What's the formula for entropy?

JavaScript
1

I counted the frequency of each character, but what do I do now?
As I understand the formula chould be
(char_count/length)*log(2, (char_count/length))
for each character in the object, anyone please clarify what the formula is? i tried to look it up but rather than the whole topic of information theory or code that i still don’t really understand I didn’t find much
Your code so far


function entropy(str) {
var chars = {}, numChars = 0, len = str.length, 
res = 0, tmp;
for(let i of str) {
    if(chars[i]) chars[i]++;
    else {chars[i] = 1; numChars++;}
}
for(let i in chars) {
    tmp = chars[i]/len;
    res += tmp;
    //Math.log(2,tmp), what do you do with this?
}
return res;
}
console.log(entropy("0123"))

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2

each term of the summation is

so you need you calculate that before adding to your total

also note the minus sign in front of the summation

1 Like
3

yeah, i did that in my code

still doesn’t work

function entropy(str) {
    var chars = {}, len = str.length, 
    res = 0, tmp;
    for(let i of str) {
        if(chars[i]) chars[i]++;
        else chars[i] = 1;
    }
    for(let i in chars) {
        tmp = chars[i]/len;
        res -= tmp*Math.log(2,tmp);
    }
    return res;
}
console.log(entropy("0123456789abcdef"));
4

I think you’re looking for this: Math.log2

1 Like
5

The Math.log() function returns the natural logarithm (base e ) of a number

that is the wrong method if you want base 2

1 Like
6

I thought it takes to arguments, one as the base and other as “destination” (you know what i mean) lol, ok, thanks than

7

I thought it takes to arguments, one as the base and other as “destination” (you know what i mean) lol, so i can use log2 or that rule of log a (b) = log m (b)/log m(a), but log2 is better cuz it’s less actions and your’e dealing with more precise numbers

8

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