by Richard Russell, July 2015

The SUM() function returns the sum of all the elements in a numeric array, and it is much faster than summing the elements yourself in a loop. However you have no control over how many elements are summed: it is always the entire array. To find the sum of the first n elements of an array you can use a user-defined function as follows:

        DIM array(999)
        array() = 1
        PRINT FNsum(array(), 500)

This program prints the value 500, being the sum of the first 500 elements of the array (0 to 499 inclusive). Here is the FNsum() function itself:

        DEF FNsum(a(), N%)
        IF DIM(a())<>1 ERROR 0, "Array must be one-dimensional"
        LOCAL !(!^a()+1)
        !(!^a()+1) = N%
        = SUM(a())