Here is a sample design for a function named Average which uses an array numList of type ListType.   The C++ code follows the design.

The DESIGN:

Function: Average
PreCond: numList contains numNums integers to be averaged
PostCond: the average of the integers in numList has been returned
Data passed in:     numList     ListType     Array of integers
                                numNums  integer        Number of items in array to be averaged
Data passed out:     None
Returns: (Decimal) average of the integers in numList
Local variables:
                                ctr                integer         For loop counter
                                sum             integer         sum of numbers to be averaged

Algorithm: (pseudocode)

sum <-- 0
for ctr going from 0 to numNums-1:
        sum <-- sum + numList[ctr]
return (sum)/numNums)
 

Note:  you could have used a while loop to sum the integers.  If you did, you'd have to initialize the counter and show the step where it gets incremented each time in the loop.  For this reason, the for loop seems a lot simpler to use.
 

// The C++ code:
// (function) Average
// PreCond: numList contains numNums integers
// PostCond:  the average of the integers has been returned
//===============================================
float Average (ListType numList, int numNums)
{
        int
            sum = 0; // accumulator for sum

        // add up the numbers in the list
        for (int ctr=0; ctr < numNums; ctr++)
                sum = sum + numList[ctr];

        // return the average
        return (float(sum)/numNums);
}