How to use fold and reduce

Use fold to accumulate a value from an iterator into a new type, and reduce to combine all items in an iterator into a single item of the same type. fold requires an initial value, while reduce uses the first item as the starting point and returns None if the iterator is empty.

let numbers = vec![1, 2, 3, 4];

// fold: starts with 0, adds each number, returns 10
let sum = numbers.iter().fold(0, |acc, &x| acc + x);

// reduce: starts with first item, adds remaining, returns Some(10)
let product = numbers.iter().reduce(|acc, &x| acc * x);

Think of fold as a bucket where you start with a specific amount and add items one by one until you have a final total. reduce is like a line of people where the first person grabs the next person's hand, then that pair grabs the next person, until everyone is holding hands in one group. Use fold when you need a starting point (like zero) and reduce when you want to combine existing items without an external starter.