for loops

for and range

The for in construct can be used to iterate through an Iterator. One of the easiest ways to create an iterator is to use the range notation a..b. This yields values from a (inclusive) to b (exclusive) in steps of one.

Let's write FizzBuzz using for instead of while.

fn main() {
    // `n` will take the values: 1, 2, ..., 100 in each iteration
    for n in 1..101 {
        if n % 15 == 0 {
            println!("fizzbuzz");
        } else if n % 3 == 0 {
            println!("fizz");
        } else if n % 5 == 0 {
            println!("buzz");
        } else {
            println!("{}", n);
        }
    }
}

Alternatively, a..=b can be used for a range that is inclusive on both ends. The above can be written as:

fn main() {
    // `n` will take the values: 1, 2, ..., 100 in each iteration
    for n in 1..=100 {
        if n % 15 == 0 {
            println!("fizzbuzz");
        } else if n % 3 == 0 {
            println!("fizz");
        } else if n % 5 == 0 {
            println!("buzz");
        } else {
            println!("{}", n);
        }
    }
}

for and iterators

The for in construct is able to interact with an Iterator in several ways. As discussed in the section on the Iterator trait, by default the for loop will apply the into_iter function to the collection. However, this is not the only means of converting collections into iterators.

into_iter, iter and iter_mut all handle the conversion of a collection into an iterator in different ways, by providing different views on the data within.

• iter - This borrows each element of the collection through each iteration. Thus leaving the collection untouched and available for reuse after the loop.

fn main() {
    let names = vec!["Bob", "Frank", "Ferris"];

    for name in names.iter() {
        match name {
            &"Ferris" => println!("There is a rustacean among us!"),
            // TODO ^ Try deleting the & and matching just "Ferris"
            _ => println!("Hello {}", name),
        }
    }
    
    println!("names: {:?}", names);
}

• into_iter - This consumes the collection so that on each iteration the exact data is provided. Once the collection has been consumed it is no longer available for reuse as it has been 'moved' within the loop.

fn main() {
    let names = vec!["Bob", "Frank", "Ferris"];

    for name in names.into_iter() {
        match name {
            "Ferris" => println!("There is a rustacean among us!"),
            _ => println!("Hello {}", name),
        }
    }
    
    println!("names: {:?}", names);
    // FIXME ^ Comment out this line
}

• iter_mut - This mutably borrows each element of the collection, allowing for the collection to be modified in place.

fn main() {
    let mut names = vec!["Bob", "Frank", "Ferris"];

    for name in names.iter_mut() {
        *name = match name {
            &mut "Ferris" => "There is a rustacean among us!",
            _ => "Hello",
        }
    }

    println!("names: {:?}", names);
}

In the above snippets note the type of match branch, that is the key difference in the types of iteration. The difference in type then of course implies differing actions that are able to be performed.

See also:

Iterator