Blocks

  sum = 0 
  arr.each do |x|
    if yield(x)
      sum += x 
    end 
  end 
  sum 
end 

# now we can sum_if with various blocks, ex: 

# sum if even: 

a = sum_if([1,2,3,4,5]) do |i|
  i % 2 == 0 
end 
puts a 

# sum if > 3 
b = sum_if([1,2,3,4,5]) do |i| 
  i > 3 
end 
puts b