Entropy

WSH.echo('Type str in console');
var str = WScript.StdIn.ReadLine();
var alf = new Array();
/*Initialization*/
for (var i = 0; i < str.length; i++){
alf[str.charAt(i)] = 0;
}
/*frq calc*/
for (var i = 0; i < str.length; i++){
alf[str.charAt(i)]++;
}

var sum = 0.0;
var entropy = 0.0;
var n = 0;
for (i in alf){
    sum += alf[i];
    n++;
}
//WSH.echo(alf.length);
for (i in alf){
    alf[i] = alf[i]/sum;
}
if (n>1)
for (i in alf){
    entropy += (alf[i])*((Math.log(alf[i]))/(Math.log(n)));
}

else
    entropy = -1;
entropy *= -1;
if (str.length == 0)
    WSH.echo("str is null");
else
WSH.echo(entropy);