Entry API
- Rust course
- Intermediate
- Lesson 57/115
- 4 min
Insert into a map safely with the Entry API.
The Entry API
For "insert if missing, update if present", entry is used. It does a single lookup and gives a mutable reference to the value:
let metin = "elma armut elma kiraz elma armut";
let mut sayim = HashMap::new();
for k in metin.split_whitespace() {
*sayim.entry(k).or_insert(0) += 1;
}or_insert(0) puts 0 if the key is missing and returns &mut V in every case; it's dereferenced with * and incremented.
Other entry methods
or_default() puts the type's default; and_modify changes only an existing value:
let mut gruplar: HashMap<char, Vec<&str>> = HashMap::new();
for k in ["elma", "erik", "ayva"] {
gruplar.entry(k.chars().next().unwrap()).or_default().push(k);
}
let mut puan = HashMap::new();
puan.entry("ali").and_modify(|p| *p += 10).or_insert(50); // 50 if missing, +10 if presentPractice Word count
Count the words in "elma armut elma kiraz elma armut" with entry(..).or_insert(0). Collect the result into a Vec, sort it and print it with {:?}.
Starter code
use std::collections::HashMap;
fn main() {
// buraya yaz
}Sample solution
use std::collections::HashMap;
fn main() {
let metin = "elma armut elma kiraz elma armut";
let mut sayim = HashMap::new();
for k in metin.split_whitespace() {
*sayim.entry(k).or_insert(0) += 1;
}
let mut v: Vec<(&str, i32)> = sayim.into_iter().collect();
v.sort();
println!("{:?}", v);
}Expected output
[("armut", 2), ("elma", 3), ("kiraz", 1)]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is the difference between <code>or_insert_with(Vec::new)</code> and <code>or_insert(Vec::new())</code>?
- or_insert errors
- or_insert_with sorts
- or_insert_with produces the value only if the key is missing Correct answer
- No difference
Explanation: The argument of or_insert is always computed; for expensive defaults prefer or_insert_with or or_default.
2 Initialize if missing
*sayim.entry(k).____(0) += 1;*sayim.entry(k).or_insert(0) += 1;Explanation: or_insert puts the value if missing and returns &mut V.
3 Method and behaviour
| Term | Meaning |
|---|---|
or_insert(v) |
Put v if missing |
or_insert_with(f) |
Call f() if missing |
or_default() |
Default if missing |
and_modify(f) |
Change if present |