The inject and reduce Method

The doc for those two methods is a bit unclear, therfore an example is better than thousands words.

Let’s start with an example without argument to the #reduce method.

list = (1..5).to_a

result = list.reduce do |sum, n|
  sum + n
end

puts result # 15

In this case, #reduce will pass the first element of the array (1) to the first argument (sum) and the second element (2) to the second argument (n).

The block execution then kicks in and return 1 + 2 = 3.

In the next iteration, that 3 will become the value of sum and the next element of the array (here, 3) will become the value of n.

The block execution then kicks in and return 3 + 3 = 6.

And so on until the end of the array.

Example with one numerical argument

list = (1..5).to_a

result = list.reduce(6) do |sum, n|
  sum + n
end

puts result # 21

The only difference here is the first element passed to sum is not the first element of the array list, but the argument 6 instead. The first element of the array (1) is passed to n.

In this scenario, when the block execution kicks in for the first time, it returns 6 + 1 = 7.

Example with one symbol argument

list = (1..5).to_a

result = list.reduce(:*)

puts result # 120

In this case, #reduce will perform a multiplication istead of an addition.

The result will be 1 * 2 * 3 * 4 * 5 = 120.

It is possible to add a first numerical argument as well.