Losing Data in NSMutableArray during Loop - ios

Question

I am trying to create a NSMutableArray so I can populate a map. But immediately after "[row removeAllObjects];" it removes all of the objects in the "layer" array also.
NSMutableArray *row = [[NSMutableArray alloc] init];
NSMutableArray *layer = [[NSMutableArray alloc] init];
NSMutableArray *all = [[NSMutableArray alloc] init];
int blockType = 0;
for (int y = 0; y<10; y++) {
for (int x = 0; x<10; x++) {
for (int z = 0; z<10; z++) {
if (y<5) blockType = 0;
else blockType = arc4random() % 2 + 1;
[row addObject:[NSNumber numberWithInt:blockType]];
}
[layer addObject:row];
[row removeAllObjects];
}
[all addObject:layer];
[layer removeAllObjects];
}
is this because
[layer addObject:row];
[row removeAllObjects];
are being performed at the same time? If so, how do I carry out the actions one after another?
If not, what am I doing wrong?

Answer 1

Try this,
[layer addObject:[row copy]];

Answer 2

You are adding row to layer, but then you're emptying it.
I understand why you are doing this, as you want to re-use both row and layer. However that's not how it works in Objective-C, as variables are just references (C pointers) to objects.
What you need to do is to create all outside the loops, and re-create row and layer inside the inner loops. You can re-use the variables, but the underlying objects need to change.
Try this:
NSMutableArray *all = [[NSMutableArray alloc] init];
NSMutableArray *row;
NSMutableArray *layer;
int blockType = 0;
for (int y = 0; y < 10; y++) {
layer = [[NSMutableArray alloc] init];
for (int x = 0; x < 10; x++) {
row = [[NSMutableArray alloc] init];
for (int z = 0; z < 10; z++) {
if (y < 5)
blockType = 0;
else
blockType = arc4random() % 2 + 1;
[row addObject:[NSNumber numberWithInt:blockType]];
}
[layer addObject: row];
}
[all addObject: layer];
}

Answer 3

Move NSMutableArray *row = [[NSMutableArray alloc] init]; to just after for (int x = 0; x<10; x++) {, and delete [row removeAllObjects]; entirely.

Answer 4

You create a problematic program structure, and it bites you in the behind.
for (int z = 0; z<10; z++) {
if (y<5) blockType = 0;
else blockType = arc4random() % 2 + 1;
[row addObject:[NSNumber numberWithInt:blockType]];
}
[layer addObject:row];
[row removeAllObjects];
This code is supposed to create an array with 10 numbers and add it to the array "layer". But that's not what it does. The array has been created a lot earlier. And add the end, it empties out the array to be ready for the next iteration. That's an awful structure. The correct structure would be:
NSMutableArray* row = [NSMutableArray array];
for (int z = 0; z<10; z++) {
if (y<5) blockType = 0;
else blockType = arc4random() % 2 + 1;
[row addObject:[NSNumber numberWithInt:blockType]];
}
[layer addObject:row];
This does exactly what it is supposed to do: Create an array, fill it with data, add it to layer. No reliance on previous and following code. The complete working code is:
NSMutableArray *all = [[NSMutableArray alloc] init];
for (int y = 0; y<10; y++) {
NSMutableArray *layer = [[NSMutableArray alloc] init];
for (int x = 0; x<10; x++) {
NSMutableArray *row = [[NSMutableArray alloc] init];
for (int z = 0; z<10; z++) {
int blockType = y < 5 ? 0 : arc4random() % 2 + 1;
[row addObject:[NSNumber numberWithInt:blockType]];
}
[layer addObject:row];
}
[all addObject:layer];
}
Well, someone else typed quicker. Which is even better, because you see twice an identical program structure, with almost identical code. That's because the task is clear, and there is exactly one way how any experienced programmer would structure the code.