How to merge two HashMaps

Use the extend method to merge one HashMap into another, overwriting existing keys with values from the source map.

use std::collections::HashMap;

let mut map1 = HashMap::new();
map1.insert("a", 1);

let mut map2 = HashMap::new();
map2.insert("b", 2);
map2.insert("a", 10); // Overwrites map1's "a"

map1.extend(map2);
// map1 now contains: {"a": 10, "b": 2}

Note: extend consumes map2, so it cannot be used afterward.

The extend method copies all key-value pairs from one HashMap into another. If a key exists in both maps, the value from the second map replaces the first. Think of it like pouring one bucket of items into another, where duplicate items get swapped out for the new ones.