.