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## Control Flow

The ability to run some code depending on whether a condition is `true` and to run some code repeatedly while a condition is `true` are basic building blocks in most programming languages. The most common constructs that let you control the flow of execution of Rust code are `if` expressions and loops.

### `if` Expressions

An `if` expression allows you to branch your code depending on conditions. You provide a condition and then state, “If this condition is met, run this block of code. If the condition is not met, do not run this block of code.”

Create a new project called branches in your projects directory to explore the `if` expression. In the src/main.rs file, input the following:

Filename: src/main.rs

    fn main() {
        let number = 3;

        if number < 5 {
            println!("condition was true");
        } else {
            println!("condition was false");
        }
    }

All `if` expressions start with the keyword `if`, followed by a condition. In this case, the condition checks whether or not the variable `number` has a value less than 5. We place the block of code to execute if the condition is `true` immediately after the condition inside curly brackets. Blocks of code associated with the conditions in `if` expressions are sometimes called arms, just like the arms in `match` expressions that we discussed in the “Comparing the Guess to the Secret Number” section of Chapter 2.

Optionally, we can also include an `else` expression, which we chose to do here, to give the program an alternative block of code to execute should the condition evaluate to `false`. If you don’t provide an `else` expression and the condition is `false`, the program will just skip the `if` block and move on to the next bit of code.

Try running this code; you should see the following output:

    $ cargo run
       Compiling branches v0.1.0 (file:///projects/branches)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.31s
         Running `target/debug/branches`
    condition was true

Let’s try changing the value of `number` to a value that makes the condition `false` to see what happens:

    fn main() {
        let number = 7;

        if number < 5 {
            println!("condition was true");
        } else {
            println!("condition was false");
        }
    }

Run the program again, and look at the output:

    $ cargo run
       Compiling branches v0.1.0 (file:///projects/branches)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.31s
         Running `target/debug/branches`
    condition was false

It’s also worth noting that the condition in this code must be a `bool`. If the condition isn’t a `bool`, we’ll get an error. For example, try running the following code:

Filename: src/main.rs

    fn main() {
        let number = 3;

        if number {
            println!("number was three");
        }
    }

The `if` condition evaluates to a value of `3` this time, and Rust throws an error:

    $ cargo run
       Compiling branches v0.1.0 (file:///projects/branches)
    error[E0308]: mismatched types
     --> src/main.rs:4:8
      |
    4 |     if number {
      |        ^^^^^^ expected `bool`, found integer

    For more information about this error, try `rustc --explain E0308`.
    error: could not compile `branches` (bin "branches") due to 1 previous error

The error indicates that Rust expected a `bool` but got an integer. Unlike languages such as Ruby and JavaScript, Rust will not automatically try to convert non-Boolean types to a Boolean. You must be explicit and always provide `if` with a Boolean as its condition. If we want the `if` code block to run only when a number is not equal to `0`, for example, we can change the `if` expression to the following:

Filename: src/main.rs

    fn main() {
        let number = 3;

        if number != 0 {
            println!("number was something other than zero");
        }
    }

Running this code will print `number was something other than zero`.

#### Handling Multiple Conditions with `else if`

You can use multiple conditions by combining `if` and `else` in an `else if` expression. For example:

Filename: src/main.rs

    fn main() {
        let number = 6;

        if number % 4 == 0 {
            println!("number is divisible by 4");
        } else if number % 3 == 0 {
            println!("number is divisible by 3");
        } else if number % 2 == 0 {
            println!("number is divisible by 2");
        } else {
            println!("number is not divisible by 4, 3, or 2");
        }
    }

This program has four possible paths it can take. After running it, you should see the following output:

    $ cargo run
       Compiling branches v0.1.0 (file:///projects/branches)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.31s
         Running `target/debug/branches`
    number is divisible by 3

When this program executes, it checks each `if` expression in turn and executes the first body for which the condition evaluates to `true`. Note that even though 6 is divisible by 2, we don’t see the output `number is divisible by 2`, nor do we see the `number is not divisible by 4, 3, or 2` text from the `else` block. That’s because Rust only executes the block for the first `true` condition, and once it finds one, it doesn’t even check the rest.

Using too many `else if` expressions can clutter your code, so if you have more than one, you might want to refactor your code. Chapter 6 describes a powerful Rust branching construct called `match` for these cases.

#### Using `if` in a `let` Statement

Because `if` is an expression, we can use it on the right side of a `let` statement to assign the outcome to a variable, as in Listing 3-2.

Filename: src/main.rs

    fn main() {
        let condition = true;
        let number = if condition { 5 } else { 6 };

        println!("The value of number is: {number}");
    }

Listing 3-2: Assigning the result of an `if` expression to a variable

The `number` variable will be bound to a value based on the outcome of the `if` expression. Run this code to see what happens:

    $ cargo run
       Compiling branches v0.1.0 (file:///projects/branches)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.30s
         Running `target/debug/branches`
    The value of number is: 5

Remember that blocks of code evaluate to the last expression in them, and numbers by themselves are also expressions. In this case, the value of the whole `if` expression depends on which block of code executes. This means the values that have the potential to be results from each arm of the `if` must be the same type; in Listing 3-2, the results of both the `if` arm and the `else` arm were `i32` integers. If the types are mismatched, as in the following example, we’ll get an error:

Filename: src/main.rs

    fn main() {
        let condition = true;

        let number = if condition { 5 } else { "six" };

        println!("The value of number is: {number}");
    }

When we try to compile this code, we’ll get an error. The `if` and `else` arms have value types that are incompatible, and Rust indicates exactly where to find the problem in the program:

    $ cargo run
       Compiling branches v0.1.0 (file:///projects/branches)
    error[E0308]: `if` and `else` have incompatible types
     --> src/main.rs:4:44
      |
    4 |     let number = if condition { 5 } else { "six" };
      |                                 -          ^^^^^ expected integer, found `&str`
      |                                 |
      |                                 expected because of this

    For more information about this error, try `rustc --explain E0308`.
    error: could not compile `branches` (bin "branches") due to 1 previous error

The expression in the `if` block evaluates to an integer, and the expression in the `else` block evaluates to a string. This won’t work because variables must have a single type, and Rust needs to know at compile time what type the `number` variable is, definitively. Knowing the type of `number` lets the compiler verify the type is valid everywhere we use `number`. Rust wouldn’t be able to do that if the type of `number` was only determined at runtime; the compiler would be more complex and would make fewer guarantees about the code if it had to keep track of multiple hypothetical types for any variable.

### Repetition with Loops

It’s often useful to execute a block of code more than once. For this task, Rust provides several loops, which will run through the code inside the loop body to the end and then start immediately back at the beginning. To experiment with loops, let’s make a new project called loops.

Rust has three kinds of loops: `loop`, `while`, and `for`. Let’s try each one.

#### Repeating Code with `loop`

The `loop` keyword tells Rust to execute a block of code over and over again forever or until you explicitly tell it to stop.

As an example, change the src/main.rs file in your loops directory to look like this:

Filename: src/main.rs

    fn main() {
        loop {
            println!("again!");
        }
    }

When we run this program, we’ll see `again!` printed over and over continuously until we stop the program manually. Most terminals support the keyboard shortcut `ctrl`-`c` to interrupt a program that is stuck in a continual loop. Give it a try:

    $ cargo run
       Compiling loops v0.1.0 (file:///projects/loops)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.08s
         Running `target/debug/loops`
    again!
    again!
    again!
    again!
    ^Cagain!

The symbol `^C` represents where you pressed `ctrl`-`c`.

You may or may not see the word `again!` printed after the `^C`, depending on where the code was in the loop when it received the interrupt signal.

Fortunately, Rust also provides a way to break out of a loop using code. You can place the `break` keyword within the loop to tell the program when to stop executing the loop. Recall that we did this in the guessing game in the “Quitting After a Correct Guess” section of Chapter 2 to exit the program when the user won the game by guessing the correct number.

We also used `continue` in the guessing game, which in a loop tells the program to skip over any remaining code in this iteration of the loop and go to the next iteration.

#### Returning Values from Loops

One of the uses of a `loop` is to retry an operation you know might fail, such as checking whether a thread has completed its job. You might also need to pass the result of that operation out of the loop to the rest of your code. To do this, you can add the value you want returned after the `break` expression you use to stop the loop; that value will be returned out of the loop so you can use it, as shown here:

    fn main() {
        let mut counter = 0;

        let result = loop {
            counter += 1;

            if counter == 10 {
                break counter * 2;
            }
        };

        println!("The result is {result}");
    }

Before the loop, we declare a variable named `counter` and initialize it to `0`. Then we declare a variable named `result` to hold the value returned from the loop. On every iteration of the loop, we add `1` to the `counter` variable, and then check whether the `counter` is equal to `10`. When it is, we use the `break` keyword with the value `counter * 2`. After the loop, we use a semicolon to end the statement that assigns the value to `result`. Finally, we print the value in `result`, which in this case is `20`.

You can also `return` from inside a loop. While `break` only exits the current loop, `return` always exits the current function.

#### Loop Labels to Disambiguate Between Multiple Loops

If you have loops within loops, `break` and `continue` apply to the innermost loop at that point. You can optionally specify a loop label on a loop that you can then use with `break` or `continue` to specify that those keywords apply to the labeled loop instead of the innermost loop. Loop labels must begin with a single quote. Here’s an example with two nested loops:

    fn main() {
        let mut count = 0;
        'counting_up: loop {
            println!("count = {count}");
            let mut remaining = 10;

            loop {
                println!("remaining = {remaining}");
                if remaining == 9 {
                    break;
                }
                if count == 2 {
                    break 'counting_up;
                }
                remaining -= 1;
            }

            count += 1;
        }
        println!("End count = {count}");
    }

The outer loop has the label `'counting_up`, and it will count up from 0 to 2. The inner loop without a label counts down from 10 to 9. The first `break` that doesn’t specify a label will exit the inner loop only. The `break 'counting_up;` statement will exit the outer loop. This code prints:

    $ cargo run
       Compiling loops v0.1.0 (file:///projects/loops)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.58s
         Running `target/debug/loops`
    count = 0
    remaining = 10
    remaining = 9
    count = 1
    remaining = 10
    remaining = 9
    count = 2
    remaining = 10
    End count = 2

#### Conditional Loops with `while`

A program will often need to evaluate a condition within a loop. While the condition is `true`, the loop runs. When the condition ceases to be `true`, the program calls `break`, stopping the loop. It’s possible to implement behavior like this using a combination of `loop`, `if`, `else`, and `break`; you could try that now in a program, if you’d like. However, this pattern is so common that Rust has a built-in language construct for it, called a `while` loop. In Listing 3-3, we use `while` to loop the program three times, counting down each time, and then, after the loop, print a message and exit.

Filename: src/main.rs

    fn main() {
        let mut number = 3;

        while number != 0 {
            println!("{number}!");

            number -= 1;
        }

        println!("LIFTOFF!!!");
    }

Listing 3-3: Using a `while` loop to run code while a condition evaluates to `true`

This construct eliminates a lot of nesting that would be necessary if you used `loop`, `if`, `else`, and `break`, and it’s clearer. While a condition evaluates to `true`, the code runs; otherwise, it exits the loop.

#### Looping Through a Collection with `for`

You can choose to use the `while` construct to loop over the elements of a collection, such as an array. For example, the loop in Listing 3-4 prints each element in the array `a`.

Filename: src/main.rs

    fn main() {
        let a = [10, 20, 30, 40, 50];
        let mut index = 0;

        while index < 5 {
            println!("the value is: {}", a[index]);

            index += 1;
        }
    }

Listing 3-4: Looping through each element of a collection using a `while` loop

Here, the code counts up through the elements in the array. It starts at index `0`, and then loops until it reaches the final index in the array (that is, when `index < 5` is no longer `true`). Running this code will print every element in the array:

    $ cargo run
       Compiling loops v0.1.0 (file:///projects/loops)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.32s
         Running `target/debug/loops`
    the value is: 10
    the value is: 20
    the value is: 30
    the value is: 40
    the value is: 50

All five array values appear in the terminal, as expected. Even though `index` will reach a value of `5` at some point, the loop stops executing before trying to fetch a sixth value from the array.

However, this approach is error prone; we could cause the program to panic if the index value or test condition is incorrect. For example, if you changed the definition of the `a` array to have four elements but forgot to update the condition to `while index < 4`, the code would panic. It’s also slow, because the compiler adds runtime code to perform the conditional check of whether the index is within the bounds of the array on every iteration through the loop.

As a more concise alternative, you can use a `for` loop and execute some code for each item in a collection. A `for` loop looks like the code in Listing 3-5.

Filename: src/main.rs

    fn main() {
        let a = [10, 20, 30, 40, 50];

        for element in a {
            println!("the value is: {element}");
        }
    }

Listing 3-5: Looping through each element of a collection using a `for` loop

When we run this code, we’ll see the same output as in Listing 3-4. More importantly, we’ve now increased the safety of the code and eliminated the chance of bugs that might result from going beyond the end of the array or not going far enough and missing some items. Machine code generated from `for` loops can be more efficient as well, because the index doesn’t need to be compared to the length of the array at every iteration.

Using the `for` loop, you wouldn’t need to remember to change any other code if you changed the number of values in the array, as you would with the method used in Listing 3-4.

The safety and conciseness of `for` loops make them the most commonly used loop construct in Rust. Even in situations in which you want to run some code a certain number of times, as in the countdown example that used a `while` loop in Listing 3-3, most Rustaceans would use a `for` loop. The way to do that would be to use a `Range`, provided by the standard library, which generates all numbers in sequence starting from one number and ending before another number.

Here’s what the countdown would look like using a `for` loop and another method we’ve not yet talked about, `rev`, to reverse the range:

Filename: src/main.rs

    fn main() {
        for number in (1..4).rev() {
            println!("{number}!");
        }
        println!("LIFTOFF!!!");
    }

This code is a bit nicer, isn’t it?

## Summary

You made it! This was a sizable chapter: you learned about variables, scalar and compound data types, functions, comments, `if` expressions, and loops! To practice with the concepts discussed in this chapter, try building programs to do the following:

- Convert temperatures between Fahrenheit and Celsius.
- Generate the nth Fibonacci number.
- Print the lyrics to the Christmas carol “The Twelve Days of Christmas,” taking advantage of the repetition in the song.

When you’re ready to move on, we’ll talk about a concept in Rust that doesn’t commonly exist in other programming languages: ownership.