Christian Heilmann

Simple things: Storing a list of booleans as a single number

Wednesday, February 25th, 2015 at 1:10 pm

This blog started as a scratch pad of simple solutions to problems I encountered. So why not go back to basics?

Yesterday I was asked by someone if there is a possibility to store the state of a collection of checkboxes in a single value. The simplest way I could think of doing this is by using binary conversion.

Tron binary

You can see the result of my approach in this JSBin:

Storing a list of booleans as a single number

What’s going on here? The state of a checkbox is a Boolean, meaning it is checked or unchecked. This could be true or false, or, as JavaScript is a lenient language, 1 or 0. That’s why we can loop over all the checkboxes and assemble a string of their state that compromises of zeros and ones:

var inputs = document.querySelectorAll('input');
var all = inputs.length;
for (var i = 0; i < all; i++){
  state += inputs[i].checked ? '1' : '0';
}

This results in a string like 1001010101. This could be our value to store, but looks pretty silly and with 50 or more checkboxes becomes very long to store, for example, as a URL parameter string.

That’s why we can use parseInt() to convert this binary number into a decimal one. That’s what the second parameter in parseInt() is for – not only to please Douglas Crockford and JSLint (as it is preset to decimal – 10 – people keep omitting it). The counterpart of parseInt() in this case is toString() and that one also takes an optional parameter that is the radix of the number system you convert from. That way you can convert this state back and forth:

x = parseInt('1001010101',2);
// x -> 579
x.toString(2);
// "1001010101"

Once converted, you turn it back into a string and loop over the values to set the checked state of the checkboxes accordingly.

A small niggle: leading zeroes don’t work

One little problem here is that if the state results in a string with leading zeroes, you get a wrong result back as toString() doesn’t create them (there is no knowing how long the string needs to be, all it does is convert the number).

x = parseInt('00001010101',2);
x.toString(2);
"1010101"

You can avoid this is in two ways: either pad the string by always starting it with a 1 or by reversing the string and looping over the checkboxes in reverse. In the earlier example I did the padding part, in this JSBin you can see the reversing trick:

Storing a list of booleans as a single number (reverse)r

Personally, I like the reversing method better, it just feels cleaner. It does rely a lot on falsy/truthy though as the size of the resulting arrays differs.

Limitation

In any case, this only works when the amount of checkboxes doesn’t change in between the storing and re-storing, but that’s another issue.

As pointed out by Matthias Reuter on Twitter this is also limited to 52 checkboxes, so if you need more, this is not the solution.

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