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rustlings-exercises-completed/exercises/error_handling/errors2.rs

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// errors2.rs
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// Say we're writing a game where you can buy items with tokens. All items cost
// 5 tokens, and whenever you purchase items there is a processing fee of 1
// token. A player of the game will type in how many items they want to buy,
// and the `total_cost` function will calculate the total number of tokens.
// Since the player typed in the quantity, though, we get it as a string-- and
// they might have typed anything, not just numbers!
// Right now, this function isn't handling the error case at all (and isn't
// handling the success case properly either). What we want to do is:
// if we call the `parse` function on a string that is not a number, that
// function will return a `ParseIntError`, and in that case, we want to
// immediately return that error from our function and not try to multiply
// and add.
// There are at least two ways to implement this that are both correct-- but
// one is a lot shorter!
// Execute `rustlings hint errors2` or use the `hint` watch subcommand for a hint.
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use std::num::ParseIntError;
pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
let processing_fee = 1;
let cost_per_item = 5;
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match item_quantity.parse::<i32>() {
Ok(q) => Ok(q * cost_per_item + processing_fee),
Err(e) => return Err(e)
}
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}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn item_quantity_is_a_valid_number() {
assert_eq!(total_cost("34"), Ok(171));
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}
#[test]
fn item_quantity_is_an_invalid_number() {
assert_eq!(
total_cost("beep boop").unwrap_err().to_string(),
"invalid digit found in string"
);
}
}