Rust: What is the difference between map and flat_map?

map — transform each item. flat_map — transform and flatten.

map:

• One-to-one transformation.
• Returns iterator of iterators.
• Simple transformation.

flat_map:

• One-to-many transformation.
• Flattens result.
• Complex transformation.

1let v = vec![1, 2, 3];
2
3// map: returns nested
4let nested: Vec<Vec<i32>> = v.iter().map(|x| vec![*x, *x * 2]).collect();
5// [[1,2], [2,4], [3,6]]
6
7// flat_map: returns flat
8let flat: Vec<i32> = v.iter().flat_map(|x| vec![*x, *x * 2]).collect();
9// [1, 2, 2, 4, 3, 6]

Key: map for one-to-one, flat_map for one-to-many.