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58 lines
1.8 KiB
Rust
58 lines
1.8 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, and
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// the `total_cost` function will calculate the total cost of the items. Since
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// the player typed in the quantity, we get it as a string. They might have
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// typed anything, not just numbers!
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//
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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: If we call
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// the `total_cost` function on a string that is not a number, that function
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// will return a `ParseIntError`. In that case, we want to immediately return
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// that error from our function and not try to multiply and add.
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//
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// There are at least two ways to implement this that are both correct. But one
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// is a lot shorter!
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use std::num::ParseIntError;
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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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// Added `?` to propagate the error.
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let qty = item_quantity.parse::<i32>()?;
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// ^ added
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// Equivalent to this verbose version:
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let qty = match item_quantity.parse::<i32>() {
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Ok(v) => v,
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Err(e) => return Err(e),
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};
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Ok(qty * cost_per_item + processing_fee)
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}
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fn main() {
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// You can optionally experiment here.
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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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use std::num::IntErrorKind;
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#[test]
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fn item_quantity_is_a_valid_number() {
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assert_eq!(total_cost("34"), Ok(171));
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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().kind(),
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&IntErrorKind::InvalidDigit,
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);
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}
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}
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