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[rustc.git] / src / doc / book / 2018-edition / src / ch03-05-control-flow.md

## Control Flow

Deciding whether or not to run some code depending on if a condition is true
and deciding 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:

<span class="filename">Filename: src/main.rs</span>

```rust
fn main() {
    let number = 3;

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

<!-- NEXT PARAGRAPH WRAPPED WEIRD INTENTIONALLY SEE #199 -->

All `if` expressions start with the keyword `if`, which is followed by a
condition. In this case, the condition checks whether or not the variable
`number` has a value less than 5. The block of code we want to execute if the
condition is true is placed 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:

```text
$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31 secs
     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:

```rust,ignore
let number = 7;
```

Run the program again, and look at the output:

```text
$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31 secs
     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:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore
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:

```text
error[E0308]: mismatched types
 --> src/main.rs:4:8
  |
4 |     if number {
  |        ^^^^^^ expected bool, found integral variable
  |
  = note: expected type `bool`
             found type `{integer}`
```

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:

<span class="filename">Filename: src/main.rs</span>

```rust
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 have multiple conditions by combining `if` and `else` in an `else if`
expression. For example:

<span class="filename">Filename: src/main.rs</span>

```rust
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:

```text
$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31 secs
     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 holds 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, as in Listing 3-2:

<span class="filename">Filename: src/main.rs</span>

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

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

<span class="caption">Listing 3-2: Assigning the result of an `if` expression
to a variable</span>

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

```text
$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
    Finished dev [unoptimized + debuginfo] target(s) in 0.30 secs
     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:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore
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:

```text
error[E0308]: if and else have incompatible types
 --> src/main.rs:4:18
  |
4 |       let number = if condition {
  |  __________________^
5 | |         5
6 | |     } else {
7 | |         "six"
8 | |     };
  | |_____^ expected integral variable, found &str
  |
  = note: expected type `{integer}`
             found type `&str`
```

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. Rust needs to know at compile time what type the `number`
variable is, definitively, so it can verify at compile time that its 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*. A loop runs through the code inside the loop
body to the end and then starts 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:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore
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 a keyboard shortcut,
<span class="keystroke">ctrl-c</span>, to halt a program that is stuck in a
continual loop. Give it a try:

```text
$ cargo run
   Compiling loops v0.1.0 (file:///projects/loops)
    Finished dev [unoptimized + debuginfo] target(s) in 0.29 secs
     Running `target/debug/loops`
again!
again!
again!
again!
^Cagain!
```

The symbol `^C` represents where you pressed <span class="keystroke">ctrl-c
</span>. 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 halt signal.

Fortunately, Rust provides another, more reliable way to break out of a loop.
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.


#### Returning from loops

One of the uses of a `loop` is to retry an operation you know can fail, such as
checking if a thread completed its job. However, you might need to pass the
result of that operation to the rest of your code. If you add it to the `break`
expression you use to stop the loop, it will be returned by the broken loop:

```rust
fn main() {
    let mut counter = 0;

    let result = loop {
        counter += 1;

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

    assert_eq!(result, 20);
}
```

#### Conditional Loops with `while`

It’s often useful for a program 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. This loop type could be implemented
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. Listing 3-3 uses `while`: the program loops
three times, counting down each time, and then, after the loop, it prints
another message and exits.

<span class="filename">Filename: src/main.rs</span>

```rust
fn main() {
    let mut number = 3;

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

        number = number - 1;
    }

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

<span class="caption">Listing 3-3: Using a `while` loop to run code while a
condition holds true</span>

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 holds
true, the code runs; otherwise, it exits the loop.

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

You could use the `while` construct to loop over the elements of a collection,
such as an array. For example, let’s look at Listing 3-4:

<span class="filename">Filename: src/main.rs</span>

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

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

        index = index + 1;
    }
}
```

<span class="caption">Listing 3-4: Looping through each element of a collection
using a `while` loop</span>

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:

```text
$ cargo run
   Compiling loops v0.1.0 (file:///projects/loops)
    Finished dev [unoptimized + debuginfo] target(s) in 0.32 secs
     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.

But this approach is error prone; we could cause the program to panic if the
index length is incorrect. It’s also slow, because the compiler adds runtime
code to perform the conditional check on every element 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 this code in Listing 3-5:

<span class="filename">Filename: src/main.rs</span>

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

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

<span class="caption">Listing 3-5: Looping through each element of a collection
using a `for` loop</span>

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.

For example, in the code in Listing 3-4, if you removed an item from the `a`
array but forgot to update the condition to `while index < 4`, the code would
panic. 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.

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`, which is a type provided by the standard library
that 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:

<span class="filename">Filename: src/main.rs</span>

```rust
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! That was a sizable chapter: you learned about variables, scalar
and compound data types, functions, comments, `if` expressions, and loops! If
you want 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.