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right let's try this one again

This commit is contained in:
olivia 2018-11-09 20:31:14 +01:00
commit f7846af7ac
60 changed files with 130 additions and 939 deletions

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// errors1.rs
// This function refuses to generate text to be printed on a nametag if
// you pass it an empty string. It'd be nicer if it explained what the problem
// was, instead of just sometimes returning `None`. The 2nd test currently
// does not compile or pass, but it illustrates the behavior we would like
// this function to have.
// Scroll down for hints!!!
pub fn generate_nametag_text(name: String) -> Option<String> {
if name.len() > 0 {
Some(format!("Hi! My name is {}", name))
} else {
// Empty names aren't allowed.
None
}
}
#[cfg(test)]
mod tests {
use super::*;
// This test passes initially if you comment out the 2nd test.
// You'll need to update what this test expects when you change
// the function under test!
#[test]
fn generates_nametag_text_for_a_nonempty_name() {
assert_eq!(
generate_nametag_text("Beyoncé".into()),
Some("Hi! My name is Beyoncé".into())
);
}
#[test]
fn explains_why_generating_nametag_text_fails() {
assert_eq!(
generate_nametag_text("".into()),
Err("`name` was empty; it must be nonempty.".into())
);
}
}
// `Err` is one of the variants of `Result`, so what the 2nd test is saying
// is that `generate_nametag_text` should return a `Result` instead of an
// `Option`.
// To make this change, you'll need to:
// - update the return type in the function signature to be a Result that
// could be the variants `Ok(String)` and `Err(String)`
// - change the body of the function to return `Ok(stuff)` where it currently
// returns `Some(stuff)`
// - change the body of the function to return `Err(error message)` where it
// currently returns `None`
// - change the first test to expect `Ok(stuff)` where it currently expects
// `Some(stuff)`.

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// errors2.rs
// 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! Scroll down for hints to both ways.
use std::num::ParseIntError;
pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
let processing_fee = 1;
let cost_per_item = 5;
let qty = item_quantity.parse::<i32>();
Ok(qty * cost_per_item + processing_fee)
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn item_quantity_is_a_valid_number() {
assert_eq!(
total_cost("34"),
Ok(171)
);
}
#[test]
fn item_quantity_is_an_invalid_number() {
assert_eq!(
total_cost("beep boop").unwrap_err().to_string(),
"invalid digit found in string"
);
}
}
// One way to handle this is using a `match` statement on
// `item_quantity.parse::<i32>()` where the cases are `Ok(something)` and
// `Err(something)`. This pattern is very common in Rust, though, so there's
// a `?` operator that does pretty much what you would make that match statement
// do for you! Take a look at this section of the Error Handling chapter:
// https://doc.rust-lang.org/stable/book/second-edition/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
// and give it a try!

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// errors3.rs
// This is a program that is trying to use a completed version of the
// `total_cost` function from the previous exercise. It's not working though--
// we can't use the `?` operator in the `main()` function! Why not?
// What should we do instead? Scroll for hints!
use std::num::ParseIntError;
fn main() {
let mut tokens = 100;
let pretend_user_input = "8";
let cost = total_cost(pretend_user_input)?;
if cost > tokens {
println!("You can't afford that many!");
} else {
tokens -= cost;
println!("You now have {} tokens.", tokens);
}
}
pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
let processing_fee = 1;
let cost_per_item = 5;
let qty = item_quantity.parse::<i32>()?;
Ok(qty * cost_per_item + processing_fee)
}
// Since the `?` operator returns an `Err` early if the thing it's trying to
// do fails, you can only use the `?` operator in functions that have a
// `Result` as their return type.
// Hence the error that you get if you run this code is:
// ```
// error[E0277]: the `?` operator can only be used in a function that returns `Result` (or another type that implements `std::ops::Try`)
// ```
// So we have to use another way of handling a `Result` within `main`.
// Decide what we should do if `pretend_user_input` has a string value that does
// not parse to an integer, and implement that instead of using the `?`
// operator.

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// errorsn.rs
// This is a bigger error exercise than the previous ones!
// You can do it! :)
//
// Edit the `read_and_validate` function so that it compiles and
// passes the tests... so many things could go wrong!
//
// - Reading from stdin could produce an io::Error
// - Parsing the input could produce a num::ParseIntError
// - Validating the input could produce a CreationError (defined below)
//
// How can we lump these errors into one general error? That is, what
// type goes where the question marks are, and how do we return
// that type from the body of read_and_validate?
//
// Scroll down for hints :)
use std::error;
use std::fmt;
use std::io;
// PositiveNonzeroInteger is a struct defined below the tests.
fn read_and_validate(b: &mut io::BufRead) -> Result<PositiveNonzeroInteger, ???> {
let mut line = String::new();
b.read_line(&mut line);
let num: i64 = line.trim().parse();
let answer = PositiveNonzeroInteger::new(num);
answer
}
// This is a test helper function that turns a &str into a BufReader.
fn test_with_str(s: &str) -> Result<PositiveNonzeroInteger, Box<error::Error>> {
let mut b = io::BufReader::new(s.as_bytes());
read_and_validate(&mut b)
}
#[test]
fn test_success() {
let x = test_with_str("42\n");
assert_eq!(PositiveNonzeroInteger(42), x.unwrap());
}
#[test]
fn test_not_num() {
let x = test_with_str("eleven billion\n");
assert!(x.is_err());
}
#[test]
fn test_non_positive() {
let x = test_with_str("-40\n");
assert!(x.is_err());
}
#[test]
fn test_ioerror() {
struct Broken;
impl io::Read for Broken {
fn read(&mut self, _buf: &mut [u8]) -> io::Result<usize> {
Err(io::Error::new(io::ErrorKind::BrokenPipe, "uh-oh!"))
}
}
let mut b = io::BufReader::new(Broken);
assert!(read_and_validate(&mut b).is_err());
assert_eq!("uh-oh!", read_and_validate(&mut b).unwrap_err().to_string());
}
#[derive(PartialEq,Debug)]
struct PositiveNonzeroInteger(u64);
impl PositiveNonzeroInteger {
fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
if value == 0 {
Err(CreationError::Zero)
} else if value < 0 {
Err(CreationError::Negative)
} else {
Ok(PositiveNonzeroInteger(value as u64))
}
}
}
#[test]
fn test_positive_nonzero_integer_creation() {
assert!(PositiveNonzeroInteger::new(10).is_ok());
assert_eq!(Err(CreationError::Negative), PositiveNonzeroInteger::new(-10));
assert_eq!(Err(CreationError::Zero), PositiveNonzeroInteger::new(0));
}
#[derive(PartialEq,Debug)]
enum CreationError {
Negative,
Zero,
}
impl fmt::Display for CreationError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.write_str((self as &error::Error).description())
}
}
impl error::Error for CreationError {
fn description(&self) -> &str {
match *self {
CreationError::Negative => "Negative",
CreationError::Zero => "Zero",
}
}
}
// First hint: To figure out what type should go where the ??? is, take a look
// at the test helper function `test_with_str`, since it returns whatever
// `read_and_validate` returns and`test_with_str` has its signature fully
// specified.
// Next hint: There are three places in `read_and_validate` that we call a
// function that returns a `Result` (that is, the functions might fail).
// Apply the `?` operator on those calls so that we return immediately from
// `read_and_validate` if those function calls fail.
// Another hint: under the hood, the `?` operator calls `From::from`
// on the error value to convert it to a boxed trait object, a Box<error::Error>,
// which is polymorphic-- that means that lots of different kinds of errors
// can be returned from the same function because all errors act the same
// since they all implement the `error::Error` trait.
// Check out this section of the book:
// https://doc.rust-lang.org/stable/book/second-edition/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
// Another another hint: Note that because the `?` operator returns
// the *unwrapped* value in the `Ok` case, if we want to return a `Result` from
// `read_and_validate` for *its* success case, we'll have to rewrap a value
// that we got from the return value of a `?`ed call in an `Ok`-- this will
// look like `Ok(something)`.
// Another another another hint: `Result`s must be "used", that is, you'll
// get a warning if you don't handle a `Result` that you get in your
// function. Read more about that in the `std::result` module docs:
// https://doc.rust-lang.org/std/result/#results-must-be-used

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// option1.rs
// This example panics because the second time it calls `pop`, the `vec`
// is empty, so `pop` returns `None`, and `unwrap` panics if it's called
// on `None`. Handle this in a more graceful way than calling `unwrap`!
// Scroll down for hints :)
fn main() {
let mut list = vec![3];
let last = list.pop().unwrap();
println!("The last item in the list is {:?}", last);
let second_to_last = list.pop().unwrap();
println!("The second-to-last item in the list is {:?}", second_to_last);
}
// Try using a `match` statement where the arms are `Some(thing)` and `None`.
// Or set a default value to print out if you get `None` by using the
// function `unwrap_or`.
// Or use an `if let` statement on the result of `pop()` to both destructure
// a `Some` value and only print out something if we have a value!

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// result1.rs
// Make this test pass! Scroll down for hints :)
#[derive(PartialEq,Debug)]
struct PositiveNonzeroInteger(u64);
#[derive(PartialEq,Debug)]
enum CreationError {
Negative,
Zero,
}
impl PositiveNonzeroInteger {
fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
Ok(PositiveNonzeroInteger(value as u64))
}
}
#[test]
fn test_creation() {
assert!(PositiveNonzeroInteger::new(10).is_ok());
assert_eq!(Err(CreationError::Negative), PositiveNonzeroInteger::new(-10));
assert_eq!(Err(CreationError::Zero), PositiveNonzeroInteger::new(0));
}
// `PositiveNonzeroInteger::new` is always creating a new instance and returning an `Ok` result.
// It should be doing some checking, returning an `Err` result if those checks fail, and only
// returning an `Ok` result if those checks determine that everything is... okay :)