Rust lang notes - variables and data types

Variables

Variables are defined using the let keyword

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let x = 5;

By default are inmutables, the way to make a variable mutable is with the mut keyword.

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

• type def ( explicit / implicit )

In Rust every variable in Rust has a type, and the compiler can infer the type most of the times. If we want to annotate the type of a variable, we can by putting a colon after the name and then the type after.

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let x: i32 = 5;

Constants

Constants are declared using the const keyword ( instead of let ) and the type must be annotated.

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const x: i32 = 5

constants are always immutable and you can not use mut. You can declared it in any scope, including the global scope, which makes them useful for values that many parts of code need to know about.

Shadowing

Rust let you declare a new variable with the same name as a previous variable, and the new variable shadows the previous variable.

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let x = 5;
println!("{}", x);

let x = x +1;
println!("{}", x);

Shadowing is different from marking a variable as mut

• We get compile errors if we forget the let keyword.

• We can change the type of the value but reuse the same name.

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  let spaces = " "; println!("{}",spaces); let spaces = spaces.len(); println!("{}",spaces);

Rust book chapter

Data types

Every value in Rust is of a certain data type, which tells Rust what kind of data is being specified so it knows how to work with that data.

• Scalar

A scalar type represents a single value. Rust has four primary scalar types: integers, floating-point ( f64 / f32 ), Booleans, and characters.

Rust has different types of integers

Note on characters, in Rust char literals are specified with single quotes, as opposed to string literals, which use double quotes.

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let c: char = 'A';
let s: String = String::from( "A" );

• Compound

Compound types can group multiple values into one type.

Tuples

Tuples let you group multiple values together within parentheses. These multiple values don’t have to be the same type and you can use the index ( with dot notation ) for access the item you want ( starting from 0 ).

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let my_tuple = ( 1, 'a', false );
println!("{}", my_tuple.1); // will print a

Arrays

Arrays in Rust are collections where all of the elements have the same type. and a fixed length set when you initialize them. They can’t get bigger or smaller, even if you declare as mut.

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let fruits = [ "apple", "banana" ];

If you need a sequence of values that can change in size, you can use a Vec type ( provided by the standard library ).

Slice

One primitive type you will see a lot is called a slice. Slices let us reference a contiguous subset of data in another data structure.

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    let numbers = [ 1, 2, 3, 4 ];
    let subset = &numbers[0..2];
    println!("{}", subset.last().unwrap() );

Rust book chapter

Also posted in collected notes.