Variables and values · Coding Class

Basics · Build a program← 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 · Trace a changing value

Start with price 12 and quantity 2. Increase quantity by 1 and save the total. Increase quantity again, then print the saved total and the new total.

Completion: The lines are 36 and 48; explain why changing quantity does not update the saved total.

Hint: A calculation stores its result at that moment.

2. Build a program · Build a receipt calculator

Use variables for price 7, quantity 4, and shipping 3. Print subtotal: 28 and total: 31. Also run with quantity 0; no condition is needed yet.

Completion: Quantity 0 gives subtotal 0 and total 3. Use int_to_str when joining numbers to strings.

Hint: Reuse the subtotal when calculating the total.

3. Debug · Choose mutable variables

Check the broken program below and fix just the declaration that needs to change.

Start with this deliberately broken program.

class_mutability.xen

fn main() {
    let quantity = 2;
    quantity = quantity + 1;
    println(quantity);
}

xen check class_mutability.xen

Diagnostic excerpt:

class_mutability.stderr

cannot assign immutable local 'quantity'

Completion: Checking succeeds and output is 3, without redeclaring quantity.

Hint: A name assigned a new value needs let mut.

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.