Control flow with if

Two roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveler, long I stood
And looked down one as far as I could
To where it bent in the undergrowth;

  • Robert Frost, “The Road Not Taken”

In Rust, there are a few ways to cause our code to branch. The most fundamental way is by using if. An if expression gives us two paths forward, and asks the question, “Which one should I take?”

Let’s make a new project to explore if. Navigate to your projects directory, and use Cargo to make a new project called branches:

$ cargo new --bin branches
$ cd branches

Here’s a sample program using if:

fn main() {
    let condition = true;

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

Let's try running it:

$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
     Running `target/debug/branches`
condition was true

We can change the value of condition:

    let condition = false;

And then run it again:

$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
     Running `target/debug/branches`
condition was false

This is the very basic structure of if: if the condition is true, then execute some code. If it’s not true, then execute some other code, after else.

An else is not required:

fn main() {
    let condition = false;

    if condition {
        println!("condition was true");
    }
}

In this case, nothing is printed.

It’s also worth noting that condition here must be a bool. Let’s try an example with something else:

fn main() {
    let condition = 5;

    if condition {
        println!("condition was five");
    }
}

If we try to run this program, Rust will complain:

   Compiling branches v0.1.0 (file:///projects/branches)
src/main.rs:4:8: 4:17 error: mismatched types:
 expected `bool`,
    found `_`
(expected bool,
    found integral variable) [E0308]
src/main.rs:4     if condition {
                     ^~~~~~~~~
src/main.rs:4:8: 4:17 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to previous error
Could not compile `branches`.

We expected a bool, but got an integer. Rust will not automatically try to convert non-boolean types to a boolean here. We must be explicit.

else if

We can make multiple decisions by combining if and else in another way:

fn main() {
    let number = 5;

    if number == 3 {
        println!("condition was 3");
    } else if number == 4 {
        println!("condition was 4");
    } else if number == 5 {
        println!("condition was 5");
    } else {
        println!("condition was something else");
    }
}

Let's try running it:

$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
     Running `target/debug/branches`
condition was 5

When this program executes, it will check each if in turn, and execute the first body for which the condition holds true.

Using a single else if can be okay, but if you find yourself with more than one, you may want to refactor your code. Rust has a more powerful branching construct called match for these cases. We'll cover it later, when we talk about enums.

if as an expression

There’s one last detail we need to learn about if: it’s an expression. That means that we can use it on the right hand side of a let binding, for instance:

fn main() {
    let condition = true;

    let number = if condition {
        5
    } else {
        6
    };

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

Let’s run this:

$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
     Running `target/debug/branches`
The value of number is: 5

Remember, blocks of code evaluate to the last expression in them. And numbers by themselves are also expressions. So in this case, the value of the whole if expression depends on which block of code executes.

There’s another small detail involved here: this means that if you use if in this way, both arms of the if must be the same type. This doesn’t work:

fn main() {
    let condition = true;

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

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

If we try to run this, we’ll get an error:

   Compiling branches v0.1.0 (file:///projects/branches)
src/main.rs:4:18: 8:6 error: if and else have incompatible types:
 expected `_`,
    found `&‘static str`
(expected integral variable,
    found &-ptr) [E0308]
src/main.rs:4     let number = if condition {
src/main.rs:5         5
src/main.rs:6     } else {
src/main.rs:7         "six"
src/main.rs:8     };
src/main.rs:4:18: 8:6 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to previous error
Could not compile `branches`.

if and else have incompatible types. This can’t work.