Functions and control flow · 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 · Check a range endpoint

Sum 1..5, compare with 1..6, and also run 5..5.

Completion: The totals are 10, 15, and 0.

Hint: The endpoint is excluded.

2. Build a program · Build an even-number total

Write sum_even(limit: Int) -> Int to sum even numbers from zero up to, but excluding, limit. Print calls with 7, 1, and 0.

Completion: Output is 12, 0, 0. Keep calculation in the function and calls/output in main.

Hint: Select even numbers with number % 2 == 0.

3. Debug · Fix a missing return path

The positive function below does not return for every input. Make all paths return a Bool.

Start with this deliberately broken program.

class_return.xen

fn positive(value: Int) -> Bool {
    if value > 0 { return true; }
}
fn main() { println(positive(0)); }

xen check class_return.xen

Diagnostic excerpt:

class_return.stderr

function 'positive' may fall through without returning a value

Completion: Inputs 1, 0, and -1 yield true, false, and false.

Hint: Return false after the if, or return from an else branch.

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.