if/else
main thing to note here is that boolean expression not wrapped in parentheses
fn main() {
let big_n =
if n< 10 && n > -10 {
10 * n
} else {
n / 2
}; // <--- semicolon here is for let binding
}Loop
loop keyword indicates an infinite loop.
fn main() {
let mut count = 0u32;
loop {
count += 1;
if count == 2 {
continue; // will continue to next loop, skipping the check of count == 3
}
if count == 3 {
break;
}
}
}Nesting and Labels
you can label the loops so that you can break and continue a specific loop
fn main() {
'outer: loop {
'inner: loop {
break 'outer; // this will break the whole thing
}
}
}Returning from Loops
break keyword can actually return values
fn main() {
let mut count = 0;
let result = loop {
count += 1;
if count == 3 {
break count * 10;
}
};
// result will be 30
}While
loops until a condition is met
fn main() {
let mut count = 0;
while count < 10 {
count += 1;
}
}For and Range
defined as for <something> in <Iterator>, where an iterator is something you create. The easiest way it by defining a range use ..
fn main() {
for n in 1..100 {
println!("{}", n);
}
}specific iterators
There are a number of ways to iterate over lists, each with varying degrees of permissions. More in Traits
fn main() {
let names = vec!["bob", "francis", "ferris"];
// iter borrows each element, cannot mut
for name in names.iter() {
match name {
&"Ferris" => println!("There is a rustacean among us!"),
_ => println!("Hello {}", name),
}
}
// into_iter consumes the collection, meaning it is no longer available for reuse after it has been moved into the loop
for name in names.into_iter() {
match name {
"Ferris" => println!("There is a rustacean among us!"),
_ => println!("Hello {}", name),
}
}
println!("names: {:?}", names); // <-- not possible
// must rebind
let names = vec!["bob", "francis", "ferris"];
// iter_mut mutably borrows each element of the collection, allowing for inplace modifications
for name in names.iter_mut() {
*name = match name {
&mut "Ferris" => "There is a rustacean among us!",
_ => "Hello",
}
}
println!("names: {:?}", names);
}Match
provides pattern matching via match, similar to switch case in C
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"),
// Match an inclusive range
13..=19 => println!("A teen"),
// Handle the rest of cases
_ => println!("Ain't special"),
}
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,
};
println!("{} -> {}", boolean, binary);
}Its alot more powerful though since the cases can be expressed in various creative ways:
- They can destructure tuples, arrays, enums, pointers, and structs
- They can have further conditional guards
- They can be bound
Destructuring
fn main() {
// tuples
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
}
// array
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
),
}
// enum
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 and references
// 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);
},
}
struct Foo {
x: (u32, u32),
y: u32,
}
// structs
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),
}
}Guards
enum Temperature {
Celsius(i32),
Fahrenheit(i32),
}
fn main() {
let temperature = Temperature::Celsius(35);
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),
}
}Binding
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?
// Could `match` n and use an `if` guard, but would
// not contribute to exhaustiveness checks.
// (Although in this case that would not matter since
// a "catch-all" pattern is present at the bottom)
// 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),
// A similar binding can be done when matching several values.
n @ (1 | 7 | 15 | 13) => println!("I'm a teen of age {:?}", n),
// Nothing bound. Return the result.
n => println!("I'm an old person of age {:?}", n),
}
}If Let
If the let expression succeeds, evaluate the block with that let
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 :)!");
// 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");
}
}Let Else
allows for a fallback from let if it doesnt workout
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"));
}While let
similar to if let, while ‘let’ expression succeeds, keep looping with it.
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.
}