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rustlings-exercises-completed/exercises/conversions/from_str.rs

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// from_str.rs
// This is similar to from_into.rs, but this time we'll implement `FromStr`
// and return errors instead of falling back to a default value.
// Additionally, upon implementing FromStr, you can use the `parse` method
// on strings to generate an object of the implementor type.
// You can read more about it at https://doc.rust-lang.org/std/str/trait.FromStr.html
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// Execute `rustlings hint from_str` or use the `hint` watch subcommand for a hint.
use std::num::ParseIntError;
use std::str::FromStr;
#[derive(Debug, PartialEq)]
struct Person {
name: String,
age: usize,
}
// We will use this error type for the `FromStr` implementation.
#[derive(Debug, PartialEq)]
enum ParsePersonError {
// Empty input string
Empty,
// Incorrect number of fields
BadLen,
// Empty name field
NoName,
// Wrapped error from parse::<usize>()
ParseInt(ParseIntError),
}
// Steps:
// 1. If the length of the provided string is 0, an error should be returned
// 2. Split the given string on the commas present in it
// 3. Only 2 elements should be returned from the split, otherwise return an error
// 4. Extract the first element from the split operation and use it as the name
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// 5. Extract the other element from the split operation and parse it into a `usize` as the age
// with something like `"4".parse::<usize>()`
// 6. If while extracting the name and the age something goes wrong, an error should be returned
// If everything goes well, then return a Result of a Person object
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//
// As an aside: `Box<dyn Error>` implements `From<&'_ str>`. This means that if you want to return a
// string error message, you can do so via just using return `Err("my error message".into())`.
impl FromStr for Person {
type Err = ParsePersonError;
fn from_str(s: &str) -> Result<Person, Self::Err> {
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// Attempt to split input str
let mut split = s.split(",");
// Return an error if there are not exactly two resulting elements
if split.clone().count() != 2 {
if s.is_empty() {
// There's nothing here
return Err(ParsePersonError::Empty);
} else {
// There's either not enough or too much here
return Err(ParsePersonError::BadLen);
}
}
// Attempt to grab name
let name = split.next().unwrap().to_string();
// Make sure the name isn't empty
if name.is_empty() {
return Err(ParsePersonError::NoName);
}
// Attempt to grab age
let age = match split.next().unwrap().parse::<usize>() {
// Yay we have a usable age
Ok(a) => a,
// Wait this isn't a number
Err(e) => return Err(ParsePersonError::ParseInt(e)),
};
Ok(Person { name, age })
}
}
fn main() {
let p = "Mark,20".parse::<Person>().unwrap();
println!("{:?}", p);
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn empty_input() {
assert_eq!("".parse::<Person>(), Err(ParsePersonError::Empty));
}
#[test]
fn good_input() {
let p = "John,32".parse::<Person>();
assert!(p.is_ok());
let p = p.unwrap();
assert_eq!(p.name, "John");
assert_eq!(p.age, 32);
}
#[test]
fn missing_age() {
assert!(matches!(
"John,".parse::<Person>(),
Err(ParsePersonError::ParseInt(_))
));
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}
#[test]
fn invalid_age() {
assert!(matches!(
"John,twenty".parse::<Person>(),
Err(ParsePersonError::ParseInt(_))
));
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}
#[test]
fn missing_comma_and_age() {
assert_eq!("John".parse::<Person>(), Err(ParsePersonError::BadLen));
}
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#[test]
fn missing_name() {
assert_eq!(",1".parse::<Person>(), Err(ParsePersonError::NoName));
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}
#[test]
fn missing_name_and_age() {
assert!(matches!(
",".parse::<Person>(),
Err(ParsePersonError::NoName | ParsePersonError::ParseInt(_))
));
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}
#[test]
fn missing_name_and_invalid_age() {
assert!(matches!(
",one".parse::<Person>(),
Err(ParsePersonError::NoName | ParsePersonError::ParseInt(_))
));
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}
#[test]
fn trailing_comma() {
assert_eq!("John,32,".parse::<Person>(), Err(ParsePersonError::BadLen));
}
#[test]
fn trailing_comma_and_some_string() {
assert_eq!(
"John,32,man".parse::<Person>(),
Err(ParsePersonError::BadLen)
);
}
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}