calculating the average of arrays

Hello, I want to calculate an average from the following variables/arrays a - $f[$index][f_d1_perc] with $f[$index][f_d1_weight] b - $f[$index][f_d2_perc] with $f[$index][f_d2_weight] c - $f[$index][f_d3_perc] with $f[$index][f_d3_weight] d - $f[$index][f_d4_perc] with $f[$index][f_d4_weight] e - $f[$index][f_d5_perc] with $f[$index][f_d5_weight] All varables are integers: - $f[$index][f_d _perc] = 0 - 100 - $f[$index][f_d1_weight] = 0 - 10 I want the folowing: total score = (a * weight) + (b * weight) + (c * weight) + (d * weight) + (e * weight). total vars = weight a + weight b + weight c + weight d + weight e. Average = total score / total average. The calulated average has to be stored in "$f[$index][f_perc]". The problem is that not always all the variables are set. How do I check these arrays and leave them out of the calculation when they don't contain a value. Hope anyone can help me...