Set an array with unknown dimensions as global

Hi! In my code I use a 2d array which I need in more than two subroutines. This array doesn't have specific dimensions. The user gives them. Is there any way to declare it so it wouldn't be changed every time I need it? I've tried to declare it as global but I couldn't.

You can't CHANGE the dimensions of an array in C (or in C++ for that matter).

You CAN use dynamic allocation to build a 2D array - there will be plenty of examples of that.

Or, if you prefer that method, you can GUESS a "huge" size and just use whatever portion of it that the user wishes to use, e.g. make it 10000 x 10000 (400MB), but only use 1% of it if the user asks for 100 x 100. This of course assumes the code isn't supposed to run on a system with limited memory space.

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Mats

Come on, matsp, 400 MB? People complain at me for bloat :rolleyes:
A iaximum of 1000 x 1000 I can recommend, but preferably smaller.
Perferbly a dynamic allocated array with just enough necessary space.

Ok! Thank you!!!! I'll do it using dynamic allocated array!

Quote:

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Originally Posted by Elysia

Come on, matsp, 400 MB? People complain at me for bloat :rolleyes:
A iaximum of 1000 x 1000 I can recommend, but preferably smaller.
Perferbly a dynamic allocated array with just enough necessary space.

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But in any decent OS, the array-space will only be committed to memory if it's ACTUALLY used, so a 100 x 100 usage of a 10000 x 10000 array will only use physical memory for a small amount of the 400MB [roughly 100/10000 -> 1%].

Yes, it's excessive, but it's definitely a simpler solution.

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Mats

You may be right, but I don't know if we can rely on this always happening. As you say, a good OS won't mind, but a bad one might.
Well, I think dynamic arrays can be good practice anyway.

This works to consume seemingly little memory, but I can't access the entire array either. Somewhere around the middle, it gives an access violation.
But you allocate it in one big, single array, it consumes 400 MB of memory.

Quote:

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Originally Posted by Elysia

You may be right, but I don't know if we can rely on this always happening. As you say, a good OS won't mind, but a bad one might.
Well, I think dynamic arrays can be good practice anyway.

Code:

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        int** pNewInt = new int*[10000];
        for (int i = 0; i < 10000; pNewInt[i++] = new int);

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This works to consume seemingly little memory, but I can't access the entire array either. Somewhere around the middle, it gives an access violation.
But you allocate it in one big, single array, it consumes 400 MB of memory.

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But you are only allocating one int for each row...

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Mats

Quote:

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Originally Posted by matsp

But you are only allocating one int for each row...

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Mats

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As you can see, I'm not very experienced with 2D-arrays :p

Code:

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        int** pNewInt = malloc(sizeof(int*) * 10000);
        for (int i = 0; i < 10000; pNewInt[i++] = malloc(sizeof(int) * 10000));
        for (int i = 0; i < 10000; i++)
        {
                for (int j = 0; j < 10000; pNewInt[i][j++] = 0);
        }

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OK, so here's the new test.
Results: 400 MB physical memory gobbled up, memory use by app +400 MB and swap file grew by 400 MB too.
Took some time to complete on the first run too, but not the second or third. It may just be due to the debugger.

So in conclusion: it's bad to allocate such a huge array.

Isn't it a C forum?

Point to be demonstrated. Do you rather I rewrite it with malloc?

Quote:

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Originally Posted by Elysia

Point to be demonstrated. Do you rather I rewrite it with malloc?

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I'd rather to point you to post the correct code on the correct forum in the future. There are people here that can be misleaded by such demonstrations.

Code updated to C code.