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HashSet implements the set data structure in Rust. Just like a set, it allows us to store values without duplicates.
Hashset is part of the Rust standard collections library, so we must include the HashSet module in our program.
use std::collections::HashSet;We have imported the HashSet module using the use declaration. It should be at the top of the program.
Now, we can create a hashset using the new() method of the HashSet module. For example,
let mut color: HashSet<String> = HashSet::new();Here,
let mut color- declares a mutable variablecolorHashSet<String>- type of the hashset where the values are of typeStringHashSet::new()- creates a new hashset
// import HashSet from Rust standard collections library
use std::collections::HashSet;
fn main() {
// create a new HashSet
let mut color: HashSet<String> = HashSet::new();
println!("HashSet = {:?}", color);
}HashSet = {}Here, we create an empty HashSet and print it to the screen.
We use :? in the println! macro to print a hashset.
The HashSet module provides various methods to perform basic operations in a hashset.
-
Add Values
-
Check Values
-
Remove Values
-
Iterate over Values
We can use the insert() method to add an element to the hashset. For example,
let mut colors: HashSet<&str> = HashSet::new();
// insert elements to hashset
colors.insert("Red");
colors.insert("Yellow");Here, we insert two values in the HashSet bound to the variable colors.
Adding a new value to the hashset is only possible because of the mut variable declaration.
use std::collections::HashSet;
fn main() {
let mut colors: HashSet<&str> = HashSet::new();
// insert values in a HashSet
colors.insert("Red");
colors.insert("Yellow");
colors.insert("Green");
println!("colors = {:?}", colors);
}colors = {"Yellow", "Red", "Green"}Here, the output has the elements in a different order. It's because a hashset doesn't preserve the insertion order of values.
We use the contains() method to check if a value is present in a hashset. The method returns true if the specified element is present in the hashset, otherwise returns false.
Let's see an example,
use std::collections::HashSet;
fn main() {
let mut colors: HashSet<&str> = HashSet::new();
colors.insert("Red");
colors.insert("Yellow");
println!("colors = {:?}", colors);
// check for a value in a HashSet
if colors.contains("Red") {
println!("We have the color \"Red\" in the HashSet.")
}
}colors = {"Red", "Yellow"}
We have the color "Red" in the HashSet.In the above example, we have used the colors.contains("Red") as a condition to the if statement.
Here, the element Red is present inside the hashset, so the condition is true. Hence, we get the desired output.
We can use the remove() method to remove the specified element from the hashset. For example,
use std::collections::HashSet;
fn main() {
let mut colors: HashSet<&str> = HashSet::new();
colors.insert("Red");
colors.insert("Yellow");
colors.insert("Green");
println!("colors before remove operation = {:?}", colors);
// remove value from a HashSet
colors.remove("Yellow");
println!("colors after remove operation = {:?}", colors);
}colors before remove operation = {"Yellow", "Red", "Green"}
colors after remove operation = {"Red", "Green"}In the above example, we have used
colors.remove("Yellow");to remove the element Yellow from the hashset.
We can use the Rust for Loop to iterate over values of a hashset. For example,
use std::collections::HashSet;
fn main() {
let mut colors: HashSet<&str> = HashSet::new();
colors.insert("Red");
colors.insert("Yellow");
colors.insert("Green");
// iterate over a hashset
for color in colors {
// print each value in the hashset
println!("{}", color);
}
}Yellow
Green
RedHere, we iterate over the hashset named colors and print each element.
Order of colors in Output varies with each run.
We can also create a hashset with default values using the from() method when creating it. For example,
use std::collections::HashSet;
fn main() {
// Create HashSet with default set of values using from() method
let numbers = HashSet::from([2, 7, 8, 10]);
println!("numbers = {:?}", numbers);
}numbers = {8, 7, 10, 2}Here, we create a hashset using the HashSet::from() method with default values and print it to the screen.
Order of values in Output varies with each run.
Besides the basic methods, here are some more commonly used HashSet methods.
| Method | Description |
|---|---|
len() |
returns the length of a hashset |
is_empty() |
checks if the hashset is empty |
clear() |
removes all elements from the hashset |
drain() |
returns all the elements as an iterator and clears the hashset |
The HashSet module also provides various methods used to perform different set operations.
We can use the union() method to find the union of two sets. For example,
use std::collections::HashSet;
fn main() {
let hashset1 = HashSet::from([2, 7, 8]);
let hashset2 = HashSet::from([1, 2, 7]);
// Union of hashsets
let result: HashSet<_> = hashset1.union(&hashset2).collect();
println!("hashset1 = {:?}", hashset1);
println!("hashset2 = {:?}", hashset2);
println!("union = {:?}", result);
}hashset1 = {7, 8, 2}
hashset2 = {1, 2, 7}
union = {8, 2, 1, 7}Here, we have used the union() method to find the union between two sets: hashset1 and hashset2.
hashset1.union(&hashset2).collect();The union() method returns an iterator, so we have used the collect() method to get the actual result.
We have passed &hashset2 as an argument to the union() method because it takes a reference as an argument.
We can use the intersection() method to find the intersection between two sets. For example,
use std::collections::HashSet;
fn main() {
let hashset1 = HashSet::from([2, 7, 8]);
let hashset2 = HashSet::from([1, 2, 7]);
// Intersection of hashsets
let result: HashSet<_> = hashset1.intersection(&hashset2).collect();
println!("hashset1 = {:?}", hashset1);
println!("hashset2 = {:?}", hashset2);
println!("intersection = {:?}", result);
}hashset1 = {7, 8, 2}
hashset2 = {2, 1, 7}
intersection = {7, 2}Order of values in Output vary but the actual values themselves are the same.
Here, we have used the union() method to find the union between two sets: hashset1 and hashset2.
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hashset1.union(&hashset2).collect();The union() method returns an iterator, so we have used the collect() method to get the actual result.
Note: We have passed &hashset2 as an argument to the union() method because it takes a reference as an argument.
35-Tutorial-Hashset-continued-onwards.md