Beginner += in Ruby

Question

Looking at this block, I can follow the whole program until I hit, sum += square.

What is he point of this line, what does it say???

sum = 0

[1, 2, 3, 4].each do |value|

square = value * value

sum += square

end

puts sum

Answer 1

sum = sum + square

Answer 2

For Ex

sum=4 
sum += 5  #9

Answer 3

Big explination of 'compound operators' here.

http://stackoverflow.com/questions/2493140/meaning-of/2589389#2589389