| An integral part of any programming language are ways to modify control flow: if/else, for, and others. Let's talk about them in Rust. if/elseBranching with if-else is similar to other languages. Unlike many of them, the boolean condition doesn't need to be surrounded by parentheses, and each condition is followed by a block. if-else conditionals are expressions, and, all branches must return the same type. fn main() {
let n = 5;
if n < 0 {
print!("{} is negative", n);
} else if n > 0 {
print!("{} is positive", n);
} else {
print!("{} is zero", n);
}
let big_n =
if n < 10 && n > -10 {
println!(", and is a small number, increase ten-fold");
// This expression returns an `i32`.
10 * n
} else {
println!(", and is a big number, halve the number");
// This expression must return an `i32` as well.
n / 2
// TODO ^ Try suppressing this expression with a semicolon.
};
// ^ Don't forget to put a semicolon here! All `let` bindings need it.
println!("{} -> {}", n, big_n);
}
loopRust provides a loop keyword to indicate an infinite loop. The break statement can be used to exit a loop at anytime, whereas the continue statement can be used to skip the rest of the iteration and start a new one. fn main() {
let mut count = 0u32;
println!("Let's count until infinity!");
// Infinite loop
loop {
count += 1;
if count == 3 {
println!("three");
// Skip the rest of this iteration
continue;
}
println!("{}", count);
if count == 5 {
println!("OK, that's enough");
// Exit this loop
break;
}
}
}
Nesting and labelsIt's possible to break or continue outer loops when dealing with nested loops. In these cases, the loops must be annotated with some 'label, and the label must be passed to the break/continue statement. #![allow(unreachable_code, unused_labels)]
fn main() {
'outer: loop {
println!("Entered the outer loop");
'inner: loop {
println!("Entered the inner loop");
// This would break only the inner loop
//break;
// This breaks the outer loop
break 'outer;
}
println!("This point will never be reached");
}
println!("Exited the outer loop");
}
Returning from loopsOne of the uses of a loop is to retry an operation until it succeeds. If the operation returns a value though, you might need to pass it to the rest of the code: put it after the break, and it will be returned by the loop expression. fn main() {
let mut counter = 0;
let result = loop {
counter += 1;
if counter == 10 {
break counter * 2;
}
};
assert_eq!(result, 20);
}
whileThe while keyword can be used to run a loop while a condition is true. Let's write the infamous FizzBuzz using a while loop. fn main() {
// A counter variable
let mut n = 1;
// Loop while `n` is less than 101
while n < 101 {
if n % 15 == 0 {
println!("fizzbuzz");
} else if n % 3 == 0 {
println!("fizz");
} else if n % 5 == 0 {
println!("buzz");
} else {
println!("{}", n);
}
// Increment counter
n += 1;
}
}
for loops
for and rangeThe for in construct can be used to iterate through an Iterator. One of the easiest ways to create an iterator is to use the range notation a..b. This yields values from a (inclusive) to b (exclusive) in steps of one. Let's write FizzBuzz using for instead of while. fn main() {
// `n` will take the values: 1, 2, ..., 100 in each iteration
for n in 1..101 {
if n % 15 == 0 {
println!("fizzbuzz");
} else if n % 3 == 0 {
println!("fizz");
} else if n % 5 == 0 {
println!("buzz");
} else {
println!("{}", n);
}
}
}
Alternatively, a..=b can be used for a range that is inclusive on both ends. The above can be written as: fn main() {
// `n` will take the values: 1, 2, ..., 100 in each iteration
for n in 1..=100 {
if n % 15 == 0 {
println!("fizzbuzz");
} else if n % 3 == 0 {
println!("fizz");
} else if n % 5 == 0 {
println!("buzz");
} else {
println!("{}", n);
}
}
}
for and iteratorsThe for in construct is able to interact with an Iterator in several ways. As discussed in the section on the Iterator trait, by default the for loop will apply the into_iter function to the collection. However, this is not the only means of converting collections into iterators. into_iter, iter and iter_mut all handle the conversion of a collection into an iterator in different ways, by providing different views on the data within.
iter - This borrows each element of the collection through each iteration. Thus leaving the collection untouched and available for reuse after the loop.
fn main() {
let names = vec!["Bob", "Frank", "Ferris"];
for name in names.iter() {
match name {
&"Ferris" => println!("There is a rustacean among us!"),
// TODO ^ Try deleting the & and matching just "Ferris"
_ => println!("Hello {}", name),
}
}
println!("names: {:?}", names);
}
into_iter - This consumes the collection so that on each iteration the exact data is provided. Once the collection has been consumed it is no longer available for reuse as it has been 'moved' within the loop.
fn main() {
let names = vec!["Bob", "Frank", "Ferris"];
for name in names.into_iter() {
match name {
"Ferris" => println!("There is a rustacean among us!"),
_ => println!("Hello {}", name),
}
}
println!("names: {:?}", names);
// FIXME ^ Comment out this line
}
iter_mut - This mutably borrows each element of the collection, allowing for the collection to be modified in place.
fn main() {
let mut names = vec!["Bob", "Frank", "Ferris"];
for name in names.iter_mut() {
*name = match name {
&mut "Ferris" => "There is a rustacean among us!",
_ => "Hello",
}
}
println!("names: {:?}", names);
}
In the above snippets note the type of match branch, that is the key difference in the types of iteration. The difference in type then of course implies differing actions that are able to be performed. matchRust provides pattern matching via the match keyword, which can be used like a C switch. The first matching arm is evaluated and all possible values must be covered. fn main() {
let number = 13;
// TODO ^ Try different values for `number`
println!("Tell me about {}", number);
match number {
// Match a single value
1 => println!("One!"),
// Match several values
2 | 3 | 5 | 7 | 11 => println!("This is a prime"),
// TODO ^ Try adding 13 to the list of prime values
// Match an inclusive range
13..=19 => println!("A teen"),
// Handle the rest of cases
_ => println!("Ain't special"),
// TODO ^ Try commenting out this catch-all arm
}
let boolean = true;
// Match is an expression too
let binary = match boolean {
// The arms of a match must cover all the possible values
false => 0,
true => 1,
// TODO ^ Try commenting out one of these arms
};
println!("{} -> {}", boolean, binary);
}
DestructuringA match block can destructure items in a variety of ways. tuplesTuples can be destructured in a match as follows: fn main() {
let triple = (0, -2, 3);
// TODO ^ Try different values for `triple`
println!("Tell me about {:?}", triple);
// Match can be used to destructure a tuple
match triple {
// Destructure the second and third elements
(0, y, z) => println!("First is `0`, `y` is {:?}, and `z` is {:?}", y, z),
(1, ..) => println!("First is `1` and the rest doesn't matter"),
(.., 2) => println!("last is `2` and the rest doesn't matter"),
(3, .., 4) => println!("First is `3`, last is `4`, and the rest doesn't matter"),
// `..` can be used to ignore the rest of the tuple
_ => println!("It doesn't matter what they are"),
// `_` means don't bind the value to a variable
}
}
arrays/slicesLike tuples, arrays and slices can be destructured this way: fn main() {
// Try changing the values in the array, or make it a slice!
let array = [1, -2, 6];
match array {
// Binds the second and the third elements to the respective variables
[0, second, third] =>
println!("array[0] = 0, array[1] = {}, array[2] = {}", second, third),
// Single values can be ignored with _
[1, _, third] => println!(
"array[0] = 1, array[2] = {} and array[1] was ignored",
third
),
// You can also bind some and ignore the rest
[-1, second, ..] => println!(
"array[0] = -1, array[1] = {} and all the other ones were ignored",
second
),
// The code below would not compile
// [-1, second] => ...
// Or store them in another array/slice (the type depends on
// that of the value that is being matched against)
[3, second, tail @ ..] => println!(
"array[0] = 3, array[1] = {} and the other elements were {:?}",
second, tail
),
// Combining these patterns, we can, for example, bind the first and
// last values, and store the rest of them in a single array
[first, middle @ .., last] => println!(
"array[0] = {}, middle = {:?}, array[2] = {}",
first, middle, last
),
}
}
enumsAn enum is destructured similarly: // `allow` required to silence warnings because only
// one variant is used.
#[allow(dead_code)]
enum Color {
// These 3 are specified solely by their name.
Red,
Blue,
Green,
// These likewise tie `u32` tuples to different names: color models.
RGB(u32, u32, u32),
HSV(u32, u32, u32),
HSL(u32, u32, u32),
CMY(u32, u32, u32),
CMYK(u32, u32, u32, u32),
}
fn main() {
let color = Color::RGB(122, 17, 40);
// TODO ^ Try different variants for `color`
println!("What color is it?");
// An `enum` can be destructured using a `match`.
match color {
Color::Red => println!("The color is Red!"),
Color::Blue => println!("The color is Blue!"),
Color::Green => println!("The color is Green!"),
Color::RGB(r, g, b) =>
println!("Red: {}, green: {}, and blue: {}!", r, g, b),
Color::HSV(h, s, v) =>
println!("Hue: {}, saturation: {}, value: {}!", h, s, v),
Color::HSL(h, s, l) =>
println!("Hue: {}, saturation: {}, lightness: {}!", h, s, l),
Color::CMY(c, m, y) =>
println!("Cyan: {}, magenta: {}, yellow: {}!", c, m, y),
Color::CMYK(c, m, y, k) =>
println!("Cyan: {}, magenta: {}, yellow: {}, key (black): {}!",
c, m, y, k),
// Don't need another arm because all variants have been examined
}
}
pointers/refFor pointers, a distinction needs to be made between destructuring and dereferencing as they are different concepts which are used differently from languages like C/C++. - Dereferencing uses
* - Destructuring uses
&, ref, and ref mut
fn main() {
// Assign a reference of type `i32`. The `&` signifies there
// is a reference being assigned.
let reference = &4;
match reference {
// If `reference` is pattern matched against `&val`, it results
// in a comparison like:
// `&i32`
// `&val`
// ^ We see that if the matching `&`s are dropped, then the `i32`
// should be assigned to `val`.
&val => println!("Got a value via destructuring: {:?}", val),
}
// To avoid the `&`, you dereference before matching.
match *reference {
val => println!("Got a value via dereferencing: {:?}", val),
}
// What if you don't start with a reference? `reference` was a `&`
// because the right side was already a reference. This is not
// a reference because the right side is not one.
let _not_a_reference = 3;
// Rust provides `ref` for exactly this purpose. It modifies the
// assignment so that a reference is created for the element; this
// reference is assigned.
let ref _is_a_reference = 3;
// Accordingly, by defining 2 values without references, references
// can be retrieved via `ref` and `ref mut`.
let value = 5;
let mut mut_value = 6;
// Use `ref` keyword to create a reference.
match value {
ref r => println!("Got a reference to a value: {:?}", r),
}
// Use `ref mut` similarly.
match mut_value {
ref mut m => {
// Got a reference. Gotta dereference it before we can
// add anything to it.
*m += 10;
println!("We added 10. `mut_value`: {:?}", m);
},
}
}
structsSimilarly, a struct can be destructured as shown: fn main() {
struct Foo {
x: (u32, u32),
y: u32,
}
// Try changing the values in the struct to see what happens
let foo = Foo { x: (1, 2), y: 3 };
match foo {
Foo { x: (1, b), y } => println!("First of x is 1, b = {}, y = {} ", b, y),
// you can destructure structs and rename the variables,
// the order is not important
Foo { y: 2, x: i } => println!("y is 2, i = {:?}", i),
// and you can also ignore some variables:
Foo { y, .. } => println!("y = {}, we don't care about x", y),
// this will give an error: pattern does not mention field `x`
//Foo { y } => println!("y = {}", y),
}
let faa = Foo { x: (1, 2), y: 3 };
// You do not need a match block to destructure structs:
let Foo { x : x0, y: y0 } = faa;
println!("Outside: x0 = {x0:?}, y0 = {y0}");
// Destructuring works with nested structs as well:
struct Bar {
foo: Foo,
}
let bar = Bar { foo: faa };
let Bar { foo: Foo { x: nested_x, y: nested_y } } = bar;
println!("Nested: nested_x = {nested_x:?}, nested_y = {nested_y:?}");
}
GuardsA match guard can be added to filter the arm. #[allow(dead_code)]
enum Temperature {
Celsius(i32),
Fahrenheit(i32),
}
fn main() {
let temperature = Temperature::Celsius(35);
// ^ TODO try different values for `temperature`
match temperature {
Temperature::Celsius(t) if t > 30 => println!("{}C is above 30 Celsius", t),
// The `if condition` part ^ is a guard
Temperature::Celsius(t) => println!("{}C is equal to or below 30 Celsius", t),
Temperature::Fahrenheit(t) if t > 86 => println!("{}F is above 86 Fahrenheit", t),
Temperature::Fahrenheit(t) => println!("{}F is equal to or below 86 Fahrenheit", t),
}
}
Note that the compiler won't take guard conditions into account when checking if all patterns are covered by the match expression. fn main() {
let number: u8 = 4;
match number {
i if i == 0 => println!("Zero"),
i if i > 0 => println!("Greater than zero"),
// _ => unreachable!("Should never happen."),
// TODO ^ uncomment to fix compilation
}
}
BindingIndirectly accessing a variable makes it impossible to branch and use that variable without re-binding. match provides the @ sigil for binding values to names: // A function `age` which returns a `u32`.
fn age() -> u32 {
15
}
fn main() {
println!("Tell me what type of person you are");
match age() {
0 => println!("I haven't celebrated my first birthday yet"),
// Could `match` 1 ..= 12 directly but then what age
// would the child be? Instead, bind to `n` for the
// sequence of 1 ..= 12. Now the age can be reported.
n @ 1 ..= 12 => println!("I'm a child of age {:?}", n),
n @ 13 ..= 19 => println!("I'm a teen of age {:?}", n),
// Nothing bound. Return the result.
n => println!("I'm an old person of age {:?}", n),
}
}
You can also use binding to "destructure" enum variants, such as Option: fn some_number() -> Option<u32> {
Some(42)
}
fn main() {
match some_number() {
// Got `Some` variant, match if its value, bound to `n`,
// is equal to 42.
Some(n @ 42) => println!("The Answer: {}!", n),
// Match any other number.
Some(n) => println!("Not interesting... {}", n),
// Match anything else (`None` variant).
_ => (),
}
}
if letFor some use cases, when matching enums, match is awkward. For example: // Make `optional` of type `Option<i32>`
let optional = Some(7);
match optional {
Some(i) => println!("This is a really long string and `{:?}`", i),
_ => {},
// ^ Required because `match` is exhaustive. Doesn't it seem
// like wasted space?
};
if let is cleaner for this use case and in addition allows various failure options to be specified:
fn main() {
// All have type `Option<i32>`
let number = Some(7);
let letter: Option<i32> = None;
let emoticon: Option<i32> = None;
// The `if let` construct reads: "if `let` destructures `number` into
// `Some(i)`, evaluate the block (`{}`).
if let Some(i) = number {
println!("Matched {:?}!", i);
}
// If you need to specify a failure, use an else:
if let Some(i) = letter {
println!("Matched {:?}!", i);
} else {
// Destructure failed. Change to the failure case.
println!("Didn't match a number. Let's go with a letter!");
}
// Provide an altered failing condition.
let i_like_letters = false;
if let Some(i) = emoticon {
println!("Matched {:?}!", i);
// Destructure failed. Evaluate an `else if` condition to see if the
// alternate failure branch should be taken:
} else if i_like_letters {
println!("Didn't match a number. Let's go with a letter!");
} else {
// The condition evaluated false. This branch is the default:
println!("I don't like letters. Let's go with an emoticon :)!");
}
}
In the same way, if let can be used to match any enum value: // Our example enum
enum Foo {
Bar,
Baz,
Qux(u32)
}
fn main() {
// Create example variables
let a = Foo::Bar;
let b = Foo::Baz;
let c = Foo::Qux(100);
// Variable a matches Foo::Bar
if let Foo::Bar = a {
println!("a is foobar");
}
// Variable b does not match Foo::Bar
// So this will print nothing
if let Foo::Bar = b {
println!("b is foobar");
}
// Variable c matches Foo::Qux which has a value
// Similar to Some() in the previous example
if let Foo::Qux(value) = c {
println!("c is {}", value);
}
// Binding also works with `if let`
if let Foo::Qux(value @ 100) = c {
println!("c is one hundred");
}
}
Another benefit is that if let allows us to match non-parameterized enum variants. This is true even in cases where the enum doesn't implement or derive PartialEq. In such cases if Foo::Bar == a would fail to compile, because instances of the enum cannot be equated, however if let will continue to work. Would you like a challenge? Fix the following example to use if let: // This enum purposely neither implements nor derives PartialEq.
// That is why comparing Foo::Bar == a fails below.
enum Foo {Bar}
fn main() {
let a = Foo::Bar;
// Variable a matches Foo::Bar
if Foo::Bar == a {
// ^-- this causes a compile-time error. Use `if let` instead.
println!("a is foobar");
}
}
let-else
🛈 stable since: rust 1.65 🛈 you can target specific edition by compiling like this rustc --edition=2021 main.rs
With let-else, a refutable pattern can match and bind variables in the surrounding scope like a normal let, or else diverge (e.g. break, return, panic!) when the pattern doesn't match. use std::str::FromStr;
fn get_count_item(s: &str) -> (u64, &str) {
let mut it = s.split(' ');
let (Some(count_str), Some(item)) = (it.next(), it.next()) else {
panic!("Can't segment count item pair: '{s}'");
};
let Ok(count) = u64::from_str(count_str) else {
panic!("Can't parse integer: '{count_str}'");
};
(count, item)
}
fn main() {
assert_eq!(get_count_item("3 chairs"), (3, "chairs"));
}
The scope of name bindings is the main thing that makes this different from match or if let-else expressions. You could previously approximate these patterns with an unfortunate bit of repetition and an outer let: let (count_str, item) = match (it.next(), it.next()) {
(Some(count_str), Some(item)) => (count_str, item),
_ => panic!("Can't segment count item pair: '{s}'"),
};
let count = if let Ok(count) = u64::from_str(count_str) {
count
} else {
panic!("Can't parse integer: '{count_str}'");
};
while let
Similar to if let, while let can make awkward match sequences more tolerable. Consider the following sequence that increments i: // Make `optional` of type `Option<i32>`
let mut optional = Some(0);
// Repeatedly try this test.
loop {
match optional {
// If `optional` destructures, evaluate the block.
Some(i) => {
if i > 9 {
println!("Greater than 9, quit!");
optional = None;
} else {
println!("`i` is `{:?}`. Try again.", i);
optional = Some(i + 1);
}
// ^ Requires 3 indentations!
},
// Quit the loop when the destructure fails:
_ => { break; }
// ^ Why should this be required? There must be a better way!
}
}
Using while let makes this sequence much nicer: fn main() {
// Make `optional` of type `Option<i32>`
let mut optional = Some(0);
// This reads: "while `let` destructures `optional` into
// `Some(i)`, evaluate the block (`{}`). Else `break`.
while let Some(i) = optional {
if i > 9 {
println!("Greater than 9, quit!");
optional = None;
} else {
println!("`i` is `{:?}`. Try again.", i);
optional = Some(i + 1);
}
// ^ Less rightward drift and doesn't require
// explicitly handling the failing case.
}
// ^ `if let` had additional optional `else`/`else if`
// clauses. `while let` does not have these.
}
|
