//@ for that, but there is a catch: What happens if there is some other piece of code running
//@ concurrently, that also reads from standard input? The result would be a mess. Hence
//@ Rust requires us to `lock` standard input if we want to perform large operations on
- //@ it. (See [the documentation](https://doc.rust-lang.org/stable/std/io/struct.Stdin.html) for more
- //@ details.)
+ //@ it. (See [the documentation](https://doc.rust-lang.org/stable/std/io/struct.Stdin.html) for
+ //@ more details.)
for line in stdin.lock().lines() {
// Rust's type for (dynamic, growable) strings is `String`. However, our variable `line`
// here is not yet of that type: It has type `io::Result<String>`.
- //@ The problem with I/O is that it can always go wrong. The type of `line` is a lot like `Option<String>` ("a `String` or
- //@ nothing"), but in the case of "nothing", there is additional information about the error.
- //@ Again, I recommend to check [the documentation](https://doc.rust-lang.org/stable/std/io/type.Result.html).
- //@ You will see that `io::Result` is actually just an alias for `Result`, so click on that to obtain
+ //@ The problem with I/O is that it can always go wrong. The type of `line` is a lot like
+ //@ `Option<String>` ("a `String` or nothing"), but in the case of "nothing", there is
+ //@ additional information about the error. Again, I recommend to check
+ //@ [the documentation](https://doc.rust-lang.org/stable/std/io/type.Result.html). You will
+ //@ see that `io::Result` is actually just an alias for `Result`, so click on that to obtain
//@ the list of all constructors and methods of the type.
- //@ We will be lazy here and just assume that nothing goes wrong: `unwrap` returns the `String` if there is one,
- //@ and panics the program otherwise. Since a `Result` carries some details about the error that occurred,
- //@ there will be a somewhat reasonable error message. Still, you would not want a user to see such
- //@ an error, so in a "real" program, we would have to do proper error handling.
+ //@ We will be lazy here and just assume that nothing goes wrong: `unwrap` returns the
+ //@ `String` if there is one, and panics the program otherwise. Since a `Result` carries
+ //@ some details about the error that occurred, there will be a somewhat reasonable error
+ //@ message. Still, you would not want a user to see such an error, so in a "real" program,
+ //@ we would have to do proper error handling.
//@ Can you find the documentation of `Result::unwrap`?
//@
- // I chose the same name (`line`) for the new variable to ensure that I will never, accidentally,
- // access the "old" `line` again.
+ // I chose the same name (`line`) for the new variable to ensure that I will never,
+ // accidentally, access the "old" `line` again.
let line = line.unwrap();
// Now that we have our `String`, we want to make it an `i32`.
//@ We first `trim` the `line` to remove leading and trailing whitespace.
- //@ `parse` is a method on `String` that can convert a string to anything. Try finding its documentation!
+ //@ `parse` is a method on `String` that can convert a string to anything. Try finding its
+ //@ documentation!
- //@ In this case, Rust *could* figure out automatically that we need an `i32` (because of the return type
- //@ of the function), but that's a bit too much magic for my taste. We are being more explicit here:
- //@ `parse::<i32>` is `parse` with its generic type set to `i32`.
+ //@ In this case, Rust *could* figure out automatically that we need an `i32` (because of
+ //@ the return type of the function), but that's a bit too much magic for my taste. We are
+ //@ being more explicit here: `parse::<i32>` is `parse` with its generic type set to `i32`.
match line.trim().parse::<i32>() {
- //@ `parse` returns again a `Result`, and this time we use a `match` to handle errors (like, the user entering
- //@ something that is not a number).
- //@ This is a common pattern in Rust: Operations that could go wrong will return `Option` or `Result`.
- //@ The only way to get to the value we are interested in is through pattern matching (and through helper functions
- //@ like `unwrap`). If we call a function that returns a `Result`, and throw the return value away,
- //@ the compiler will emit a warning. It is hence impossible for us to *forget* handling an error,
- //@ or to accidentally use a value that doesn't make any sense because there was an error producing it.
+ //@ `parse` returns again a `Result`, and this time we use a `match` to handle errors
+ //@ (like, the user entering something that is not a number).
+ //@ This is a common pattern in Rust: Operations that could go wrong will return
+ //@ `Option` or `Result`. The only way to get to the value we are interested in is
+ //@ through pattern matching (and through helper functions like `unwrap`). If we call
+ //@ a function that returns a `Result`, and throw the return value away, the compiler
+ //@ will emit a warning. It is hence impossible for us to *forget* handling an error,
+ //@ or to accidentally use a value that doesn't make any sense because there was an
+ //@ error producing it.
Ok(num) => {
vec.push(num) /*@*/
},
vec
}
-//@ So much for `read_vec`. If there are any questions left, the documentation of the respective function
-//@ should be very helpful. Try finding the one for `Vec::push`. I will not always provide the links,
-//@ as the documentation is quite easy to navigate and you should get used to that.
+//@ So much for `read_vec`. If there are any questions left, the documentation of the respective
+//@ function should be very helpful. Try finding the one for `Vec::push`. I will not always provide
+//@ the links, as the documentation is quite easy to navigate and you should get used to that.
// For the rest of the code, we just re-use part 02 by importing it with `use`.
//@ I already sneaked a bunch of `pub` in part 02 to make this possible: Only
min.print(); /*@*/
}
-// **Exercise 03.1**: Define a trait `Print` to write a generic version of `SomethingOrNothing::print`.
-// Implement that trait for `i32`, and change the code above to use it.
+// **Exercise 03.1**: The goal is to write a generic version of `SomethingOrNothing::print`.
+// To this end, define a trait `Print` that provides (simple) generic printing, and implement
+// that trait for `i32`. Then define `SomethingOrNothing::print2` to use that trait, and change
+// `main` above to use the new generic `print2` function.
// I will again provide a skeleton for this solution. It also shows how to attach bounds to generic
// implementations (just compare it to the `impl` block from the previous exercise).
// You can read this as "For all types `T` satisfying the `Print` trait, I provide an implementation
// for `SomethingOrNothing<T>`".
//
-// Notice that I called the function on `SomethingOrNothing` `print2` to disambiguate from the `print` defined previously.
+// Notice that I called the function on `SomethingOrNothing` `print2` to disambiguate from the
+// `print` defined previously.
//
// *Hint*: There is a macro `print!` for printing without appending a newline.
pub trait Print {
/* Add things here */
}
+impl Print for i32 {
+ /* Add things here */
+}
impl<T: Print> SomethingOrNothing<T> {
fn print2(self) {
unimplemented!()
}
}
-// **Exercise 03.2**: Building on exercise 02.2, implement all the things you need on `f32` to make your
-// program work with floating-point numbers.
+// **Exercise 03.2**: Building on exercise 02.2, implement all the things you need on `f32` to make
+// your program work with floating-point numbers.
-//@ [index](main.html) | [previous](part02.html) | [raw source](workspace/src/part03.rs) | [next](part04.html)
+//@ [index](main.html) | [previous](part02.html) | [raw source](workspace/src/part03.rs) |
+//@ [next](part04.html)
projects / rust-101.git / blobdiff