Variable Array Size

[bchudomelka]

In this code, I scanf to get the NxN array size. Then I try to create an array of that size, then make it a zero matrix. Any clues?

#include <stdio.h>
#include <math.h>

int main (){
	/* Create a matrix of NxN dimensions */
	int i, j, k;
	printf("\nWhat NxN size of array do you want?\n");
	printf("N=");
	scanf("%d",&k);
	getchar();
	printf("You desire a %dx%d Matrix.\n" ,k,k);
	/* Create a NxN matrix */
	/* Array is now an NxN matrix. */
	/*					   */
	/*      Display the matrix     */
	
	int M[k][k];
	for (i=0; i<k; i++){
		for (j=0;j<k;j++)
			M[i][j]=0;
	}
	printf("\nHere is a zero array with the specified dimension %dx%d." , k,k);
	for (i=0; i<k; i++){
		for (j=0;j<k;j++)
			printf("%f", M[i][j]);
		printf("\n");
	}
	getchar();
	return 0;
}

[laserlight]

How does it not work? In case you did not know, the variable length array language feature is relatively new, having been introduced to standard C in the 1999 edition of the C standard.

[bchudomelka]

This is the error report from Microsoft Visual C++ 2010 Express.

1>c:\users\bryce\documents\visual studio 2010\projects\taylorsrsexpmatrix\taylorsrsexpmatrix\expmatrix.cpp(13): error C2057: expected constant expression
1>c:\users\bryce\documents\visual studio 2010\projects\taylorsrsexpmatrix\taylorsrsexpmatrix\expmatrix.cpp(13): error C2466: cannot allocate an array of constant size 0
1>c:\users\bryce\documents\visual studio 2010\projects\taylorsrsexpmatrix\taylorsrsexpmatrix\expmatrix.cpp(13): error C2057: expected constant expression
1>c:\users\bryce\documents\visual studio 2010\projects\taylorsrsexpmatrix\taylorsrsexpmatrix\expmatrix.cpp(13): error C2466: cannot allocate an array of constant size 0
1>c:\users\bryce\documents\visual studio 2010\projects\taylorsrsexpmatrix\taylorsrsexpmatrix\expmatrix.cpp(13): error C2087: 'M' : missing subscript
1>c:\users\bryce\documents\visual studio 2010\projects\taylorsrsexpmatrix\taylorsrsexpmatrix\expmatrix.cpp(13): error C2133: 'M' : unknown size

It won't even compile. I agree that it should run also.

[laserlight]

Well, I notice the ".cpp" file extension, which means that you are very likely compiling as C++, and C++ does not have a variable length array language feature (although it may be supported as a compiler extension). But even if you are compiling as C, if I remember correctly, MSVC does not support this particular C language feature (hence my mention that it is relatively new, from C99).

You may want to consider the use of the likes of malloc(), realloc() and free() from <stdlib.h>. Alternatively, if the upper bound of N is small enough, you can simply create a fixed size 2D array of that largest possible size, but only use a portion of it as needed.

[bchudomelka]

Thank you for the quick replies!!

[bchudomelka]

I am not aware of the malloc, realloc or free functions. If this is a way to create my variable size array then I'm going to need to learn those commands.

[Brafil]

You do certainly, as they are one of the most useful - if not the most useful - functions out there. And are very closely related to pointers, which are a very important concept in C.

[AdnanShaheen]

Use double pointer to int (int** pMatrix) to solve your problem. You have to create array and then array on each item of array.

I hope you will write it, hint is enough.

[bchudomelka]

Is that before the

int M[k][k];

[dayalsoap]

I am not aware of the malloc, realloc or free functions. If this is a way to create my variable size array then I'm going to need to learn those commands.

You can do this real easily with malloc

Here's a goodT overview of how pointers, arrays, and malloc work

http://c-faq.com/aryptr/dynmuldimary.html

	#include <stdlib.h>

        int nrows = 10;
        int ncolumns = 20;
 
        // create a [10][20] array
	int **array1 = malloc(nrows * sizeof(int *));
	for(i = 0; i < nrows; i++)
		array1[i] = malloc(ncolumns * sizeof(int));

then you can index the elements like array[3][4] for example

[bchudomelka]

So, the int **array1 assigns the array1 pointer to be of int type? malloc allocates the memory of the nrows in the 1st instance? the for loop causes array element i to be allocate the memory for ncolumns?

[MK27]

So, the int **array1 assigns the array1 pointer to be of int type? malloc allocates the memory of the nrows in the 1st instance? the for loop causes array element i to be allocate the memory for ncolumns?

**array1 indicates a pointer to a pointer (**), which is why the malloc looks like malloc(nrows * sizeof(int *)). This is space for an array, so array1 is a pointer to an array of pointers. Each element of the array is an int pointer which has space for an array of ints allocated to it -- malloc(ncolumns * sizeof(int)). So this is an array of "row pointers" to rows of columns of ints.

I'd recommend you try a 1D array first, which would be a single row (*array).

[dayalsoap]

**array1 indicates a pointer to a pointer (**), which is why the malloc looks like malloc(nrows * sizeof(int *)). This is space for an array, so array1 is a pointer to an array of pointers. Each element of the array is an int pointer which has space for an array of ints allocated to it -- malloc(ncolumns * sizeof(int)). So this is an array of "row pointers" to rows of columns of ints.

I'd recommend you try a 1D array first, which would be a single row (*array).

This is correct.

The website I gave gives a pretty good image
http://c-faq.com/aryptr/array1.gif

Try a 1d array first

int col = 10;
int *array1d;
array1d = (int*)malloc(sizeof(int) * col);

This will create a 1d array of size 10...

[bchudomelka]

Thanks so much. I was able to create the malloc. It's starting to make more sense.

[bchudomelka]

This is an example I create with the code you gave.

#include <stdio.h>
#include <stdlib.h>
 int main(){
	 int k;
	 int col = 10;
	int row = 10;
	int *array1d;
	array1d = (int*)malloc(sizeof(int) * col*row);
	printf("Which element do you want to see?\n");
	printf("Element:");
	scanf("%d", &k);
	printf("The first element of array1d:%d\n", array1d[k]=k);
	scanf("%d", &k);
	if (k==1) {main();}
	return 0;
 }

How does that expand to 2x2?