how to add value (array[i]+1) to NSMutableArray

Question

i'm from poland so sorry for word mistakes.

what's my problem?

i've got array with representation of red color, its:

NSMutableArray *redColor = [[NSMutableArray alloc] initWithCapacity: 255];

i've got FOR loop for fill my array value: 0.

for (int i=0; i < 255; i++) {
    [redArray insertObject:[NSNumber numberWithInt:0] atIndex:i];
}

ok it's fine. but problem is when i want to add value in next FOR loop.

for (NSUInteger ii = 0 ; ii < width * height ; ++ii)
{
 //blablabla


[redArray insertObject:[NSNumber numberWithInt:redArray[ii]+1] atIndex:redArray[ii]];
//example: ii=4
//insertObject: (numberWithInt:value for redArray[4] + 1) at index:4.

//it's not work because i've got ERROR: Subscript requiers size of interface
//"NSMutableArray", which is non constant in non-fragile ABI.
}

how i can fix it? i don't know how to easy increment value in NSMutableArray cell.

thanks you for help, it's my first post here.

regards, Tomek

Josh Rosen · Accepted Answer · 2011-11-15 06:54:32Z

3

Unlike C/C++, NSArray is not accessed using a [] operator. To access an element of an NSArray, use objectAtIndex:.

[redArray objectAtIndex:ii]

Additionally, when you want to add 1 to the element in the array, you must convert the original value back to an int from NSNumber.

[[redArray objectAtIndex:ii] intValue] +1

Alternatively, you could just use a normal C array.

int redArray[255];
redArray[index] = redArray[index] + 1

Objective-C is a super-set of C (meaning it contains all of C), so standard C arrays are valid.

Good luck!

answered Nov 15, 2011 at 6:54

Josh Rosen

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Comments

zneak · Accepted Answer · 2011-11-15 06:54:00Z

2

There are three problems with your code. First, you cannot access array elements with the subscript syntax. To access objects contained in a NSArray, you need to use the objectAtIndex: method.

You need to replace:

redArray[ii]

with:

[redArray objectAtIndex:ii]

And then you'll get a NSNumber instance, with which you can't directly do arithmetic. You'll need to call intValue on it first:

[[redArray objectAtIndex:ii] intValue] + 1

Also, the atIndex: parameter of insertObject:atIndex: accepts a NSInteger, not a NSNumber. Therefore, it makes no sense to query the NSArray. You will have to replace:

redArray[ii]

with just:

ii

This call will insert a new object in the array. In case you meant to replace the previous NSNumber, use replaceObjectAtIndex:withObject:.

With all the changes, the line should look like this (I've split it on several lines for the sake of clarity):

int red = [[redArray objectAtIndex:ii] intValue] + 1;
NSNumber* redObject = [NSNumber numberWithInt:red];
[redArray insertObject:redObject atIndex:ii];
// or possibly:
// [redArray replaceObjectAtIndex:ii withObject:redObject];

answered Nov 15, 2011 at 6:54

zneak

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1 Comment

JohnK Over a year ago

One little correction: insertObject:atIndex: is a method (instance method) for NSMutableArray, not NSArray.

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