combinations of a set with given number elements

Question

i have tried to do that but could not figure it out ,

lets say i have a set : {1,2,3,4,5}

and i want to have the combinations of 2 elements like :

{1,2}
{1,3}
{1,4}
{1,5}
{2,3}
{2,4}
{2,5}
{3,4}
{3,5}
{4,5}

how can i implement that in objective-c?

i have checked the algorithms but i could not figure out how can i do that.. here's the main url that i have checked previously : Algorithm to return all combinations of k elements from n

if anyone could help me with that i will be really happy..

regards.

yeah but only for 2 elemented combination. i need a generic code that will work for "n" elemented combinations.. — iremk, Sep 02 '12 at 19:12
I believe the answers from your link work for the general case, though I guess i'm not dead certain. — argentage, Sep 02 '12 at 23:52

score 4 · Accepted Answer · answered Sep 02 '12 at 18:49

Just a nested loop to walk over the array's elements and writing the combinations to a result array should work (this code is tested and works):

NSArray *set = [[NSArray alloc] initWithObjects:
                [NSNumber numberWithInteger:1],
                [NSNumber numberWithInteger:2],
                [NSNumber numberWithInteger:3],
                [NSNumber numberWithInteger:4],
                [NSNumber numberWithInteger:5], nil];

NSMutableArray *combinations = [[NSMutableArray alloc] init];

for (NSInteger i=0; i<[set count]; i++) {
    for(NSInteger j=i+1; j<[set count]; j++){
        NSArray *newCombination = [[NSArray alloc] initWithObjects:
                                   [set objectAtIndex:i],
                                   [set objectAtIndex:j],
                                   nil];
        [combinations addObject:newCombination];
        NSLog(@"added combination %@", newCombination);
    }
}

At the end of this nested loop, NSMutableArray combinations contains all your combinations.

thank you so much , i have an array of items but i will use the i'th and the j'th elements so i will have the required combinations.. — iremk, Sep 02 '12 at 18:51
but i have a problem right now , if i want to get 3 element possibilities than i need to use them in a while loop? here starts my problem again :) how can i implement this? — iremk, Sep 02 '12 at 18:58

score 1 · Answer 2 · edited Feb 15 '13 at 16:23

1

You can accomplish this using 2 for-loops. In the first loop, you iterate through the elements as i and in the second loop from the value j=i+1 to the end of the count of the number of elements in the set.

It could be something like this:

 for (i = 0; i < length_set; i++)
{
    for (j = i + 1;length_set; j++)
    {
        print ("%d%d\n", set[i], set[j]);
    }
}

}

NOTE: Its just a pseudocode and i have not checked for syntax, It is just to show the logic.

edited Feb 15 '13 at 16:23

uchuugaka

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answered Sep 02 '12 at 18:38

heretolearn

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Wrap the for loop in a recursive function or method and it can call itself based on the number of elements needed. – uchuugaka Feb 15 '13 at 16:18

combinations of a set with given number elements

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