How to Merge Two HashMaps in Rust

You merge two HashMaps by iterating over the second map and inserting its key-value pairs into the first, using entry().or_insert() to handle collisions.

use std::collections::HashMap;

let mut map1: HashMap<&str, i32> = HashMap::from([("a", 1), ("b", 2)]);
let map2: HashMap<&str, i32> = HashMap::from([("b", 3), ("c", 4)]);

for (k, v) in map2 {
    map1.entry(k).or_insert(v);
}

// map1 now contains: {"a": 1, "b": 2, "c": 4}

Note: This keeps the original value from map1 if keys collide. To overwrite with map2's value, use map1.insert(k, v); instead.

Merging HashMaps means combining two collections of key-value pairs into one. You take every item from the second map and add it to the first. If a key already exists in the first map, you decide whether to keep the old value or replace it with the new one.