Most efficient way to write Combination and Permutation calculator in Javascript

Question

I have a math website http://finitehelp.com that teaches students Finite Math. I thought it would be cool to include a calculator so I made one for combinations and permutations in Javascript. Live calculator is at http://finitehelp.com/finite-calculator.html. I know next to nothing about Javascript and would venture to guess there is a much more efficient way to write the following particularly because of the excessive use of variables. If someone could please help me I would be very grateful.

<script type="text/javascript">
// calculate n!
Math.factorial = function(n)
{
    if(typeof n == 'string') n = Number(n);
    if(typeof n != 'number' || isNaN(n))
    {
        alert("Factorial requires a numeric argument.");
        return null;
    }
    if (n < 2) return 1;
    return (n * Math.factorial(n-1));
}
Math.divide = function(a,b)
{
    return a/b;
}
</script>

<form class="form" name="combination" action="">
    <p>C(<input type="text" value="n" name="T1" size="1">,<input type="text" value="r" name="T2" size="1">)
    <input type="button" value="Calculate"
     onclick="var n = T1.value; var r = T2.value; var n_minus_r = parseFloat(n) - parseFloat(r); var numerator = Math.factorial(T1.value); var n_minus_r_fact = Math.factorial(n_minus_r); var r_fact = Math.factorial(r); var denominator = n_minus_r_fact * r_fact; T3.value = Math.divide(numerator,denominator); return true;">
    = <input type="text" name="T3" size="12" readonly></p>
</form>

Answer 1

Well, here we go!

First of all, why would you ever need to write this?

Math.divide = function(a,b)
{
    return a/b;
}

I would do away with it completely.

You can also clean up your Math.factorial a little bit:

Math.factorial = function(n)
{
    n = Number(n);

    if (isNAN(n)) {
        alert("Factorial requires a numeric argument.");
        return null;
    } else if (n < 2) {
        return 1;
    } else {
        return (n * Math.factorial(n - 1));
    }
}

But the main problem is your onclick() code:

onclick="var n = T1.value; var r = T2.value; var n_minus_r = parseFloat(n) - parseFloat(r); var numerator = Math.factorial(T1.value); var n_minus_r_fact = Math.factorial(n_minus_r); var r_fact = Math.factorial(r); var denominator = n_minus_r_fact * r_fact; T3.value = Math.divide(numerator,denominator); return true;

This is way too complicated. I'd make it a function and bind it to the element, which would get rid of all of the crap in your HTML and make it a bit easier to work with:

window.onload = function()
{
    document.getElementById('calculate').onclick = function() {
        var n = T1.value,
            r = T2.value;

        T3.value = Math.factorial(n) / (Math.factorial(r) * Math.factorial(n - r));
    }
}

And just get rid of the onclick= code.

Answer 2

If you're concerned about efficiency, you'd probably want to re-implement the factorial as an iterative function rather than a recursive one. The recursive version will use a lot more memory and CPU time than the iterative version.

function factorial(n) { 
  var x=1; 
  var f=1;
  while (x<=n) {
    f*=x; x++;
  }
    return f;
}

You also shouldn't be adding your own functions to the Math namespace. It's not a good habit to get into.

Answer 3

As we know, combinations is short for(https://en.wikipedia.org/wiki/Combination):

So the fastest combinations implement is below(only if n is small):

// r*(r-1)*...*2*1
function factorial(r) {
  let s = 1;
  while (r > 1) s *= r--;
  return s;
}

function combinations(n,r){
    let s = 1;
    let i = r;
    // n*(n-1)*....*(n-r+1)
    while(i<n) s*=++i;
    return s/factorial(n-r)
}
console.log(combinations(100,2) === 4950) // false

Note: math float operation has Precision limit, for example combinations(100,2)==4950 is false. But https://www.wolframalpha.com/input?i=100+choose+2 tells us it's true result is 4950.

So we should use BigInt to replace float operation:

// r*(r-1)*...*2*1
function factorial(r) {
    let s = BigInt(1);
    var i = BigInt(r)
    while (i > 1) s *= i--;
    return s;
}

// n*(n-1)*....*(n-r+1) / factorial(r)
function combinations(n, r){
    let s = BigInt(1);
    let i = BigInt(r);
    while(i<n) s*=++i;
    return s/factorial(n-r)
}
console.log(combinations(100,2) === 4950n) // true

Answer 4

Math.factorial= function(n){
    var i= n;
    while(--i) n*= i;
    return n;
}

Math.combinations= function(n, r, repeats){
    if(n< r) return 0;
    if(n=== r) return 1;
    if(repeats){
        return Math.factorial(n+r-1)/((Math.factorial(r)*Math.factorial(n-1)));
    }
    return Math.factorial(n)/((Math.factorial(r)*Math.factorial(n-r)));
}


var a= [
    'aqua', 'black', 'blue', 'fuchsia', 'gray', 'green', 'lime', 'maroon',
    'navy', 'olive', 'orange', 'purple', 'red', 'silver', 'teal', 'white',
    'yellow'
]
//how many 3 color combinations are there?
//[red,green,blue] is different than [green,red,blue]
// Math.combinations(a.length,3,true) >>969
// how many unique combinations (ignoring order) are there?
// Math.combinations(a.length,3)>>680