Control flow, part 1

All our programs so far have been pretty straightforward.
A sequence of instructions is executed from top to bottom, and that's it.

It's time to introduce some branching.

if clauses

The if keyword is used to execute a block of code only if a condition is true.

Here's a simple example:

let number = 3;
if number < 5 {
    println!("`number` is smaller than 5");
}

This program will print number is smaller than 5 because the condition number < 5 is true.

else clauses

Like most programming languages, Rust supports an optional else branch to execute a block of code when the condition in an if expression is false.
For example:

let number = 3;

if number < 5 {
    println!("`number` is smaller than 5");
} else {
    println!("`number` is greater than or equal to 5");
}

else if clauses

Your code drifts more and more to the right when you have multiple if expressions, one nested inside the other.

let number = 3;

if number < 5 {
    println!("`number` is smaller than 5");
} else {
    if number >= 3 {
        println!("`number` is greater than or equal to 3, but smaller than 5");
    } else {
        println!("`number` is smaller than 3");
    }
}

You can use the else if keyword to combine multiple if expressions into a single one:

let number = 3;

if number < 5 {
    println!("`number` is smaller than 5");
} else if number >= 3 {
    println!("`number` is greater than or equal to 3, but smaller than 5");
} else {
    println!("`number` is smaller than 3");
}

Booleans

The condition in an if expression must be of type bool, a boolean.
Booleans, just like integers, are a primitive type in Rust.

A boolean can have one of two values: true or false.

No truthy or falsy values

If the condition in an if expression is not a boolean, you'll get a compilation error.

For example, the following code will not compile:

let number = 3;
if number {
    println!("`number` is not zero");
}

You'll get the following compilation error:

error[E0308]: mismatched types
 --> src/main.rs:3:8
  |
3 |     if number {
  |        ^^^^^^ expected `bool`, found integer

This follows from Rust's philosophy around type coercion: there's no automatic conversion from non-boolean types to booleans. Rust doesn't have the concept of truthy or falsy values, like JavaScript or Python.
You have to be explicit about the condition you want to check.

Comparison operators

It's quite common to use comparison operators to build conditions for if expressions.
Here are the comparison operators available in Rust when working with integers:

• ==: equal to
• !=: not equal to
• <: less than
• >: greater than
• <=: less than or equal to
• >=: greater than or equal to

if/else is an expression

In Rust, if expressions are expressions, not statements: they return a value.
That value can be assigned to a variable or used in other expressions. For example:

let number = 3;
let message = if number < 5 {
    "smaller than 5"
} else {
    "greater than or equal to 5"
};

In the example above, each branch of the if evaluates to a string literal, which is then assigned to the message variable.
The only requirement is that both if branches return the same type.

Exercise

The exercise for this section is located in 02_basic_calculator/03_if_else