a bad way to calculate probabilities

a-bad-way-to-calculate-probabilities.markdown

a bad way to calculate probabilities

A Pen by Evan Goode on CodePen.

License.

index.pug

p Calculates the probability of the occurence of one or more events in a set (#[i P(A∪B∪C...)]).
p Enter probabilities, #[i 0 < P(X) < 1], separated by commas.
input#input
button#go Go
br
pre#result

script.js

document.querySelector("#go").addEventListener("click", function() {
	document.querySelector("#result").textContent = or(JSON.parse("[" + document.querySelector("#input").value + "]"))
})


function or (array) {
  var probability = 0
	var combos = []

	for (var i = 1; i < array.length + 1; i++) {
		var combinations = getCombinations(array, i)
		for(var j = 0; j < combinations.length; j++) {
			combos.push(combinations[j])
		}
	}
	for (var i = 0; i < combos.length; i++) {
		product = combos[i][0]
		for(var j = 1; j < combos[i].length; j++) {
			product *= combos[i][j]
		}
		if(combos[i].length % 2 == 0) {
			probability -= product
		} else {
			probability += product
		}
	}
	return probability
}

function getCombinations (array, k) {
	if (k > array.length || k <= 0) {
		return []
	}
	if (k == array.length) {
		return [array]
	}
	if (k == 1) {
		var combinations = []
		for(i = 0; i < array.length; i++) {
			combinations.push([array[i]]);
		}
		return combinations;
	}

	combinations = [];
	for (var i = 0; i < array.length - k + 1; i++) {
		var head = array.slice(i, i + 1);
		var tail = getCombinations(array.slice(i + 1), k - 1);
		for (var j = 0; j < tail.length; j++) {
			combinations.push(head.concat(tail[j]));
		}
	}
	return combinations;
}

style.scss

/*

I now realize this is dumb and you can just take 1 - P(A' ∩ B' ∩ C'). Oh well.

*/