title: What is the difference between cumulative summation and simple summation?
body: <p>Let's say I have a 1-D array which I would integrate it along the z direction. The easiest way, is to replace the integral by a summation times the grid spacing in the z direction, in this case I just get a scalar. But if instead of simple summation I use the cumulative summation I get an array. 
  My question what's the difference? and why do I get a array? </p>