The Entry API
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Using entry with or_insert
.entry(key).or_insert(default) inserts the default value only if the key does not already exist, and returns a mutable reference to the value either way -- ready to use or modify immediately.
Example: Using entry with or_insert
use std::collections::HashMap;
fn main() {
let mut scores = HashMap::new();
scores.entry("Alice").or_insert(0);
println!("{:?}", scores.get("Alice"));
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Counting Word Occurrences
A classic use of the entry API is counting occurrences: for each item, get its entry, default to 0 if missing, then increment through the returned mutable reference.
Example: Counting Word Occurrences
use std::collections::HashMap;
fn main() {
let words = ["a", "b", "a", "c", "b", "a"];
let mut counts = HashMap::new();
for word in words.iter() {
let count = counts.entry(*word).or_insert(0);
*count += 1;
}
println!("{:?}", counts.get("a"));
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Lazy Defaults with or_insert_with
.or_insert_with(|| default()) only runs the closure to compute a default value if the key is actually missing, which avoids unnecessary work for expensive default values.
Example: Lazy Defaults with or_insert_with
use std::collections::HashMap;
fn main() {
let mut groups: HashMap<&str, Vec<i32>> = HashMap::new();
groups.entry("evens").or_insert_with(Vec::new).push(2);
groups.entry("evens").or_insert_with(Vec::new).push(4);
println!("{:?}", groups.get("evens"));
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Modifying Existing Values in Place
Because .entry().or_insert() returns a mutable reference, you can dereference it with * to update the value directly, which is the standard pattern for accumulating totals per key.
Example: Modifying Existing Values in Place
use std::collections::HashMap;
fn main() {
let mut totals: HashMap<&str, i32> = HashMap::new();
for (name, amount) in vec![("tea", 3), ("coffee", 5), ("tea", 2)] {
*totals.entry(name).or_insert(0) += amount;
}
println!("{:?}", totals.get("tea"));
}
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- Manually checking with
.contains_key()and then calling.insert()separately, instead of the more idiomatic single.entry()call. - Forgetting
.or_insert()returns a mutable reference, which you can then modify directly, like incrementing a counter. - Using
.or_insert(expensive_computation())when the default value is costly to compute --.or_insert_with()only runs the closure when needed.
.entry(key)returns a handle representing that key's slot, whether it currently exists or not..or_insert(default)inserts a default value only if the key is missing, and always returns a mutable reference to the value..or_insert_with(|| ...)lazily computes the default only when actually needed.- The entry API is the idiomatic way to implement counting or accumulating patterns with a HashMap.
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