Rust Conditional Statements

Conditional statements in Rust are written in this format:

Example

fn main() {
    let number = 3;
    if number < 5 {
        println!("Condition is true");
    } else {
        println!("Condition is false");
    }
}

In the above program, there is an if statement, which is common in many other languages, but there are some differences: First, the condition expression number < 5 does not need to be enclosed in parentheses (note, not needed does not mean not allowed); however, Rust's if does not have the rule where a single statement does not need curly braces {}, and it does not allow a single statement to replace a block. Nevertheless, Rust still supports the traditional else-if syntax:

Example

fn main() {
    let a = 12;
    let b;
    if a > 0 {
        b = 1;
    } else if a < 0 {
        b = -1;
    } else {
        b = 0;
    }
    println!("b is {}", b);
}

Output:

b is 1

In Rust, the condition expression must be of type bool. For example, the following program is incorrect:

Example

fn main() {
    let number = 3;
    if number { // Error, expected `bool`, found integer rustc(E0308)
        println!("Yes");
    }
}

Although in C/C++ languages, the condition expression is represented by an integer, where non-zero means true, this rule is prohibited in many languages that focus on code safety.

Considering the function body expressions we learned in previous chapters, we can think about this:

if <condition> { block 1 } else { block 2 }

Can { block 1 } and { block 2 } be function body expressions?

The answer is yes! This means that in Rust, we can use if-else structures to achieve effects similar to the ternary conditional operator (A ? B : C):

Example

fn main() {
    let a = 3;
    let number = if a > 0 { 1 } else { -1 };
    println!("number is {}", number);
}

Output:

number is 1

Note: Both function body expressions must be of the same type! And there must be an else followed by an expression block.