Modify C-array in objective-c function

Question

As an iOS programmer I sometimes delve into C to achieve faster results (well actually for fun) and I'm struggling to modify a C-array's values inside a function call. Although I think this answer may help (Modifying a array in a function in C), I don't know where to begin in implementing it.

Currently I have:

[object.guestlists enumerateObjectsUsingBlock:^(id obj, BOOL *stop) {

   NSObject *someNSObject = [NSObject new];

   NSInteger array[3] = {totalIncomingMales,totalIncomingFemales, 
                        totalIncomingGuestsCount};

   [self callMethod:object withCArray:cArray];

}];


- (void) callMethod:(NSObject*) object withCArray:(NSInteger*) cArray {

    // do something with that object

    NSInteger   totalIncomingMales = cArray[0],
                totalIncomingFemales = cArray[1],
                totalIncomingGuestsCount = cArray[2];

    // modify these integers internally so that the C-array passed in is also modified

}

Obviously this passes a pointer and therefore doesn't modify the value. I tried replacing the NSinteger * with NSInteger ** and making, e.g. totalIncomingMales = * cArray[0], but alas I wasn't able to pass the c-array as a parameter (even with an ampersand).

Some useful material and potentially a solution to this would be much appreciated!

You use ** only with NSObjects and its subclass. NSInteger is just a primitive int. I believe * and/ & should work. — Anoop Vaidya
– Anoop Vaidya, Commented Dec 5, 2014 at 13:29
Sorry, have edited. I tried using ampersand, I think. Let me try again — Rambatino
– Rambatino, Commented Dec 5, 2014 at 13:38
Yer, just using the ampersand gives me a compiler warning NSInteger(*)[3] to NSInteger * (aka int *) — Rambatino
– Rambatino, Commented Dec 5, 2014 at 13:40

Juraj Antas · Accepted Answer · 2014-12-05 14:13:23Z

1

Not sure if I understand your question, but it seems to be trivial one:

- (void) callMethod:(NSObject*) object withCArray:(NSInteger*) cArray {
     // modify these integers internally so that the C-array passed in is also modified
     cArray[0] = 10;
     cArray[1] = 20;
     cArray[2] = 30;
}


- (void) myMethod:(NSString *) myConstString array: (NSArray *) myArray
{
    NSInteger array[3] = {1,2,3};
    NSLog(@"%ld %ld %ld", (long)array[0],(long)array[1],(long)array[2]);
    [self callMethod:nil withCArray:array];
    NSLog(@"%ld %ld %ld", (long)array[0],(long)array[1],(long)array[2]);
}

result will be: 1,2,3 after 10,20,30. No pointer trickery needed, because you are telling it is NSInteger so compiler does it for you.

answered Dec 5, 2014 at 14:13

Juraj Antas

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1 Comment

Rambatino Over a year ago

That's it. It was a slip on my part and I shouldn't have been assigning them to new variables. So dum.

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