Copying array elements to other array with pointers

Question

Say I have NSMutableArray *array1 with 10 objects. I want to create an *array2 and add 5 objects from array1 to array2, and I want it so that when I change these object properties from array2, they also change the 5 specific objects from array1 as well. How would I do this?

Edit: Ok I think I asked the wrong question. It's more about passing by reference and pointers, which I confuse too much:

NSMutableArray *mainArray;
NSMutableArray *secondaryArray;
NSMutableDictionary *dic1;

[mainArray addObject:dic1];
[self changeValues:[mainArray lastObject]];

-(void)changeValues:(NSMutableDictionary*)someDic
{
    [secondaryArray addObject:someDic];
    NSMutableDictionary *aDic=[secondaryArray lastObject];
    ...//some code to change values of aDic
    //by changing aDic, I want to also change the same dic from mainArray

    //so [mainArray lastObject] should be the same exact thing as [secondaryArray lastObject]
}

How would I change the above code so the changes reflect in both arrays?

NSArray *array2 = [NSArray arrayWithObjects:[array1 objectAtIndex:0], [array1 objectAtIndex:2],[array1 objectAtIndex:4],[array1 objectAtIndex:6],[array1 objectAtIndex:8], nil];? — Matthias Bauch
– Matthias Bauch, Commented Feb 19, 2012 at 22:01
Change means 1) replace with a new object or 2) change the values of the object properties? — sch
– sch, Commented Feb 19, 2012 at 22:01
@mohabitar, you already did receive answers. dic1, someDic and aDic - all these values are same. Just change aDic(or someDic) and see result. — beryllium
– beryllium, Commented Feb 20, 2012 at 8:39

beryllium · Accepted Answer · 2012-02-20 08:40:25Z

1

NSMutableArray *array2 = [NSMutableArray array];
for (int i=0; i<5; ++i){
    [array2 addObject: [array1 objectAtIndex:i] ]
}

In this example you have the set of objects pointed by items of array1 as well as by items of array2, since NSMutableArray contains pointers to objects, not objects theirselves. Therefore, changing the object thru pointer in one array you may observe that change thru pointer from other array.

Edit

@mohabitar, you already receive an answers. dic1, someDic and aDic - all these values are same. Just change aDic(or someDic) and see result.

edited Feb 20, 2012 at 8:40

answered Feb 19, 2012 at 22:07

beryllium

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1 Comment

beryllium Over a year ago

Or use @MatthiasBauch's method from comments if objects will appear randomly

Monolo · Accepted Answer · 2012-02-20 16:41:33Z

This sounds like a good case for some KVC (Key-Value Coding).

With KVC you can create indexed properties and have the KVC engine create an array proxy for the indexed property, which will then allow you to operate on the indexed property as if it were an array.

Below is a quick proof-of-concept piece of code, tested on both OS X and iOS.

Interface:

@property (strong) NSMutableArray *mainArray;

Implementation:

@synthesize mainArray   = _mainArray;


- (id)init
{
    self = [super init];
    if (self) {

        // For simplicity, use strings as the example
        _mainArray = [NSMutableArray arrayWithObjects:
                      @"1st element",
                      @"2nd element",
                      @"3rd element",
                      @"4th element",
                      @"5th element",
                      @"6th element",
                      @"7th element",
                      @"8th element",
                      @"9th element",
                      @"10th element",
                      nil];
    }
    return self;
}


// KVC for a synthetic array, accessible as property @"secondaryArray"

- (NSUInteger) countOfSecondaryArray
{
    return 5;
}


- (id) objectInSecondaryArrayAtIndex: (NSUInteger) index
{
    // In practice you would need your mapping code here. For now
    // we just map through a plain C array:

    static NSUInteger mainToSecondaryMap[5] = {1,4,5,7,8};

    return [self.mainArray objectAtIndex:mainToSecondaryMap[index]];
}


- (void) watchItWork
{
    NSArray *secondaryArray = [self valueForKey:@"secondaryArray"];

    // See how the sub array contains the elements from the main array:

    NSLog(@"%@", secondaryArray);

    // Now change the main array and watch the change reflect in the sub array:

    [self.mainArray replaceObjectAtIndex:4 withObject:@"New String"];

    NSLog(@"%@", secondaryArray);
}

There is more information in the docs, specifically the part on Indexed Accessor Pattern.

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