What is reduce?

reduce.rb

arr = ["cup", "cake", "birthday", "Jesus"]

# this will print "cup\ncake\nbirthday\nJesus", as you can imagine
for word in arr
  puts word
end

def add_em_up()
  acc = 5
  for word in arr
    acc = acc + word.length
  end
  acc # returns acc from add_em_up
end

def reduce_em()
  # acc starts as 5 because it's the first parameter (usually called "initial" or similar)
  # it is *always* a number
  arr.reduce 5 do |acc, word| # word is each word; this do block gets called for each one
    acc + word.length # implicitly return the addition from the do loop
  end # because the reduction is the last value in the function, its return value is implicitly returned from reduce_em
end