Activity 5: Compiler Error Hunt

What you are doing

Working in pairs, in the Rust Playground: https://play.rust-lang.org

Break the program below as many different ways as you can, and read what the compiler says each time.

Goal: at least 8 different compiler errors. There is room for 12 on your sheet if you get on a roll.

Why we use the Playground

VS Code underlines your mistake before you ever compile. That is useful most of the time, but not today, because today's exercise is all about hitting the errors and reading them!

It's also a lot faster to get going.

The program

Paste this in and Run it first, so you know it works before you touch it.

fn main() {
    let temps_c = vec![14.0, 17.5, 21.0, 19.5, 23.0];
    let mut warm_days = 0;

    for i in 0..temps_c.len() {
        let c = temps_c[i];
        let f = c * 9.0 / 5.0 + 32.0;
        println!("Day {}: {:.1}C is {:.1}F", i + 1, c, f);

        if c > 18.0 {
            warm_days += 1;
        }
    }

    let total: f64 = temps_c.iter().sum();
    let mean = total / temps_c.len() as f64;

    println!("Mean high: {:.1}C", mean);
    println!("Warm days: {} of {}", warm_days, temps_c.len());
}

It should print five days, a mean of 19.0C, and 3 warm days.

The activity loop

Make one change that breaks the code
Run it
Read the error over
Write it on your half sheet: what you changed, and what the error is (in brief)
Undo your change, and break it a different way

Make just one change at a time so you can tell what error message belongs to what.

Some ideas

Change something's type
Take a keyword away
Mess with punctuation and variable names
Ask for something that does not exist
Give println! the wrong number of things to print

Warnings are not errors. If you run Clippy from the Tools menu it will suggest ways to rewrite code that already works. Interesting, but it does not count for the hunt.

The compiler errors we found

Drop mut from warm_days: E0384, cannot assign twice to immutable variable
Type total as i32: E0277, a value of type i32 cannot be made by summing over &{float}
Drop as f64: E0277, cannot divide f64 by usize
Delete a semicolon: expected ;, found println
Misspell temps_c in the loop: E0425, cannot find value in this scope
Write if c > 18: E0277, can't compare {float} with {integer}
Remove one argument from println!: 3 positional arguments in format string, but there are 2
Write warm_days += 1.0: E0277, cannot add-assign {float} to {integer}
Put "17.5" in the vec: E0308, mismatched types
Rename main: E0601, main function not found
Delete a closing brace: this file contains an unclosed delimiter
Index temps_c[i + 1]: compiles, then panics at runtime with index out of bounds
Delete a !: E0423: expected function, found macro
Try to index like 0[0]: E0608 cannot index into a value of type {integer}
Add .len() to mean: E0599, no method named len found for type f64 in the current scope

Warnings and non-errors too:

Add mut: warns you don't need it
Delete ln from println!: it still works (but prints funny)
Delete Vec!: nothing seems to change (but the type of temps_c changes)

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