Mysterious calculation error when multiply by 100

Question

The following script contains a very strange error. I want to check if a value is a positive integer. To do this, I multiply by 100 to enclose the value to decimal. If I test 0.07, the script does not calculated the value 7, but the value 7.00000001. I could round the value, but I would like to know why the value is calculated this way.

<script type="text/javascript">

  var isPositiveInt = function(i) {
        i = i*100;  
      return ((i % 1) == 0 && i >= 0);
  }; 

</script>

<a href="#" onclick="alert(isPositiveInt('0.07')); return false;">Try it out!</a>

0.05, 0.06 and 0.08 works fine. But what happens with 0.07? I would be happy if someone could explain that to me.

Answer 1

That's because javascript casts everything to a double internally. As a result, all calculations pick up some noise due to floating point inaccuracy: Floating point inaccuracy examples

One way to fix this issue, is to just round down to the nearest int after all intermediate calculations.