Rust variable
last modified October 18, 2023
In this article we show how to use variables in Rust.
Variable
A variable is used to store values. It is a label given to a value.
Rust uses the let keyword to define a variable.
In Rust, a variable is immutable by default. To create a mutable variable,
we use the mut keyword.
Variables can hold values of different data types. A data type is a set of values and the allowable operations on those values. Rust can infer the data type from the right side of the assignment.
Rust variable simple example
The following is a simple Rust example with variables.
fn main() {
let name: &str = "John Doe";
let age: i32 = 34;
println!("{name} is {age} years old");
}
The program defines two variables.
let name: &str = "John Doe";
This line defines a string variable. After the colon character, we specify the data type of the variable.
let age: i32 = 34;
We define an integer variable.
println!("{name} is {age} years old");
The two variables are used to build a message, which is printed to the console
with println! macro.
$ cargo run -q John Doe is 34 years old
Rust variable type inference
Rust can infer the data type of a variable from the right side of the assignment.
fn main() {
let name = "John Doe";
let age = 34;
println!("{name} is {age} years old");
}
In the program, we omit the data type declaration for our two variables.
Rust mutable variable
A mutable variable can change over the course of a program.
fn main() {
let mut name = String::from("John Doe");
let mut age = 34;
println!("{name} is {age} years old");
name = String::from("Roger Roe");
age = 54;
println!("{name} is {age} years old");
}
We define two variables. Later we assign new values to the variables.
let mut name = String::from("John Doe");
let mut age = 34;
Mutable variables are prefixed with the mut keyword.
name = String::from("Roger Roe");
age = 54;
We assign new values to the variables.
λ cargo run -q John Doe is 34 years old Roger Roe is 54 years old
Rust function parameter
A variable passed to a function is called a parameter or an argument.
fn main() {
let name = String::from("John Doe");
let age = 34;
let msg = build_msg(name, age);
println!("{msg}");
}
fn build_msg(name: String, age: i32) -> String {
return format!("{} is {} years old", name, age);
}
In the program, we define a build_msg function. It takes two
parameters. The parameters are used to build a message, which is returned from
the function.
let name = String::from("John Doe");
let age = 34;
let msg = build_msg(name, age);
We define two variables and pass them as function arguments.
fn build_msg(name: String, age: i32) -> String {
return format!("{} is {} years old", name, age);
}
Inside the function, we have two variables. These are valid in the function block.
let msg = build_msg(name, age);
The build_msg function returns the generated string. It is assigned
to the msg variable.
In this article we have worked with variables in Rust.