How to measure rounding error of array elements sum

Dear mathematicians. I am newbie to numeric methods so I want to ask and get answer to the question about the summing of array elements. So I need create a big array with about thousand elements so they should be float or double type as it need to be rounded. The main reason of such task is the comparison of such array elements in original order, them also in ascending and descending order after sorting. It difficult to understand now but I was said that there should difference in the measure of rounding error. So would you guess me about where to do this task: in C compiler or in Matlab. How to create this array in random order and how to round the this numbers. After sunning or before it. In what books or internet resources to read about it?