How can I reverse a NSArray in Objective-C

Question

I need to reverse my NSArray   As an example    1 2 3 4 5  must become   5 4 3 2 1   What is the best way to achieve this

User · Accepted Answer

For obtaining a reversed copy of an array, look at danielpunkass' solution using reverseObjectEnumerator.

For reversing a mutable array, you can add the following category to your code:

@implementation NSMutableArray (Reverse)

- (void)reverse {
    if ([self count] <= 1)
        return;
    NSUInteger i = 0;
    NSUInteger j = [self count] - 1;
    while (i < j) {
        [self exchangeObjectAtIndex:i
                  withObjectAtIndex:j];

        i++;
        j--;
    }
}

@end

User · Answer

If all you want to do is iterate in reverse  try this      iterate backwards nextIndex    currentIndex    0     myArray count  - 1    currentIndex - 1     myArray count     You can do the  myArrayCount  once and save it to a local variable  I think its expensive   but I   m also guessing that the compiler will pretty much do the same thing with the code as written above

User · Answer

Some benchmarks  1  reverseObjectEnumerator allObjects  This is the fastest method   NSArray  anArray       aa     ab     ac     ad     ae     af     ag             ah     ai     aj     ak     al     am     an     ao     ap     aq     ar     as     at             au     av     aw     ax     ay     az     ba     bb     bc     bd     bf     bg     bh             bi     bj     bk     bl     bm     bn     bo     bp     bq     br     bs     bt     bu             bv     bw     bx     by     bz     ca     cb     cc     cd     ce     cf     cg     ch             ci     cj     ck     cl     cm     cn     co     cp     cq     cr     cs     ct     cu             cv     cw     cx     cy     cz     NSDate  methodStart    NSDate date    NSArray  reversed     anArray reverseObjectEnumerator  allObjects    NSDate  methodFinish    NSDate date   NSTimeInterval executionTime    methodFinish timeIntervalSinceDate methodStart   NSLog   executionTime    f   executionTime     Result  executionTime   0 000026  2  Iterating over an reverseObjectEnumerator  This is between 1 5x and 2 5x slower   NSDate  methodStart    NSDate date   NSMutableArray  array    NSMutableArray arrayWithCapacity  anArray count    NSEnumerator  enumerator    anArray reverseObjectEnumerator   for  id element in enumerator         array addObject element     NSDate  methodFinish    NSDate date   NSTimeInterval executionTime    methodFinish timeIntervalSinceDate methodStart   NSLog   executionTime    f   executionTime     Result  executionTime   0 000071  3  sortedArrayUsingComparator  This is between 30x and 40x slower  no surprises here    NSDate  methodStart    NSDate date   NSArray  reversed    anArray sortedArrayUsingComparator    id obj1  id obj2        return  anArray indexOfObject obj1   lt   anArray indexOfObject obj2    NSOrderedDescending   NSOrderedAscending       NSDate  methodFinish    NSDate date   NSTimeInterval executionTime    methodFinish timeIntervalSinceDate methodStart   NSLog   executionTime    f   executionTime     Result  executionTime   0 001100  So   anArray reverseObjectEnumerator  allObjects  is the clear winner when it comes to speed and ease

User · Answer

As for me  have you considered how the array was populated in the first place  I was in the process of adding MANY objects to an array  and decided to insert each one at the beginning  pushing any existing objects up by one  Requires a mutable array  in this case   NSMutableArray  myMutableArray     NSMutableArray alloc  initWithCapacity 1    myMutableArray insertObject aNewObject atIndex 0

User · Answer

After reviewing the other s answers above and finding Matt Gallagher s discussion here  I propose this   NSMutableArray   reverseArray    NSMutableArray arrayWithCapacity  myArray count      for  id element in  myArray reverseObjectEnumerator          reverseArray addObject element       As Matt observes      In the above case  you may wonder if - NSArray reverseObjectEnumerator  would be run on every iteration of the loop     potentially slowing down the code   lt        Shortly thereafter  he answers thus       lt      The  collection  expression is only evaluated once  when the for loop begins  This is the best case  since you can safely put an expensive function in the  collection  expression without impacting upon the per-iteration performance of the loop

User · Answer

DasBoot has the right approach  but there are a few mistakes in his code   Here s a completely generic code snippet that will reverse any NSMutableArray in place      Algorithm  swap the object N elements from the top with the object N     elements from the bottom  Integer division will wrap down  leaving     the middle element untouched if count is odd      for int i   0  i  lt   array count    2  i          int j    array count  - i - 1        array exchangeObjectAtIndex i withObjectAtIndex j       You can wrap that in a C function  or for bonus points  use categories to add it to NSMutableArray    In that case   array  would become  self    You can also optimize it by assigning  array count  to a variable before the loop and using that variable  if you desire   If you only have a regular NSArray  there s no way to reverse it in place  because NSArrays cannot be modified   But you can make a reversed copy   NSMutableArray   copy    NSMutableArray arrayWithCapacity  array count     for int i   0  i  lt   array count   i           copy addObject  array objectAtIndex  array count  - i - 1        Or use this little trick to do it in one line   NSArray   copy     array reverseObjectEnumerator  allObjects     If you just want to loop over an array backwards  you can use a for in loop with  array reverseObjectEnumerator   but it s likely a bit more efficient to use -enumerateObjectsWithOptions usingBlock     array enumerateObjectsWithOptions NSEnumerationReverse                         usingBlock   id obj  NSUInteger idx  BOOL  stop           This is your loop body  Use the object in obj here          If you need the index  it s in idx          This is the best feature of this method  IMHO          Instead of using  continue   use  return          Instead of using  break   set   stop   YES  and then  return          Making the surrounding method block return is tricky and probably        requires a    block  variable          This is the worst feature of this method  IMHO          Note  Substantially updated in 2014 with five more years of Foundation experience  a new Objective-C feature or two  and a couple tips from the comments

User · Answer

Here is a nice macro that will work for either NSMutableArray OR NSArray    define reverseArray   theArray         if     theArray isKindOfClass  NSMutableArray class               if    NSMutableArray     theArray count   gt  1                 NSUInteger i   0               NSUInteger j     NSMutableArray     theArray count -1               while  i  lt  j                       NSMutableArray     theArray exchangeObjectAtIndex i                                                  withObjectAtIndex j                    i                     j--                                   else if     theArray isKindOfClass  NSArray class                 theArray     NSArray alloc  initWithArray    NSArray     theArray reverseObjectEnumerator  allObjects                  To use just call  reverseArray myArray

User · Answer

There is a much easier solution  if you take advantage of the built-in reverseObjectEnumerator method on NSArray  and the allObjects method of NSEnumerator   NSArray  reversedArray     startArray reverseObjectEnumerator  allObjects     allObjects is documented as returning an array with the objects that have not yet been traversed with nextObject  in order      This array contains all the remaining objects of the enumerator in enumerated order

User · Answer

I don t know of any built in method  But  coding by hand is not too difficult  Assuming the elements of the array you are dealing with are NSNumber objects of integer type  and  arr  is the NSMutableArray that you want to reverse   int n    arr count   for  int i 0  i lt n 2    i      id c      arr objectAtIndex i  retain      arr replaceObjectAtIndex i withObject  arr objectAtIndex n-i-1       arr replaceObjectAtIndex n-i-1 withObject c       Since you start with a NSArray then you have to create the mutable array first with the contents of the original NSArray   origArray     NSMutableArray   arr     NSMutableArray alloc  init    arr setArray origArray     Edit  Fixed n -  n 2 in the loop count and changed NSNumber to the more generic id due to the suggestions in Brent s answer

User · Answer

The most efficient way to enumerate an array in reverse  Use enumerateObjectsWithOptions NSEnumerationReverse usingBlock  Using  JohannesFahrenkrug s benchmark above  this completed 8x quicker than   array reverseObjectEnumerator  allObjects    NSDate  methodStart    NSDate date     anArray enumerateObjectsWithOptions NSEnumerationReverse usingBlock   id obj  NSUInteger idx  BOOL  stop                NSDate  methodFinish    NSDate date   NSTimeInterval executionTime    methodFinish timeIntervalSinceDate methodStart   NSLog   quot executionTime    f quot   executionTime

User · Answer

Swift 3 syntax    let reversedArray   array reversed

User · Answer

Or the Scala-way   - NSArray   reverse       if   self count  lt  2           return self      else         return   self tail reverse  concat  NSArray arrayWithObject self head       - id head       return self firstObject     - NSArray   tail       if   self count  gt  1           return  self subarrayWithRange NSMakeRange 1  self count - 1        else         return

User · Answer

Try this   for  int i   0  i  lt   arr count   i          NSString  str1    arr objectAtIndex  arr count -1        arr insertObject str1 atIndex i        arr removeObjectAtIndex  arr count -1

User · Answer

Reverse array and looping through it      startArray reverseObjectEnumerator  allObjects  enumerateObjectsUsingBlock   id obj  NSUInteger idx  BOOL  stop

User · Answer

NSMutableArray  objMyObject    NSMutableArray arrayWithArray  self reverseArray objArrayToBeReversed        Function reverseArray  - NSArray    reverseArray    NSArray    myArray          return   myArray reverseObjectEnumerator  allObjects

User · Answer

Georg Sch  lly s categories are very nice  However  for NSMutableArray  using NSUIntegers for the indices results in a crash when the array is empty  The correct code is    implementation NSMutableArray  Reverse   -  void reverse       NSInteger i   0      NSInteger j    self count  - 1      while  i  lt  j             self exchangeObjectAtIndex i                   withObjectAtIndex j            i            j--            end

User · Answer

To update this  in Swift it can be done easily with   array reverse

User · Answer

There is a easy way to do it       NSArray  myArray       5    4    3    2    1        NSMutableArray  myNewArray     NSMutableArray alloc  init     this object is going to be your new array with inverse order      for int i 0  i lt  myNewArray count   i              myNewArray insertObject  myNewArray objectAtIndex i  atIndex 0               other way to do it     for NSString  eachValue in myArray            myNewArray insertObject eachValue atIndex 0                in both cases your new array will look like this     NSLog   myNewArray       myNewArray            1    2    3    4    5     I hope this helps

[objective-c] How can I reverse a NSArray in Objective-C?

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