pointers to multidimensional arrays in C

[KrazyHorse]

Yes, if you have the library of programs contained in "Numerical Recipes in C" available...



I believe the syntax was ran4(-longint)

[Ari Rahikkala]

Pointers to multidimensional arrays? Congratulations, you might just be joining the (small) ranks of the three star programmers if you play your cards right (wrong) .

[Whoha]

Originally posted by Kuciwalker


There is no "new" in C.

@Asher: I steadfastly refuse to use the random number generator.

meh, I'm speaking of memory allocation in general. by the way I think I just did what you are asking how to do, the memory was zeroed though.

[Asher]

Originally posted by Kuciwalker
@Asher: I steadfastly refuse to use the random number generator.

I'm trying to help you steer clear of horrible programming practices.

[KrazyHorse]

SO instead of using a portable number generator you're using some bass-ackwards way?

Stupid.

[Asher]

Here's how I'd declare a multi-dimensional array with dynamic allocation and pointers...

void allocateMatrix(int ***pMatrix, int size) 
{
	int i, j; 

	*pMatrix = (int **) calloc (size, sizeof(int));
	**pMatrix = (int *) calloc (size * size, sizeof(int));

	for (i = 0, j = 0; i < size; i++, j += size)  	/* init array indices */
	{
		*(*pMatrix + i) = (**pMatrix + j);
	}
}


int main()
{ 
	int **pMatrix; 

	allocateMatrix(&pMatrix,100);

  return 0;
}

[St Leo]

Originally posted by Kuciwalker
I have an assignment to write a program that does a bunch of crap with matrices in C. One thing I have to do is randomize the contents of a matrix. I want to do this by copying random data from memory into the matrix (yes, I know there are other ways, but I'm not using them). So I have to create a pointer to an int[100][100] array, and then use malloc. I've tried declaring this as int temp[100][100]*, but the compiler gives me an error. I don't get an error if I declare it as int * temp[100][100], but I'm not sure if that's a point to a matrix or a matrix of pointers. Moreover, I have to cast malloc to a pointer to an int[100][100], but the compiler doesn't like temp = (int[100][100]*) malloc(40000), either.

Dude, C arrays aren't objects or types. The array's name is a pointer to the zeroth element in the array. Square brackets are a convenience notation.

Zeroth of all, the following two lines are equivalent:
array[cell] = 1;
*(array + cell) = 1;

When you say array[cell], you are telling the compiler to follow the array pointer, advance cell sizeof()s beyond it, and do whatever.

First of all, the following to lines are equivalent:
array[row][col] = 1;
*(*(array+ row) + col) = 1;

That's right. Multi-dimensional arrays are just a hack job on top of the hack job.

The code that you want is probably:

int **p; // hi, I am a two-dimensional array
p = (int**)malloc(100 * 100 / sizeof(int)); // use me, baby

int ***a; // hi, I am the address of the two dimensional address
a = &p; // functions can now change the array by reference

void doStuff(int ***array, int row, int col) {
array[3][4] = 3;
}

doStuff(a, 100, 100);
// element at [3][4] is now 3
doStuff(&p, 100, 100);
// element at [3][4] is again 3

This is why I ****ing hate C.

[Kuciwalker]

Originally posted by KrazyHorse
SO instead of using a portable number generator you're using some bass-ackwards way?

Stupid.

It's equivalent to finding out what'll happen if you hook a reflector up to a nightlight that turns off when light hits a sensor on it...

[Kuciwalker]

Originally posted by Whoha
meh, I'm speaking of memory allocation in general. by the way I think I just did what you are asking how to do, the memory was zeroed though.

malloc doesn't zero memory, according to man malloc.

[Ari Rahikkala]

calloc, then .

[Kuciwalker]

I don't want to zero the memory.

[Asher]

Originally posted by Kuciwalker
I don't want to zero the memory.

What you are doing is fundamentally evil!

[Kuciwalker]

Then I'm . Big deal

[Whoha]

Originally posted by Kuciwalker


malloc doesn't zero memory, according to man malloc.

cc was going on implicit function definitions, so perhaps that was it.

Originally posted by Kuciwalker
I don't want to zero the memory.

Why not though? isn't all zeros a random possibility?

[Kuciwalker]

I'm using data in memory for random numbers. And St Leo answered my question.