Okay, so right now, I have a character, and he just simply checks against everything that is on the screen. It works fine, but I'm reading a book on collision detection and it talks about a way to handle many more objects on the screen. It suggests creating sectors and only checking against objects that are in the same sectors. The book then proceeds to give a working example of collision detection, but doesn't seem to follow it's own method of creating the sectors.
So my problem is with creating these sectors. The book says that doing this method can decrease the checks by half or more, depending on how many objects are being checked against however many objects. It gives numbers as an example of how many checks you can save. But what confuses me is the book counts putting an object into a sector as only one check. The only way I can think of to create these sectors is to create an array of rects to keep track of where each sector is, then checking each index of the array until it finds the appropriate sector that each object is in. But assuming there's 24 sectors, and 12 objects on the screen, that would mean 12 objects will check up to 24 times each, and therefore it would be more efficient to just check against the 12 objects directly. So how does a person put an object into a sector with only one check?