Using a Math Formula in AS3

Question

Hi,
I am trying to test using a math formula (probit function) in AS3. I can't get the script to work: the dynamic text box is displaying NaN after I run the function and use a hard-coded number for the variable. Here's the button's script that is getting a NaN display. I must be doing something wrong and would appreciate any help getting it working.
stop()
var p:Number = 39 //hard-code this number for testing
function NORMSINV(p:Number):Number {
    // Coefficients in rational approximations
    var a:Array = new Array(-3.969683028665376e+01,  2.209460984245205e+02,
                          -2.759285104469687e+02,  1.383577518672690e+02,
                          -3.066479806614716e+01,  2.506628277459239e+00);
    var b:Array = new Array(-5.447609879822406e+01,  1.615858368580409e+02,
                          -1.556989798598866e+02,  6.680131188771972e+01,
                          -1.328068155288572e+01 );
    var c:Array = new Array(-7.784894002430293e-03, -3.223964580411365e-01,
                          -2.400758277161838e+00, -2.549732539343734e+00,
                          4.374664141464968e+00,  2.938163982698783e+00);
    var d:Array = new Array (7.784695709041462e-03, 3.224671290700398e-01,
                           2.445134137142996e+00,  3.754408661907416e+00);
    // Define break-points.
    var plow:Number=0.02425;
    var phigh:Number=1-plow;
    // Rational approximation for lower region:
    if (p<plow) {
        var q:Number=Math.sqrt(-2*Math.log(p));
        return (((((c[0]*q+c[1])*q+c[2])*q+c[3])*q+c[4])*q+c[5]) /
                                                     ((((d[0]*q+d[1])*q+d[2])*q+d[3])*q+1);
    // Rational approximation for upper region:
    if (phigh<p) {
        q=Math.sqrt(-2*Math.log(1-p));
        return -(((((c[0]*q+c[1])*q+c[2])*q+c[3])*q+c[4])*q+c[5]) /
                                                            ((((d[0]*q+d[1])*q+d[2])*q+d[3])*q+1);
    // Rational approximation for central region:
    q=p-0.5;
    var r:Number=q*q;
    return (((((a[0]*r+a[1])*r+a[2])*r+a[3])*r+a[4])*r+a[5])*q /
                                 (((((b[0]*r+b[1])*r+b[2])*r+b[3])*r+b[4])*r+1);
var n1:Number=NORMSINV(p)
mathBtn.addEventListener(MouseEvent.CLICK, mathButtonClick, false, 0, true);
function mathButtonClick(e:MouseEvent):void {
    T1.text = n1.toString()    //click the button and NaN displays
Regards,
saratogacoach

Answer 1

I don't know the formula that you're trying to achieve, but the log and sqrt must use positive values.
If you're uncertain if a variable (or formula result) will give you a negative result and you need a positive one, you can use the Math.abs, which return s always a positive value.
I've just altered like this in the first IF:
var q:Number=Math.sqrt(Math.abs(-2*Math.log(p)));
and in the second:
q=Math.sqrt(Math.abs(-2*Math.log(Math.abs(1-p))));
In this case the second formula has two Math.abs since I don't know the result of "1-p" (in this case is negative) and then you multiply by "-2" wich will make the number negative again, so it has to be positive for the square root. If you have the change consider changing the formula to see you can eliminate, so many Math.abs, make it simple.
Otherwise is just a question of predicting results and see if the formula fits right.