How to flatten nested Vecs

Use the iter().flatten().collect() method chain to convert a Vec<Vec<T>> into a single Vec<T>.

let nested: Vec<Vec<i32>> = vec![vec![1, 2], vec![3, 4], vec![5]];
let flat: Vec<i32> = nested.iter().flatten().collect();

If you own the data and want to avoid cloning, use into_iter() instead of iter() to move the inner vectors:

let nested: Vec<Vec<i32>> = vec![vec![1, 2], vec![3, 4], vec![5]];
let flat: Vec<i32> = nested.into_iter().flatten().collect();

The flatten() adapter works on any iterator of iterators, automatically extracting the inner elements into a single stream.

Flattening a nested vector is like taking a stack of boxes, opening each one, and dumping all the items into a single pile. You use this when you have a list of lists but need a simple list of all the items combined. It saves you from writing manual loops to move data from the inner lists to the outer one.