Reduce / Inject

Sometimes you need to collapse an entire array down to a single value, a sum, a product, a longest string. reduce (also called inject: same method, two names) is the tool.

The block takes two parameters: the accumulator and the current element. The block's return value becomes the next accumulator.

total = [1, 2, 3, 4].reduce(0) { |acc, n| acc + n }
puts total   # 10

Step by step the accumulator is 0 → 1 → 3 → 6 → 10.

The first argument is the starting value of the accumulator. If you leave it off, Ruby uses the first element of the array:

[1, 2, 3, 4].reduce { |acc, n| acc * n }   # 24

For pure operations like sum and product, Ruby gives you a shortcut, pass a symbol instead of a block:

[1, 2, 3, 4].reduce(:+)   # 10
[1, 2, 3, 4].reduce(:*)   # 24

Once you can build a custom accumulator, you can compute almost anything in one pass.