iPhone - NSArray - Pick 4 different items - iphone

Question

I've NSArray with over 100 strings.
I would like to pick 4 different strings randomly. I can write traditional way of code using for/while loops and get the task done.
But is there any better way to pick 4 different random strings?

Answer 1

Shuffle an array as described in JEFF LAMARCHE's blog and use first four items)

Answer 2

create a NSSet from your NSArray and fetch first 4 elements.

Answer 3

I wrote some utils as category on NSArray.
You could use it like this:
#import "NSArray+RandomUtils.h"
NSArray *array = [NSArray arrayWithObjects:#"aa", #"ab",#"c",#"ad",#"dd", nil];
NSSet *set = [array setWithRandomElementsSize:4];
This will give you a set of 4 unique random elements.
If you want to allow objects to be in your collection more than once you can do:
NSArray *array = [NSArray arrayWithObjects:#"aa", #"ab",#"c",#"ad",#"dd", nil];
NSArray *resultArray = [array arrayWithRandomElementsSize:4];
#import "NSArray+RandomUtils.h"
#implementation NSArray (RandomUtils)
-(NSMutableArray *)mutableArrayShuffled
{
NSMutableArray *array = [[self mutableCopy] autorelease];
[array shuffle];
return array;
}
-(NSMutableArray *)arrayShuffled
{
return [NSArray arrayWithArray:[self mutableArrayShuffled]];
}
-(id)randomElement
{
if ([self count] < 1) return nil;
NSUInteger randomIndex = arc4random() % [self count];
return [self objectAtIndex:randomIndex];
}
-(NSSet *)setWithRandomElementsSize:(NSUInteger)size
{
if ([self count]<1) return nil;
if (size > [self count])
[NSException raise:#"NSArrayNotEnoughElements"
format:#"NSArray's size (%d) is too small to fill a random set with size %d", [self count], size];
NSMutableSet *set = [NSMutableSet set];
NSMutableArray *array = [self mutableArrayShuffled];
if (size == [array count])
return [NSSet setWithArray:array];
while ([set count]< size) {
id object = [array objectAtIndex:0];
[array removeObjectAtIndex:0];
[set addObject:object];
}
return [NSSet setWithSet:set];
}
-(NSArray *)arrayWithRandomElementsSize:(NSUInteger)size
{
if ([self count]<1) return nil;
NSMutableArray *array = [NSMutableArray array];
while ([array count] < size) {
[array addObject:[self randomElement]];
}
return [NSArray arrayWithArray:array];
}
#end
#implementation NSMutableArray (RandomUtils)
-(void)shuffle
{
NSUInteger count = [self count];
for (NSUInteger i = 0; i < count; ++i) {
NSUInteger nElements = count - i;
NSUInteger n = (arc4random() % nElements) + i;
[self exchangeObjectAtIndex:i withObjectAtIndex:n];
}
}
#end

Answer 4

This has been answered multiple times in the forum. Your key to generate random numbers is
(arc4random() % 100) +1
Above code is capable of generating a random number ranging from 1 to 100. You can use this to get what you want. If you received a random number that you already got, ignore and call again to get a unique random number.