fold and reduce

Learn aggregation with fold and reduce.

fold and reduce

fold and reduce reduce a collection to a single value. They are used for sums, products, max and similar computations. Both accumulate elements one by one.

reduce takes no initial value; it uses the first element as the start. fold takes an initial value and is safe on empty collections.

Example: var sum = [1, 2, 3, 4].fold(0, (acc, n) => acc + n);. Here 0 is the initial, acc the accumulator, n the current element. Result: 10.

Product with reduce

reduce takes no initial value. The first element is treated as the start and accumulation begins from the second. On an empty collection reduce throws.

Example: var product = [1, 2, 3, 4].reduce((a, b) => a * b);. Here a is the accumulator, b the current. Steps: 1*2=2, 2*3=6, 6*4=24. Result: 24.

fold is safer because you give an initial value. reduce is shorter to write and preferred for non-empty collections.

Exercise Compute sum and product

On the list [1, 2, 3, 4] compute the sum with fold(0, (acc, n) => acc + n) and print it. On the same list compute the product with reduce((a, b) => a * b) and print it.

Show example solution

void main() {
  var numbers = [1, 2, 3, 4];
  var sum = numbers.fold(0, (acc, n) => acc + n);
  var product = numbers.reduce((a, b) => a * b);
  print(sum);
  print(product);
}

Test yourself

Match the fold parts

0

Initial value
acc

Accumulator
n

Current element
acc + n

Accumulation step
What is the difference between fold and reduce?
- fold takes an initial value, reduce does not
- reduce is faster
- fold works only on lists
- They are exactly the same
Show answer

fold takes an initial value, reduce does not — fold takes an initial value, reduce uses the first element.

The fold call

var numbers = [1, 2, 3, 4];
var sum = numbers.____(0, (acc, n) => acc + n);

Show answer

var numbers = [1, 2, 3, 4];
var sum = numbers.fold(0, (acc, n) => acc + n);

Next lesson: forEach

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