Check out example codes for "accumulate in cpp". It will help you in understanding the concepts better.

Code Example 1

accumulate(first, last, sum);
first, last : first and last elements of range 
              whose elements are to be added
sum :  initial value of the sum

Code Example 2

accumulate(start, end, initial_sum);

Code Example 3

accumulate(first, last, sum, myfun); 
myfun : a function for performing any 
        specific task. For example, we can
        find product of elements between
        first and last.

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