// Rust-101, Part 03: Input
// ========================
-// In part 00, I promised that we would eventually replace `read_vec` by a function
-// that actually asks the user to enter a bunch of numbers. Unfortunately,
-// I/O is a complicated topic, so the code to do that is not exactly pretty - but well,
-// let's get that behind us.
+//@ In part 00, I promised that we would eventually replace `read_vec` by a function
+//@ that actually asks the user to enter a bunch of numbers. Unfortunately,
+//@ I/O is a complicated topic, so the code to do that is not exactly pretty - but well,
+//@ let's get that behind us.
-// I/O is provided by the module `std::io`, so we first import that.
-// We also import the I/O *prelude*, which brings a bunch of commonly used I/O stuff
+// I/O is provided by the module `std::io`, so we first have import that with `use`.
+// We also import the I/O *prelude*, which makes a bunch of commonly used I/O stuff
// directly available.
use std::io::prelude::*;
use std::io;
-// Let's now go over this function line-by-line. First, we call the constructor of `Vec`
-// to create an empty vector. As mentioned in the previous part, `new` here is just
-// a static function with no special treatment. While it is possible to call `new`
-// for a particular type (`Vec::<i32>::new()`), the common way to make sure we
-// get the right type is to annotate a type at the *variable*. It is this variable
-// that we interact with for the rest of the function, so having its type available
-// (and visible!) is much more useful. Without knowing the return type of `Vec::new`,
-// specifying its type parameter doesn't tell us all that much.
+//@ Let's now go over this function line-by-line. First, we call the constructor of `Vec`
+//@ to create an empty vector. As mentioned in the previous part, `new` here is just
+//@ a static function with no special treatment. While it is possible to call `new`
+//@ for a particular type (`Vec::<i32>::new()`), the common way to make sure we
+//@ get the right type is to annotate a type at the *variable*. It is this variable
+//@ that we interact with for the rest of the function, so having its type available
+//@ (and visible!) is much more useful. Without knowing the return type of `Vec::new`,
+//@ specifying its type parameter doesn't tell us all that much.
fn read_vec() -> Vec<i32> {
let mut vec: Vec<i32> = Vec::<i32>::new();
// The central handle to the standard input is made available by `io::stdin()`.
let stdin = io::stdin();
println!("Enter a list of numbers, one per line. End with Ctrl-D.");
- // We would now like to iterate over standard input line-by-line. We can use a `for` loop
- // 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](http://doc.rust-lang.org/stable/std/io/struct.Stdin.html) for more
- // details.)
+ //@ We would now like to iterate over standard input line-by-line. We can use a `for` loop
+ //@ 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](http://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. The problem with I/O is that it can always go wrong, so
- // `line` has type `io::Result<String>`. This 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](http://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.
+ // 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](http://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.
- // Can you find the documentation of `Result::unwrap()`?
- //
+ //@ 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.
let line = line.unwrap();
- // Now that we have our `String`, we want to make it an `i32`. `parse` is a method on `String` that
- // can convert a string to anything. Try finding it's documentation!
+ // 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 it's 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`.
- match line.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.
- Ok(num) => vec.push(num),
- Err(_) => println!("What did I say about numbers?"),
+ //@ 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.
+ Ok(num) => {
+ vec.push(num) /*@*/
+ },
+ // We don't care about the particular error, so we ignore it with a `_`.
+ Err(_) => {
+ println!("What did I say about numbers?") /*@*/
+ },
}
}
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
-// items declared public can be imported elsewhere.
+//@ I already sneaked a bunch of `pub` in part 02 to make this possible: Only
+//@ items declared public can be imported elsewhere.
use part02::{SomethingOrNothing,Something,Nothing,vec_min};
// If you update your `main.rs` to use part 03, `cargo run` should now ask you for some numbers,
// and tell you the minimum. Neat, isn't it?
pub fn main() {
let vec = read_vec();
- let min = vec_min(vec);
- min.print();
+ let min = vec_min(vec); /*@*/
+ min.print(); /*@*/
}
// **Exercise 03.1**: Define a trait `Print` to write a generic version of `SomethingOrNothing::print`.
}
}
-// [index](main.html) | [previous](part02.html) | [next](part04.html)
+// **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) | [next](part04.html)
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