Collect. In Rust programs, iterators are often used to modify or generate data. But to have a more usable collection, like a Vec or String, we must convert the iterator.
With collect, we have a powerful built-in function that converts an iterator into a type like a Vec or String. Collect can perform many conversions.
String example. Suppose we have an iterator of char values—for example, by having a Vec of chars that we call iter() upon. We can collect() these into a String.
Here We use the TurboFish operator to collect the iterator of characters into a String.
fn main() {
let values: Vec<char> = vec!['a', 'b', 'c'];
// Convert a vector of chars into a string.
let result = values.iter().collect::<String>();
println!("{result}");
}abc
Map example. The map() function can be used to transform the elements in an iterator with a function argument. Here we uppercase all the strings in a iterator of strings (created by iter).
fn main() {
let mut values = Vec::new();
values.push("bird");
values.push("cat");
// Map a vector of strings into uppercase strings.// Then collect the result of map into a vector of strings.
let result = values.iter().map(|a| a.to_uppercase()).collect::<Vec<String>>();
println!("{:?}", result);
}["BIRD", "CAT"]
Enumerate. In Rust we often use the enumerate() function to add indexes to an iterator. Each returned element has an index and the original value in a tuple.
Tip We can invoke collect() on the result of enumerate() to transform a vector into a vector of 2-element tuples.
fn main() {
let values = vec!["abc".to_string(), "def".to_string()];
// Convert string vector into a vector of tuples of index, String pairs.
let result = values.iter().enumerate().collect::<Vec<(usize, &String)>>();
println!("{:?}", result);
}[(0, "abc"), (1, "def")]
TurboFish syntax. Rust supports the TurboFish syntax, which is a way to cast the result of a function in its calling location. Here we examine this syntax form.
Version 1 This version of the code uses the TurboFish operator on collect() to cast the result of the rev() function call.
Version 2 Here we use collect() in the same way, but avoid TurboFish and specify the type of the "result2" local variable.
fn main() {
let numbers = vec![10, 20, 30];
// Version 1: use collect with TurboFish.
let result = numbers.iter().rev().collect::<Vec<&usize>>();
println!("{:?}", result);
// Version 2: use collect with type annotation.
let result2: Vec<&usize> = numbers.iter().rev().collect();
println!("{:?}", result2);
}[30, 20, 10]
[30, 20, 10]
Collect optimization. How can we use collect() to optimize Rust programs? With collect, we can store the results of an iterator like bytes() in a vector.
Version 1 This version of the code loops over the bytes of the string directly, and sums the values of the bytes.
Version 2 This version loops over a vector that was created with collect() from the bytes of the string. It also sums the bytes.
Result It is much faster to loop over a vector of u8 values created by calling collect.
use std::time::*;
fn main() {
let source = "bird is blue";
let v = source.bytes().collect::<Vec<u8>>();
// Version 1: loop over bytes iterator.
let t0 = Instant::now();
let mut sum0 = 0;
for _ in 0..1000000000 {
for b in source.bytes() {
sum0 += b as usize;
}
}
println!("{:?}", t0.elapsed());
// Version 2: loop over bytes vector.
let t1 = Instant::now();
let mut sum1 = 0;
for _ in 0..1000000000 {
for &b in &v {
sum1 += b as usize;
}
}
println!("{:?}", t1.elapsed());
println!("{}", sum0);
println!("{}", sum1);
}4.413035666s bytes()
156.269833ms &v
1125000000000
1125000000000
Split. When calling the split() function on a string in Rust, we often want to collect() the results. But if we can avoid doing this, often the code will run faster.
Summary. Collect() is a powerful and general-purpose conversion function for iterators. It is used at the end of a chain of iterator function calls to convert the result into a more usable type.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Sep 16, 2024 (edit).
