The Rust Map Function - A Gateway to Iterators

This post looks at the Map function in the Rust programming language as one of the basic tools to use and interact with collections and iterators.

Arguably one of the main reasons the Rust language has become so popular is because the entire ecosystem builds upon a consistent experience. From toolchains to documentation and language specifications, almost every aspect of the language adheres to a certain level of consistency. Getting into the Rust mindset allows us, developers, to intuitively discover aspects of the language and gain the ability to write better and more consistent code.

In this article, we are going to focus on Iterators - one of the key language tools enabling us to write idiomatic Rust code. This may surprise you at first, especially if you are coming from other programming languages (for example Swift or JavaScript), where working with types like lists and arrays does not immediately expose the concept of Iterators. The opposite is valid for Rust. Iterators cover a very large number of applications and as such, being comfortable with their use greatly facilitates becoming a better Rust developer.

In order to follow along, you will need at least some familiarity with the Rust programming language. To make things easier, this article includes detailed code samples and links to the Rust playground where you can execute and study the provided code snippets. 

After reading this article, you will know:

A core concept for Functional programming, Map is also commonly used as a tool for dealing with lists and collections in Rust and is one of the most essential methods for working with Iterators.

 method offers a way to apply a function (or a closure) on each element from a list. This is key in functional programming methodologies and offers a way to transform a collection of type A, to a collection of elements of type B. In Rust, the 

 method is frequently used to interact with 

 and any other types which can be represented as an 

Let's illustrate this with an example. Imagine a list of numbers (a vector of numbers) and we would like to multiply each number by 10.

let numbers = vec![3, 6, 9, 12];
let result: Vec<i32> = numbers
    .iter()
    .map(|n| n * 10)
    .collect();
// result is now [30, 60, 90, 120]

 doesn't work directly on the vector of numbers.

The answer lies with the fact Rust makes it very easy to use iterators for almost everything. In the ergonomics of the language, it is generally preferred to use iterators instead of directly interacting with a vector. 

In the example above we actually have two iterators at play - the first which we obtain through the use of 

 in order to get the elements of the vector and a second one which happens to be the return type of the 

This may be a bit surprising at first, especially if you have a background in other programming languages (for example Swift or JavaScript) but indeed, in Rust 

 also returns an iterator. Let's have a closer look at the syntax we need in order to use Map.

, we can find the exact signature of the Map method:

fn map<B, F>(self, f: F) -> Map<Self, F> where F: FnMut(Self::Item) -> B

With this we can focus on several key takeaways.

 can only be applied to iterators. This means that we first need to convert a collection type to an iterator before we can transform its elements.

To obtain an iterator from a vector, the standard library provides 

let str_numbers: Vec<&str> = vec!["1", "2", "3", "4"];
let str_numbers_iterator = str_numbers.iter();

 is also an iterator. This can be very useful in cases when multiple transformations need to be chained one after the other. 

Let's illustrate this by extending the example with a second transformation of the vector of numbers. After multiplying by 10, let's divide each element by 3:

let numbers = vec![3, 6, 9, 12];
let result: Vec<i32> = numbers
    .iter()
    .map(|n| n * 10)
    .map(|n| n / 3)
    .collect();
// result is now [10, 20, 30, 40]

Since the Map function returns an Iterator, the result of the first transformation 

 will be used as input for the second transformation 

You probably notice by now that despite the return type of Map being Iterator, we somehow manage to get back a vector of results. 

 method. It acts as a consumer to the iterator, acquiring all elements and storing them into a vector.

, meaning the execution of the closure provided to Map is delayed until the values produced by the Map iterator are actually requested. To demonstrate this, let's modify the example so that we count the number of times the closure of the map method is invoked:

let numbers = vec![3, 6, 9, 12];
let mut number_of_times = 0;
let result: Vec<i32> = numbers
    .iter()
    .map(|n| {
        number_of_times += 1;
        return n * 10
    })
    .collect();
// result is now [30, 60, 90, 120]
// number_of_times equals 4

 runs, it first increments the counter and then applies the transformation of multiplying the number by 10, similar to what we did in the previous example. As expected, the counter equals 4 because this is the total number of elements on which the map function was applied.

Now let's modify this snippet by not calling 

let numbers = vec![3, 6, 9, 12];
let mut number_of_times = 0;
let result = numbers
    .iter()
    .map(|n| {
        number_of_times += 1;
        return n * 10
    });
// result is now Map { iter: Iter([3, 6, 9, 12]) }
// number_of_times equals 0

 variable, we see that what we get back is the actual Map iterator, the results of which are not materialized yet. We can prove this further by looking at the value of the counter variable which is now 0. This means the function inside the 

 gives us the opportunity to control when the transformation will be executed. You can imagine this can be significant in cases of large data sets where performance is important.

Let's have a look at several examples in which the Map function can be a good solution.

Transform a vector of strings to lowercase 

fn main() {
    let words: Vec<&str> = vec!["Hello", "from", "Rust", "!"];
    println!("Words before map: {:?}", words);
    let lowercased_words: Vec<String> = words // using String as the return type of `to_lowercase`
        .iter()
        .map(|word| word.to_lowercase())
        .collect();
    println!("Words after map: {:?}", lowercased_words);
}

Starting from a vector of string values (or "words"), we can use the Map function in order to get the lowercased equivalent of each word.

As before, we begin by accessing an iterator over the collection words. The 

 method produces an iterator which yields every element (that is, every word) of the collection.

Then we can use Map to define a closure which applies 

 the results into a new vector of String values.

Count the number of times a character occurs in a String

Let's define a string value and use Map in order to determine the number of occurrences of each character within the string.

let text = String::from("hello from rust!");

We expect an output giving the number of times each character is used, similar to the following:

The result looks like a dictionary where every key is a character and the value of that key is the number of occurrences. Let's use the 

fn main() {
    let text = String::from("hello from rust!");
    let result = "abcdefghijklmnopqrstuvwxyz"
        .chars()
        .map(|c| (c, text.matches(c).count()))
        .collect::<std::collections::HashMap<_, _>>();
    println!("{:?}", result);
}

 method in order to obtain an iterator over every letter of the alphabet. Then we count how many times each letter matches the text input. 

For example, how many times "a" appears in "hello from rust!", then how many times "b" appears in "hello from rust!" etc.

Finally, we collect the results from the mapping into a HashMap.

So far we've used examples where the mapping function always succeeds. In reality, we have to take into account the circumstances when this is not going to be the case. Such cases include transformations that have an optional return type like 

Let's illustrate the case of using Map with an operation that can potentially fail. 

For example, converting a vector of string values to a vector of numeric values of type 

. Applying what we've seen so far, we may attempt to solve the problem with the following snippet:

fn main() {
    let str_numbers: Vec<&str> = vec!["1", "2", "3", "I am not a number"];
    let numbers: Vec<u32> = str_numbers
        .iter()
        .map(|str_number| str_number.parse::<u32>())
        .collect();
}

For each element of the vector, we use the 

 method in order to convert the string value to a number. 

We realize quickly that this will not compile. 

The reason lies with the fact that Parse returns a 

 which indicates that the operation can either succeed returning a value of type 

 function is not equipped to deal with this situation directly.

, essentially forcing the fact that we are certain the parsing will never fail. If it does, the program will panic and exit:

fn main() {
    let str_numbers: Vec<&str> = vec!["1", "2", "3"];
    let numbers: Vec<u32> = str_numbers
        .iter()
        .map(|str_number| str_number.parse::<u32>().unwrap())
        .collect();
    print!("{:?}", numbers)
}

This works because all string values in the source vector can be converted to a number.

But what if we have a vector where some elements can't be expressed as a number?

fn main() {
    let str_numbers: Vec<&str> = vec!["1", "2", "3", "I am not a string"];
    let numbers: Vec<u32> = str_numbers
        .iter()
        .map(|str_number| str_number.parse::<u32>().unwrap())
        .collect();
    print!("{:?}", numbers)
}

The unwrap fails and causes the program to panic.

Luckily, the standard library provides an alternative that can handle the situation where the Map function can produce one or zero results. Meet 

fn main() {
    let str_numbers: Vec<&str> = vec!["1", "2", "3", "I am not a number"];
    let numbers: Vec<u32> = str_numbers
        .iter()
        .flat_map(|str_number| str_number.parse::<u32>())
        .collect();
    println!("{:?}", numbers);
	// [1, 2, 3]
}

 uses an iterator over the result of the mapping and as a consequence, it will skip over elements for which the mapping closure returns empty or unsuccessful values (like 

. In other words, we have a mechanism to skip over the failing elements and end up with a resulting vector of numbers for which the operation succeeded.

A note of caution regarding side effects when using the Map function.

Side effects refer to the fact that during the execution of a Map closure or a function, our program may modify state and variables which are external. We already saw an example of this, when we defined a counter which increments every time the Map function is executed:

let numbers = vec![3, 6, 9, 12];
let mut number_of_times = 0; // external state
let result: Vec<i32> = numbers
    .iter()
    .map(|n| {
        number_of_times += 1;
        return n * 10
    })
    .collect();

While this produces exactly the result we expect in this situation, it may not always be the case. Let's modify the example so that instead of transforming the numbers in the vector, we map the order in which they are processed:

let numbers = vec![3, 6, 9, 12];
let mut number_of_times = 0;
let result: Vec<(i32, i32)> = numbers
    .into_iter()
    .map(|n| {
        number_of_times += 1;
        return (n, number_of_times)
    })
    .collect();
println!("{:?}", result);

This results in the Map function to first be applied to the number 3 on position 1, followed by the number 6 on position 2, etc. 

 returns an iterator, we could choose to modify its output. For example, let's reverse the order of the Map operation:

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