Rust Development Classes

Back to the basics - making choices

In the previous chapter, you learned how to get input from the user, manipulate the input, and display it on the screen. Another common operation is to make a choice based on the input provided. In Rust, this is done with the if statement.

Let's start with a small modification to our earlier example.

fn main() {
    println!("What is your name?");
    let input = read_string();
    if input == "" {
        println!("You did not enter your name...");
    } else {
        println!("Your name is: {input}");
    }
}

fn read_string() -> String {
    let mut input = String::new();
    std::io::stdin()
        .read_line(&mut input)
        .expect("can not read user input");
    let cleaned_input = input.trim().to_string();
    cleaned_input
}

Note that we've extended the read_string() function to trim the whitespace from the input before returning.

The if statement has this format:

if [comparison] { code to execute when 'true' } else { code to execute when 'false' }

Common comparisons are:

• a == b; a equals b
• a != b; a does not equal b
• a < b; a is less than b
• a <= b; a is less than, or equal to b

The else block is optional.

The same { and } braces are used to group the statements that should be executed when the if is true or false.

You can chain if statements together to cover multiple conditions:

fn main() {
    println!("What is your name?");
    let input = read_string();
    if input == "" {
        println!("You did not enter your name...");
    } else if input == "Marcel" {
        println!("{input} is a great name!");
    } else {
        println!("Your name is: {input}");
    }
}

fn read_string() -> String {
    let mut input = String::new();
    std::io::stdin()
        .read_line(&mut input)
        .expect("can not read user input");
    let cleaned_input = input.trim().to_string();
    cleaned_input
}

Note that only one of the if branches is ever executed.

You can match on multiple conditions in a single if statement. To do this, separate the conditions with either:

• || for 'or'
• && for 'and'

So imagine we want to print "{} is a great name!" when the user types either "Marcel" or "marcel", we can use this if block:

if input == "" {
    println!("You did not enter your name...");
} else if input == "Marcel" || input == "marcel" {
    println!("{input} is a great name!");
} else {
    println!("Your name is: {input}");
}

Exercise

Add a fourth branch to the if block that prints "I love the name {}!" Pick two names for which you want to print this message, and create an appropriate condition for this branch.