Sum up fields in a matrix table

I want to auto calculate some fields from the matrix table below: ! I know the only way to do this would be by using JavaScript so I tried the below but it is not working: function setQuestionString(number, str){ var question = document.getElementById("QR~QID" + number); question.value = str; } var registration_total = "${q://QID9/ChoiceTextEntryValue/1/1}"; var reg_total = parseInt(registration_total, 10); var registration_a = "${q://QID9/ChoiceTextEntryValue/1/2}"; var a = parseInt(registration_a, 10); var registration_b = "${q://QID9/ChoiceTextEntryValue/1/3}"; var b = parseInt(registration_b, 10); var registration_c = "${q://QID9/ChoiceTextEntryValue/1/4}"; var c = parseInt(registration_c, 10); var i= 0; reg_total = a + b + c; registration_total = reg_total.toString(); while (i<12){ setQuestionString(9, "registration_total"); }