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72 lines
2.0 KiB
Rust
72 lines
2.0 KiB
Rust
// Say we're writing a game where you can buy items with tokens. All items cost
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// 5 tokens, and whenever you purchase items there is a processing fee of 1
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// token. A player of the game will type in how many items they want to buy,
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// and the `total_cost` function will calculate the total number of tokens.
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// Since the player typed in the quantity, though, we get it as a string-- and
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// they might have typed anything, not just numbers!
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// Right now, this function isn't handling the error case at all (and isn't
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// handling the success case properly either). What we want to do is:
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// if we call the `parse` function on a string that is not a number, that
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// function will return a `ParseIntError`, and in that case, we want to
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// immediately return that error from our function and not try to multiply
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// and add.
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// There are at least two ways to implement this that are both correct-- but
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// one is a lot shorter! Scroll down for hints to both ways.
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use std::num::ParseIntError;
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pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
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let processing_fee = 1;
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let cost_per_item = 5;
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let qty = item_quantity.parse::<i32>();
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Ok(qty * cost_per_item + processing_fee)
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn item_quantity_is_a_valid_number() {
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assert_eq!(
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total_cost("34"),
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Ok(171)
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);
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}
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#[test]
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fn item_quantity_is_an_invalid_number() {
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assert_eq!(
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total_cost("beep boop").unwrap_err().to_string(),
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"invalid digit found in string"
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);
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}
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}
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// One way to handle this is using a `match` statement on
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// `item_quantity.parse::<i32>()` where the cases are `Ok(something)` and
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// `Err(something)`. This pattern is very common in Rust, though, so there's
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// a `try!` macro that does pretty much what you would make that match statement
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// do for you! Take a look at this section of the Error Handling chapter:
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// https://doc.rust-lang.org/stable/book/error-handling.html#the-try-macro
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// and give it a `try!`
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