Question

I’m Trying here to add many arrays to one NSMutableArray
Actually am adding the same array with different values Many Times to one NSMutable Array
this the code:

NSMutableArray* wordsArray =[[NSMutableArray alloc] init ];
NSMutableArray* meaningsArray=[[NSMutableArray alloc]init];
NSMutableArray* wordsArrayTemp=[[NSMutableArray alloc]init];
NSMutableArray* meaningsArrayTemp=[[NSMutableArray alloc]init ];
NSMutableArray* allWords =[[NSMutableArray alloc]initWithCapacity:2];
NSMutableArray* allMeanings=[[NSMutableArray alloc]initWithCapacity:2];




 for(int i=0;i<2;i++)
{
    int wordsCounter=0;
     [wordsArrayTemp removeAllObjects];
[meaningsArrayTemp removeAllObjects];


    for(NSString *tmp in wordsArray)
        {  
            NSString *meaning =[meaningsArray objectAtIndex:wordsCounter++];

            subtmp= [ tmp substringWithRange:NSMakeRange(0,1)];

            if([currentTable isEqualToString:@"arabicToEnglish"])
            { 
                if([subtmp isEqualToString:[arabicLetters objectAtIndex:i]])
                {
                    [wordsArrayTemp addObject:tmp];
                    [meaningsArrayTemp addObject:meaning];
                }
            }
            else
                if([subtmp isEqualToString:[englishLetters objectAtIndex:i]])
                { 
                    [wordsArrayTemp addObject:tmp];
                    [meaningsArrayTemp addObject:meaning];
                }    
        }
  [allWords insertObject:wordsArrayTemp atIndex:i];
   // [allWords addObject: wordsArrayTemp];
    [allMeanings addObject:meaningsArrayTemp]; 
  NSLog(@"all words count%i",[[allWords objectAtIndex:i] count]);  
} 

The Problem :
The supposed behavior here is to have 2 different values in the allWords array .
But what Actually happens is that the 2 values filled with the same value with the last index Value.
I mean [allWords objectAtIndex:0] should have 2000 object and [allWords objectAtIndex:1] should have 3000 ,but what happens that they both have 3000 object !!

what am I missing here?!!
thnx

Answer 1

when you add an object to an array the object is not copied. You just save its memory address.

Basically you added the same temporary array to the parent array. And you did all your array manipulations to the same array.

Maybe this piece of unrolled loop code will make it a little bit clearer.

// create new array on a specific memory address. let's say this address is 0x01
NSMutableArray* wordsArrayTemp=[[NSMutableArray alloc]init];

// first iteration of your loop

// remove all objects from array at memory address 0x01
[wordsArrayTemp removeAllObjects];

// add objects to the array at address 0x01
[wordsArrayTemp addObject:tmp];

// insert array (still at address 0x01) to the parent array 
[allWords insertObject:wordsArrayTemp atIndex:i];
// your allWords array now looks like this: {array@0x01}

// second iteration of your loop

// remove all objects from array at memory address 0x01!!! (still the same array as in the first iteration)
// since it's the same array all objects from the array at [allWords objectAtIndex:0] are removed too
[wordsArrayTemp removeAllObjects];

// add objects to the array at address 0x01
[wordsArrayTemp addObject:tmp];

// insert array (still at address 0x01) to the parent array 
[allWords insertObject:wordsArrayTemp atIndex:i];
// your allWords array now looks like this {array@0x01, array@0x01}

---

the solution is pretty easy.

At the beginning of the for-loop instead of removing allObjects from the array create new arrays.
Just replace

[wordsArrayTemp removeAllObjects];
[meaningsArrayTemp removeAllObjects];

with

wordsArrayTemp = [NSMutableArray array];
meaningsArrayTemp = [NSMutableArray array];