wiki/content/20200827171554-variables_in_rust.md

date	id	title
2020-08-27	9a604084-80a1-4868-be7b-950e7f43b65d	Rust variables

Mutability

By default variables in Rust are immutable. To make a variable mutable one must explicity add `mut` in front of it

fn main() {
    let mut x = 5;
    println!("The value of x is: {}", x);
    x = 6;
    println!("The value of x is: {}", x);
}

Constants

Constatns are values that are bound to a name and are not allowed to change. Some differences with variables:

• You can't use `mut` with constants
• Constants are delared with `const` keyword instead of `let`
• Type value must be annotated
• Constants can be declared in any scope
• Constants may only be set to a constant expression, not the result of a function call of any other value that could only be computed at runtime

#![allow(unused_variables)]
fn main() {
const MAX_POINTS: u32 = 100_000;
}

Shadowing

You can declare a new variable with the same value as a previous variable. The new variable "shadows" the previous variable

fn main() {
    let x = 5;

    let x = x + 1;

    let x = x * 2;

    println!("The value of x is: {}", x);
}

Shadowing is different from marking a variable as mut, because we’ll get a compile-time error if we accidentally try to reassign to this variable without using the let keyword. By using let, we can perform a few transformations on a value but have the variable be immutable after those transformations have been completed.

The other difference between mut and shadowing is that because we’re effectively creating a new variable when we use the let keyword again, we can change the type of the value but reuse the same name. For example, say our program asks a user to show how many spaces they want between some text by inputting space characters, but we really want to store that input as a number:

fn main() {
    let spaces = "   ";
    let spaces = spaces.len();

    println!("There are {} spaces in the string", spaces)
}

Data types

Tuple

Tuple elements can be accessed directly by using a period.

fn main() {
    let tup: (i32, f64, u8) = (500, 6.4, 1);

    println!("These are the tuple values {} {} {}", tup.0, tup.1, tup.2)
}

Array

In Rust every element of an array must have the same type. They also have a fixed length, like tuples.

fn main() {
    let _a = [1, 2, 3, 4, 5];
}

You would write an array’s type by using square brackets, and within the brackets include the type of each element, a semicolon, and then the number of elements in the array, like so:

#![allow(unused_variables)]
fn main() {
    let a: [i32; 5] = [1, 2, 3, 4, 5];
}

You can also easily initialize an array that contains the same value for each element:

#![allow(unused_variables)]
fn main() {
    let a = [3; 5];
}

Accessing array elements is also straightforward:

#![allow(unused_variables)]
fn main() {
    let a = [1, 2, 3, 4, 5];

    let first = a[0];
    let second = a[1];
}

Destructuring

Rust also supports destructuring

fn main() {
    let tup = (500, 6.4, 1);

    let (x, y, z) = tup;

    println!("The value of x is: {}", x);
    println!("The value of y is: {}", y);
    println!("The value of z is: {}", z);
}