按降序对NSArray进行排序

Question

I have an NSArray with 4 objects, let's say 1, 2, 3 and 4. I want to sort this array in ascending order, but with a randomly selected starting number. For instance; 2, 3, 4 and 1 or 4, 1, 2 and 3.

How can I do this?

What I have thus far:

NSArray *playersArray = [_players allKeys];
NSSortDescriptor *sortPlayerArray = [[NSSortDescriptor alloc] initWithKey:nil ascending:YES];
playersArray = [playersArray sortedArrayUsingDescriptors:@[sortPlayerArray]];

This results in 1, 2, 3, 4, obviously. I am also able to randomly order the players, like so:

activePlayersArray = [_players allKeys];
NSMutableArray *temp = [[NSMutableArray alloc] initWithArray:activePlayersArray];

int count = (int)[temp count];

for (int i = 0; i < count; ++i) {
    int nElements = count - i;
    int n = (arc4random() % nElements) + i;
    [temp exchangeObjectAtIndex:i withObjectAtIndex:n];
}

activePlayersArray = [NSArray arrayWithArray:temp];

So how can I "combine" these two to get the results I want?

Hope you guys can help me.

Thanks!

Answer 1

This is really an algorithm problem, not an iOS problem. Here are the steps to follow

• make a note of your randomly selected number
• Sort the array in descending order as you normally would (as in Sort an NSArray in Descending Order )
• Then split the array at the location of your special number (similar to How to split an NSArray into two equal pieces? )
• after the split create a new array where the second piece now comes first

---

Another solution is to create a circular array of sorted elements and then traverse the array in reverse order.

Answer 2

I think this is what @Konsol intends, with a couple fixes: (1) it looks like the OP wants the order to be ascending, and (2) the array split in the other answer is at the midpoint. But I think the spirit is correct...

// Start with an unsorted (immutable?) input array of numbers (or any object
// that implements compare:.
// Pick a random location and produce an output array as described by the OP

NSMutableArray *mutableArray = [inputArray mutableCopy]; // if its not mutable already
[mutableArray sortUsingSelector:@selector(compare:)];

NSInteger inputIndex=arc4random_uniform(mutableArray.count);    
NSArray *start = [mutableArray subarrayWithRange:NSMakeRange(inputIndex, mutableArray.count-inputIndex)];
NSArray *end = [mutableArray subarrayWithRange:NSMakeRange(0, inputIndex)];

NSArray *outputArray = [start arrayByAddingObjectsFromArray:end];
NSLog(@"%@", outputArray);

Answer 3

int count = (int)[activePlayersArray count];
int n = (arc4random() % nElements) + i;
NSMutableArray *temp = [[NSMutableArray alloc] init];
for (int i = 0; i < count; ++i) {
    int nElements = count - i;
    [temp addObject:[activePlayersArray objectAtIndex:(n-i)%count]];
}
activePlayersArray = [NSArray arrayWithArray:temp];

Hope it works!