iOS - How To Choose Objects in Array More-So Then Others

Question

I have an array that contains a variety of strings. I want to choose a the objects at indexes 3,7 & 9 more-so then all the other objects..how can I achieve this. (Let's say every time I grab an object within the array, I want there to be a 80% chance that object at index 3,7 or 9 is selected)

Here's my array:

 NSMutableArray* animalImagesArray = [[NSMutableArray alloc] initWithObjects:@"bull.png",@"bunny.png",@"chicken.png",@"cow.png",@"crocodile.png",@"dog.png",@"donkey.png",@"duck.png",@"elephant.png",@"giraffe.png",@"goat.png",@"hippo.png",@"horse.png",@"lion.png",@"monkey.png",@"parrot.png",@"pig.png",@"rooster.png",@"sheep.png",@"snake.png",@"tiger.png",@"warthog.png",@"zebra.png", nil];

Any help would be greatly appreciated. Thanks in advance!

@LyndseyScott This won't work for me because I use the index of the specific object to generate win/loss conditions — Mike Simz, Dec 09 '14 at 16:03

Fogmeister · Accepted Answer · 2014-12-09T16:35:53.590

I like Lyndsey Scott's answer so thought I'd implement it so that you can still get the original index of the word.

If you have an array of 10 items and you want 3 of them (combined equally) to have an 80% chance of being chosen then that means the other 7 have a chance of 20%.

So...

I worked it out like this...

We know 7 is 20%. So 100% is 35. 80% of 35 is 28.

Ideally you should be doing this calculation in the app not on paper :) Maybe store the individual weights of each item in an array or something. That way you can create the weighted array dynamically for any number of items and different weights etc...

You'll need 28 of the other 3 (in total) so 9 of two of them and 10 of the other. (Let's make it 9 each for ease).

So you can do like this...

NSMutableArray *weightedArray = [NSMutableArray array];

for (int i=0 ; i<originalArray.count ; ++i) {
    if (i == 3 || i == 7 || i == 9) {
        for (int count=0 ; count<9 ; ++count) {
            [weightedArray addObject:array[i]];
        }
    } else {
        [weightedArray addObject:array[i]];
    }
}

// Now you can select from the weighted array.

id randomObject = weightedArray[arc4random_uniform(weightedArray.count)];

//Index of object

NSUInteger origIndex = [originalArray indexOfObject:randomObject];

It might be an idea to store the individual weights or something and then calculate the number you need to put at run time.

As Sandy Chapman pointed out in the comments you could also created a weighted array of indices for the original array so you would have...

[0, 1, 2, 3, 3, 3, ..., 4, 5, 6, 7, 7, 7..., 8, 9, 9, 9, 9, ..., 10]

That way you can get the index without having to get the original item.

Either way will work.

In your weighted array you could use an int instead of the object. So weighted array would look like: `[0, 1, 2, 3, 3, ..., 3, 4, 5, 6, 7, 7, ..., 7, 8, 9, 9, ..., 9, 10, ...]`. Then you get the index first, which may be more useful if you need to do some conditional code before getting the object from the original array. — Sandy Chapman, Dec 09 '14 at 16:25
@SandyChapman yeah, that's an alternative way. I'll add to the answer. — Fogmeister, Dec 09 '14 at 16:28
Yeah, people are being way too liberal with the down votes nowadays... Good answer. And I like the array of indices idea. But I'd also recommend having the program do all the math (instead of "We know 7 is 20%. So 100% is 35. 80% of 35 is 28.") so you can easily vary the inputs. — Lyndsey Scott, Dec 09 '14 at 16:33
@LyndseyScott yes definitely. By storing the weights in the app you can calculate these dynamically so you can change the weights easily. Will add that in thanks. — Fogmeister, Dec 09 '14 at 16:34
@Fogmeister Thanks for the help..and thank you to everyone else for their input — Mike Simz, Dec 09 '14 at 16:40

score 0 · Answer 2 · answered Dec 09 '14 at 16:07

How about a preliminary random variable from 1–5. If a number in [1,4] is chosen then choose an index at random from the set {3,7,9}. Otherwise choose an object at random from the rest of the array.

(Also, since every object in the array has the suffix .png, you might as well remove that and just concatenate it later using stringWithFormat)

iOS - How To Choose Objects in Array More-So Then Others

2 Answers2

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