Question

Hi
I have a javascript array object rapresenting the amount of items sold in a given country, like this:

var data = [{'c1':'USA', 'c2':'Item1', 'c3':100}, 
            {'c1':'Canada', 'c2':'Item1', 'c3':120},
            {'c1':'Italy', 'c2':'Item2', 'c3':140},
            {'c1':'Italy', 'c2':'Item2', 'c3':110}]

I need to avoid duplicates (as you may see, the last two ‘records’ have the same Country and the same Item) and sum the amounts; if I was getting data from a database I would use the DISTINCT SUM clause, but what about it in this scenario? Is there any good jquery trick?

Answer 1

You could use an object as a map of distinct values, like this:

var distincts, index, sum, entry, key;
distincts = {};
sum = 0;
for (index = 0; index < data.length; ++index) {
    entry = data[index];
    key = entry.c1 + "--sep--" + entry.c2;
    if (!distincts[key]) {
        distincts[key] = true;
        sum += entry.c3;
    }
}

How that works: JavaScript objects are maps, and since access to properties is an extremely common operation, a decent JavaScript implementation tries to make property access quite fast (by using hashing on property keys, that sort of thing). You can access object properties using a string for their name, by using brackets ([]), so obj.foo and obj["foo"] both refer to the foo property of obj.

And so:

1. We start with an object with no properties.
2. As we loop through the array, we create unique key from c1 and c2. It’s important that the “–sep–” string be something that cannot appear in c1 or c2. If case isn’t significant, you might throw a .toLowerCase in there.
3. If distincts already has a value for that key, we know we’ve seen it before and we can ignore it; otherwise, we add a value (true in this case, but it can be just about anything other than false, undefined, 0, or "") as a flag indicating we’ve seen this unique combination before. And we add c3 to the sum.

But as someone pointed out, your last two entries aren’t actually the same; I’m guessing that was just a typo in the question…