7.1 Conditional Statements

Conditional statements control whether a block of code executes based on a boolean condition. Rust’s if, else if, and else constructs will look familiar to C programmers, but there are some important differences.

7.1.1 The Basic if Statement

The simplest form of Rust’s if statement looks much like C’s:

fn main() {
    let number = 5;
    if number > 0 {
        println!("The number is positive.");
    }
}

Key Points:

  • No Implicit Conversions: The condition must be a bool.
  • Parentheses Optional: Rust does not require parentheses around the condition (though they are allowed).
  • Braces Required: Even a single statement must be enclosed in braces.

In Rust, the condition in an if statement must explicitly be of type bool. Unlike C, where any non-zero integer is treated as true, Rust will not compile code that relies on integer-to-boolean conversions.

C Example:

int number = 5;
if (number) {
    // In C, any non-zero value is considered true
    printf("Number is non-zero.\n");
}

7.1.2 if as an Expression

One noteworthy difference from C is that, in Rust, if can be used as an expression to produce a value. This allows you to assign the result of an if/else expression directly to a variable:

fn main() {
    let condition = true;
    let number = if condition { 10 } else { 20 };
    println!("The number is: {}", number);
}

Here:

  • Both Branches Must Have the Same Type: The if and else blocks must produce values of the same type, or the compiler will emit an error.
  • No Ternary Operator: Rust replaces the need for the ternary operator (?: in C) by letting if serve as an expression.

7.1.3 Multiple Branches: else if and else

As in C, you can chain multiple conditions using else if:

fn main() {
    let number = 0;
    if number > 0 {
        println!("The number is positive.");
    } else if number < 0 {
        println!("The number is negative.");
    } else {
        println!("The number is zero.");
    }
}

Key Points:

  • Conditions are checked sequentially.
  • Only the first matching true branch executes.
  • The optional else runs if no preceding conditions match.

7.1.4 Type Consistency in if Expressions

When using if as an expression to assign a value, all possible branches must return the same type:

fn main() {
    let condition = true;
    let number = if condition {
        5
    } else {
        "six" // Mismatched type!
    };
}

This code fails to compile because the if branch returns an i32, while the else branch returns a string slice. Rust’s strict type system prevents mixing these types in a single expression.