Question

My first day of Javascript and I am incredibly confused. I am passing numbers returned from a form to a function, but the result is not consistent with what it should be. My testing leaves a lot to be desired, but hopefully the following makes sense.

The function g calculates the sum of the sequence.

 <form name="gaussform">
    <input name="min" 
           type="number"
           min="1"
           value="1">
    <input name="max" 
           type="number"
           min="2"
           value="10">
    <input name="step" 
           type="number"
           min="1"
           value="1">
    <input onclick="alert_g()"
           type="submit"
           value="calculate">
  </form>

  <script type="text/javascript">

    function g(min,max,step) {
    var actualmax = max - ((max - min) % step)
    return (min + actualmax) * ((1 + ((actualmax - min) / step)) / 2)
    }

    function alert_g() {
    var frm = document.forms["gaussform"]
    var min = frm["min"].value
    var max = frm["max"].value
    var step = frm["step"].value
    if (min == 1) {
    alert("min is 1")}
    if (max == 10) {
    alert("max is 10")}
    if (step == 1) {
    alert("step is 1")}
    alert(g(min,max,step))
    // below returns the desired result
    alert(g(1,10,1))}
  </script>

the if statements are only so I can understand what is going on!

So if the user enter 1,10,1 (the default values) the result should be 55.

alert(g(1,10,1)) -> 55

alert(g(min,max,step) -> 550

alert(g(1,100,2)) -> 2500

alert(g(min,max,step) -> 4975 (obviously min,max,step == 1,100,2)

the function g is correct, but I do not understand what is happening to the values that are being passed to it.

Answer 1

This expression…

min + actualmax

…is the problem.

You probably want to make that…

+min + +actualmax

…or use parseFloat(), parseInt() or whatever suits your requirements to turn those strings into an actual Number.

jsFiddle.

JavaScript’s + operator is overloaded for arithmetic addition and string concatenation. Because user input is always a string, you’re doing string concatenation.