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152 lines
3.2 KiB
Markdown
152 lines
3.2 KiB
Markdown
---
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date: 20200827
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id: 9a604084-80a1-4868-be7b-950e7f43b65d
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title: Rust variables
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---
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# Mutability
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By default variables in Rust are immutable. To make a variable mutable
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one must explicity add \`mut\` in front of it
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``` rust
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fn main() {
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let mut x = 5;
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println!("The value of x is: {}", x);
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x = 6;
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println!("The value of x is: {}", x);
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}
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```
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# Constants
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Constatns are values that are bound to a name and are not allowed to
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change. Some differences with variables:
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- You can't use \`mut\` with constants
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- Constants are delared with \`const\` keyword instead of \`let\`
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- Type value must be annotated
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- Constants can be declared in any scope
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- Constants may only be set to a constant expression, not the result
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of a function call of any other value that could only be computed at
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runtime
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``` rust
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#![allow(unused_variables)]
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fn main() {
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const MAX_POINTS: u32 = 100_000;
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}
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```
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# Shadowing
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You can declare a new variable with the same value as a previous
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variable. The new variable "shadows" the previous variable
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``` rust
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fn main() {
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let x = 5;
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let x = x + 1;
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let x = x * 2;
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println!("The value of x is: {}", x);
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}
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```
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Shadowing is different from marking a variable as mut, because we’ll get
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a compile-time error if we accidentally try to reassign to this variable
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without using the let keyword. By using let, we can perform a few
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transformations on a value but have the variable be immutable after
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those transformations have been completed.
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The other difference between mut and shadowing is that because we’re
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effectively creating a new variable when we use the let keyword again,
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we can change the type of the value but reuse the same name. For
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example, say our program asks a user to show how many spaces they want
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between some text by inputting space characters, but we really want to
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store that input as a number:
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``` rust
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fn main() {
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let spaces = " ";
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let spaces = spaces.len();
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println!("There are {} spaces in the string", spaces)
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}
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```
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# Data types
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## Tuple
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Tuple elements can be accessed directly by using a period.
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``` rust
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fn main() {
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let tup: (i32, f64, u8) = (500, 6.4, 1);
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println!("These are the tuple values {} {} {}", tup.0, tup.1, tup.2)
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}
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```
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## Array
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In Rust every element of an array must have the same type. They also
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have a fixed length, like tuples.
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``` rust
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fn main() {
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let _a = [1, 2, 3, 4, 5];
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}
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```
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You would write an array’s type by using square brackets, and within the
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brackets include the type of each element, a semicolon, and then the
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number of elements in the array, like so:
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``` rust
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#![allow(unused_variables)]
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fn main() {
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let a: [i32; 5] = [1, 2, 3, 4, 5];
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}
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```
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You can also easily initialize an array that contains the same value for
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each element:
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``` rust
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#![allow(unused_variables)]
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fn main() {
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let a = [3; 5];
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}
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```
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Accessing array elements is also straightforward:
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``` rust
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#![allow(unused_variables)]
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fn main() {
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let a = [1, 2, 3, 4, 5];
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let first = a[0];
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let second = a[1];
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}
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```
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## Destructuring
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Rust also supports destructuring
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``` rust
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fn main() {
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let tup = (500, 6.4, 1);
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let (x, y, z) = tup;
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println!("The value of x is: {}", x);
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println!("The value of y is: {}", y);
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println!("The value of z is: {}", z);
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}
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```
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