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rust-iterators

Demonstrates basic Rust iterator use.

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The goal of this tutorial is to provide a handy reference to some of the common iterator patterns. It is not meant to be a replacement for the Iterator API reference or an overview of the core iterator concepts described in The Book. In fact, this tutorial relies on both resources.

To take full advantage of the material described here, it is recommended that you have at least cursory familiarity with Rust.

How to compile and run sample code:

git clone https://github.com/rustomax/rust-iterators.git
cd rust-iterators/
cargo run

Contents

Introduction

Life is repetitive and most things in it come as series of items. Programmatically we often need to count, enumerate, and iterate over these sequences. There are several ways to generate repetition in programming languages. One of the most prominent constructs is C-style for loop with familiar syntax:

for ( x = 0; x < 10; ++x ) {
  // do something
}

While this venerable method is powerful and flexible enough to accommodate many scenarios, it is also responsible for a fair share of bugs ranging from misplaced semicolons to unintentionally mutating the iterator variable inside the loop. In the spirit of safety and consistency with other language features, the C-style for loop is absent from Rust. Instead, Rust leverages iterators to achieve similar goals (and a lot more).

Basic Ranges

The most basic way to loop over a series of integers in Rust is the range. Range is created using .. notation and produces an iterator of integers incremented by 1:

for i in 1..11 {
    print!("{} ", i);
}
// output: 1 2 3 4 5 6 7 8 9 10

The code above will print a series of numbers from 1 to 10, and not include the last number 11. In other words, the .. produces an iterator that is inclusive on the left and exclusive on the right. In order to get a range that is inclusive on both ends, you use the ..= notation:

for i in 1..=10 {
  print!("{} ", i);
}
// output: 1 2 3 4 5 6 7 8 9 10

If you do not use the loop iterator variable, you can avoid instantiating it by leveraging the _ pattern. For instance, the following code prints out the number of elements in the iterator without instantiating a loop iterator variable:

let mut n: i32 = 0;
for _ in 0..10 {
  n += 1;
}
println!("num = {}", n);
// output: num = 10

The example above is somewhat contrived since iterators in Rust have count() function, which returns the number of elements in the iterator without the need to count them in a loop:

println!("num = {}", (0..10).count());
// output: num = 10

You will find that experienced Rust programmers are able to express in very terse iterator language what otherwise would have taken many more lines of conventional looping code. We cover some of these patterns below as we talk about adaptors, consumers and chaining iterator methods into complex statements.

Digging Deeper

If the basic incremental sequential range does not satisfy your needs, there are plenty of ways in Rust to customize the range iterators. Let's look at a few common ones.

Often, a range needs to be incremented not by 1, but by a different number.

The step_by() method allows you to do just that

for i in (0..11).step_by(2) {
    print!("{} ", i);
}

//output: 0 2 4 6 8 10

Alternatively, same result can be achieved with the filter() method. It applies a closure that can return either true or false to each element of an iterator and produces a new iterator that only contains elements for which the closure returns true.

The following iterator will produce a sequence of even numbers between 0 and 20.

for i in (0..21).filter(|x| (x % 2 == 0)) {
  print!("{} ", i);
}
// output: 0 2 4 6 8 10 12 14 16 18 20

Because filter() uses closures, it is very flexible and can be used to produce iterators that evaluate complex conditions. For instance, the following iterator produces a series of integers in the range between 0 and 20 that divide by both 2 and 3 without a remainder:

for i in (0..21).filter(|x| (x % 2 == 0) && (x % 3 == 0)) {
  print!("{} ", i);
}
// output: 0 6 12 18

While by default ranges are incremental, they can easily be reversed using the rev() method.

for i in (0..11).rev() {
  print!("{} ", i);
}
// output: 10 9 8 7 6 5 4 3 2 1 0

Another common iterator adaptor, map(), applies a closure to each element, and returns the resulting iterator. Here is an example of an iterator that produces a sequence of squares of numbers from 1 to 10:

for i in (1..11).map(|x| x * x) {
    print!("{} ", i);
}
// output: 1 4 9 16 25 36 49 64 81 100

fold() is a very powerful method. It returns the result of applying a special "accumulator" type of closure to all elements of an iterator resulting in a single value. The following iterator produces a sum of squares of numbers from 1 to 5.

let result = (1..=5).fold(0, |acc, x| acc + x * x);
println!("result = {}", result);

// output: result = 55

Perhaps the easiest way to understand what is happening here is to rewrite the example above in a more procedural fashion:

let mut acc = 0;

for x in 1..=5 {
  acc += x * x;
}

let result = acc;
println!("result = {}", result);

// output: result = 55

Wow! Isn't the fold() version so much more concise and readable?

Iterating over Arrays

Similarly to iterating over ranges, we can iterate over an array. The benefit of this is that arrays can contain values of arbitrary types, not just integrals. The only caveat is that array is not an iterator. We need to turn it into an iterator using the iter() method.

let cities = ["Toronto", "New York", "Melbourne"];

for city in cities.iter() {
  print!("{}, ", city);
}
// output: Toronto, New York, Melbourne,

Combining Iterator Adaptors

While in the previous sections we covered a good variety of methods allowing you to generate many different types of iterators, the real power of Rust shines when you start combining these approaches.

What if you wanted an inclusive range between 10 and 0 that is decremented by 2? This is easily accomplished by combining a couple of methods into a single iterator:

for i in (0..=10).rev().filter(|x| (x % 2 == 0)) {
  print!("{} ", i);
}
// output: 10 8 6 4 2 0

Need a non-contiguous range (basically a combination of two non-adjacent ranges)? You can combine multiple ranges with the chain() method:

let c = (1..4).chain(6..9);

for i in c {
  print!("{} ", i);
}
// output: 1 2 3 6 7 8

You can get very creative combining things! Below is an iterator that combines two ranges: the first one is incremented and filtered, another one - decremented. Not sure what such an abomination could be used for, but here it is nonetheless!

let r = (1..20)
  .filter(|&x| x % 5 == 0)
  .chain((6..9).rev());

for i in r {
  print!("{} ", i);
}
// output: 5 10 15 8 7 6

Notice how in the example above Rust allows us to visually better represent complex iterator statements by splitting them into multiple lines.

Another handy method is zip(). It is somewhat similar to chain() in that it combines two iterators into one. By contrast with chain(), zip() produces not a contiguous iterator, but an iterator of tuples: zip() method

let cities = ["Toronto", "New York", "Melbourne"];
let populations = [2_615_060, 8_550_405, ‎4_529_500];

let matrix = cities.iter().zip(populations.iter());

for (c, p) in matrix {
  println!("{:10}: population = {}", c, p);
}
// output:
// Toronto   : population = 2615060
// New York  : population = 8550405
// Melbourne : population = 4529500

Ranges of Characters

Programs that manipulate strings or text often require the ability to iterate over a range of characters. The char_iter crate provides convenient way to generate such ranges. char_iter supports Unicode characters.

To use the char_iter, put the following in your Cargo.toml

[dependencies]
char-iter = "0.1"

And then generate a character range with char_iter::new() method:

extern crate char_iter;

for c in char_iter::new('Д', 'П') {
  print!("{} ", c);
}
// output: Д Е Ж З И Й К Л М Н О П

Iterating over Vectors

Vector is one of Rust's fundamental structures. By its nature it is well suited to represent series of repetitive items. There are a number of language facilities in Rust that allow using vectors as iterators and vice-versa.

In the simplest case, similarly to how we created an iterator from an array, we can create an iterator from a vector using the iter() method. In fact this is considered to be the most idiomatic way in Rust to iterate over a vector.

let nums = vec![1, 2, 3, 4, 5];

for i in nums.iter() {
   print!("{} ", i);
}
// output: 1 2 3 4 5

As a matter of fact, the pattern above is so common that rust provides syntactic sugar for it in the form of the reference operator &.

let nums = vec![1, 2, 3, 4, 5];
for i in &nums {
   print!("{} ", i);
}
// output: 1 2 3 4 5

Notice that the borrows above are immutable. In other words, they are read-only. If we want to make changes to our vector, we have to use the mutable borrow &mut. For instance, the following code will mutably iterate over a vector doubling each element in the process.

let mut nums = vec![1, 2, 3, 4, 5];
for i in &mut nums {
    *i *= 2;
}
println!("{:?}", nums);

//output: [2, 4, 6, 8, 10]

However, now that you are an iterator ninja, you wouldn't use the for loop syntax above. You'd go with a map() instead, right?

let nums = vec![1, 2, 3, 4, 5];
let nums = nums.iter().map(|x| x * 2).collect::<Vec<i32>>();
println!("{:?}", nums);

//output: [2, 4, 6, 8, 10]

A slight digression. What if we wanted to use mutable iterator to add elements to the vector like so:

let mut nums = vec![1, 2, 3, 4, 5];
for i in &mut nums {
    nums.push(*i);
}
println!("{:?}", nums);

This won't compile with the error message cannot borrow nums as mutable more than once at a time. You see, our iterator (instantiated in the for loop) already borrowed nums as mutable. The push expression tries to do that again, which is prohibited in rust. This is rust's famous safety at work. If we could push something into the vector, while iterating over it, this would invalidate the iterator causing undefined behavior. Rust prevents this from happening at compile time. Not only iterators are powerful, but they are also super safe.

Now, let's do the opposite - create a vector from an iterator. In order to do that we need what is called a consumer. Consumers force lazy iterators to actually produce values.

collect() is a common consumer. It takes values from an iterator and converts them into a collection of required type. Below we are taking a range of numbers from 1 to 10 and transforming it into a vector of i32:

let v = (1..11).collect::<Vec<i32>>();
println!("{:?}", v);
// output: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

To get both the element of a vector and its index, you can use enumerate() method, which returns a tuple containing the index and the item on each iteration:

let v = vec![1, 2, 3];
for (i, n) in v.iter().enumerate() {
    println!("v[{}] = {}", i, n);
}
// output:
// v[0] = 1
// v[1] = 2
// v[2] = 3

There are a few other functions, that make using iterators on vectors particularly helpful.

min() and max(), for instance, return options, containing minimum and maximum values of the vector elements respectively:

let v = vec![3, 5, 0, -2, 3, 4, 1];
let max = v.iter().max();
let min = v.iter().min();

println!("max = {:?}, min = {:?}", max, min);

// output: max = Some(5), min = Some(-2)

sum() returns a sum of all values in an iterator. The following program leverages the sum() method to compute the grade point average of a rather mediocre student:

let grades = vec![4, 5, 6, 9, 7, 4, 8];
let sum: i32 = grades.iter().sum();
let gpa = sum as f32 / grades.len() as f32;

println!("sum = {}, gpa = {:.2}", sum, gpa);

// output: sum = 43, gpa = 6.14

Infinity and Beyond

So far we have dealt with iterators that operated on some finite range of values. Rust generalizes iterators in such a way that it is in fact possible to create an infinite range! Let us consider the following example:

let r = (1..).collect::<Vec<i32>>();

The (1..) defines a range that starts with 1 and increments indefinitely. In practice, such program compiles and runs, but eventually crashes with the error message: fatal runtime error: out of memory. Well, that's not very practical, you might say. Indeed, by themselves infinite ranges are pretty useless. What makes them useful is combining them with other adaptors and consumers.

One particularly helpful pattern involves using the take() method to limit the number of items returned by the iterator. The following iterator will return the first 10 items in a sequence of squares of integers that are divisible by 5 without a remainder.

let v = (1..)
  .map(|x| x * x)
  .filter(|x| x % 5 == 0 )
  .take(10)
  .collect::<Vec<i32>>();

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

// output: [25, 100, 225, 400, 625, 900, 1225, 1600, 2025, 2500]

Itertools

The itertools crate contains powerful additional iterator adaptors. Below are some examples.

To use itertools, add the following to your Cargo.toml:

[dependencies]
itertools = "0.10.0"

The unique() adaptor eliminates duplicates from an iterator. The duplicates do not need to be sequential.

extern crate itertools;
use itertools::Itertools;

let data = vec![1, 4, 3, 1, 4, 2, 5];
let unique = data.iter().unique();

for d in unique {
  print!("{} ", d);
}

//output: 1 4 3 2 5

The join() adaptor combines iterator elements into a single string with a separator in between the elements.

extern crate itertools;
use itertools::Itertools;

let creatures = vec!["banshee", "basilisk", "centaur"];
let list = creatures.iter().join(", ");
println!("In the enchanted forest, we found {}.", list);

// output: In the enchanted forest, we found banshee, basilisk, centaur.

The sorted_by() adaptor applies custom sorting order to iterator elements, returning a sorted vector. The following program will print out top 5 happiest countries, according to 2018 World Happiness Report.

sorted_by() uses Ordering trait to sort elements.

extern crate itertools;
use itertools::Itertools;

let happiness_index = vec![
    ("Canada", 7), ("Iceland", 4), ("Netherlands", 6),
    ("Finland", 1), ("New Zealand", 8), ("Denmark", 3),
    ("Norway", 2), ("Sweden", 9), ("Switzerland", 5)
  ];

let top_contries = happiness_index
  .into_iter()
  .sorted_by(|a, b| (&a.1).cmp(&b.1))
  .into_iter()
  .take(5);

for (country, rating) in top_contries {
  println!("# {}: {}", rating, country);
}

// output:
// # 1: Finland
// # 2: Norway
// # 3: Denmark
// # 4: Iceland
// # 5: Switzerland

Creating Your Own Iterators

Beautiful thing about Rust is that you can use generic language facilities to extend it. Let us leverage this awesome power and create our own iterator! We will build a very simple iterator that produces a series of pairs of temperatures (Fahrenheit, Celsius), represented by a tuple of floating-point numbers (f32, f32). The temperature is calculated using commonly known formula: °C = (°F - 32) / 1.8.

An iterator starts with a struct. Whatever we name the struct will also be the name of the iterator. We will call ours FahrToCelc. The struct contains fields that hold useful information that persists between subsequent iterator calls. We will have two f32 fields - the temperature in Fahrenheit, and the increment step:

struct FahrToCelc {
  fahr: f32,
  step: f32,
}

Next, we will create a convenience method new() that initializes the iterator by passing it initial values for temperature in Fahrenheit and the increment step. This method is strictly speaking not necessary and is not part of the iterator implementation, but I find it to be a nice syntactic sugar that improves overall program readability:

impl FahrToCelc {
  fn new(fahr: f32, step: f32) -> FahrToCelc {
    FahrToCelc { fahr: fahr, step: step }
  }
}

Finally, we program the behavior of the iterator by implementing the Iterator trait for our struct. The trait at a minimum needs to contain the following:

type Item = (f32, f32);

fn next (&mut self) -> Option<Self::Item>

The next() function typically also does some internal housekeeping. Ours increments Fahrenheit temperature fahr by step so that it can be returned on subsequent iteration. Making these modifications to internal fields is the reason why we need to pass a mutable reference to self as a parameter to next().

Combining things together, here is the Iterator trait implementation:

impl Iterator for FahrToCelc {
  type Item = (f32, f32);

  fn next (&mut self) -> Option<Self::Item> {
    let curr_fahr = self.fahr;
    let curr_celc = (self.fahr - 32.0) / 1.8;
    self.fahr = self.fahr + self.step;
    Some((curr_fahr, curr_celc))
  }
}

At last, the complete program:

struct FahrToCelc {
  fahr: f32,
  step: f32,
}

impl FahrToCelc {
  fn new(fahr: f32, step: f32) -> FahrToCelc {
    FahrToCelc { fahr: fahr, step: step }
  }
}

impl Iterator for FahrToCelc {
  type Item = (f32, f32);

  fn next (&mut self) -> Option<Self::Item> {
    let curr_fahr = self.fahr;
    let curr_celc = (self.fahr - 32.0) / 1.8;
    self.fahr = self.fahr + self.step;
    Some((curr_fahr, curr_celc))
  }
}

fn main() {
  // pass the starting temperature and step to the initializer function
  let ftc = FahrToCelc::new(0.0, 5.0);

  // produce the iterator table of first 5 values
  let temp_table = ftc.take(5);

  // print out the temperature table nicely
  for (f, c) in temp_table {
    println!("{:7.2} °F = {:7.2} °C", f, c);
  }
}

// output:
//  0.00 °F =  -17.78 °C
//  5.00 °F =  -15.00 °C
// 10.00 °F =  -12.22 °C
// 15.00 °F =   -9.44 °C
// 20.00 °F =   -6.67 °C