How to normalize a list of positive numbers in JavaScript?

Question

I have an array filled with positive int values, how could I normalize this list so the max value is always 100? Thank you in advance!

Answer 1

The idea is to first find the highest number in your array (using apply on Math.max), then find the ratio between that highest number and 100.

After that, it's just a matter of looping through your array and dividing all your numbers by that ratio:

var numbers = [3, 8, 45, 74, 123],
    ratio = Math.max.apply(Math, numbers) / 100,
    l = numbers.length,
    i;

for (i = 0; i < l; i++) {
    numbers[i] = Math.round(numbers[i] / ratio);
}

Here's the fiddle: http://jsfiddle.net/XpRR8/

---

Note: I'm using Math.round to round the numbers to the nearest integer. If you instead prefer to keep them as floats, just remove the function call:

for ( i = 0; i < l; i++ ) {
    numbers[i] /= ratio;
}

Here's the fiddle: http://jsfiddle.net/XpRR8/1/

---

If you don't have to support IE8 and below, you can use Array.prototype.map():

var numbers = [3, 8, 45, 74, 123],
    ratio = Math.max.apply(Math, numbers) / 100;

numbers = numbers.map(function (v) {
    return Math.round(v / ratio);
});

Here's the fiddle: http://jsfiddle.net/XpRR8/2/

---

If you do support IE8, but are anyhow using jQuery, you can use $.map() instead:

numbers = $.map(numbers, function (v) {
    return Math.round(v / ratio);
});

Here's the fiddle: http://jsfiddle.net/XpRR8/3/

---

Update: As pointed out by @wvxvw in the comments below, if you're concerned about fringe implementations that impose an artificial limit on the amount of arguments apply will handle, then use a loop instead of Math.max.apply. Here's an example (assuming neither Array.prototype.map nor $.map are available):

var numbers = [3, 8, 45, 74, 123],
    ratio = 0,
    i = numbers.length;

while (i--) numbers[i] > ratio && (ratio = numbers[i]);

ratio /= 100;
i = numbers.length;

while (i--) numbers[i] = Math.round(numbers[i] / ratio);

Here's the fiddle: http://jsfiddle.net/XpRR8/4/

---

If you're using ES6, this becomes laughably simple:

var numbers = [3, 8, 45, 74, 123];
var ratio = Math.max(...numbers) / 100;

numbers = numbers.map(v => Math.round(v / ratio));