2d runtime arrays

[bitwise]

How do I create a 2-dimensional runtime array in C? I know how many "columns" it has and that is 3 but the number of rows is determined at runtime. How do I properly allocate the room for that?

Thanks,
-bitwise

 

[Cagliostro]

Is the following what you need?

char* y="hello";
char*** x;
int i,j;
x=(char***)malloc(num_cols*num_rows*sizeof(char*));
for(i=0;i<numcols;i++)
for(j=0;j<numcols;j++)
x[j]=y; John Fill

http://ournet.md/~ifilipski/1c.bmp

ivfmd@mail.md

 

[Lim]

More clear way for compiler:

/* this is pointer to array of 3 integers */
int (*paiArray)[3] = NULL;
/* !!! this is not a *apiArray[3] what is the array of 3 pointers to int */


/* allocate by one call to malloc x is a number of your rows*/
paiArray = (int(*)[3])malloc(x*sizeof(int[3]));

/* Easy to use */
paiArray[0][1] = 3;
printf(&quot;A:%d\n&quot;,paiArray[0][1]);

/* free array */
free(paiArray);

This is the easy way to use 2 dim array if you know the number of colums. And it makes code more understandable for programmer and compiler.

Regards.

 

[rbobbitt]

One thing that would make it even more understandable is to drop the cast on malloc():

paiArray=malloc(x*sizeof(int[3]));

In C, pointers to void can be converted back and forth to other pointer types implicitly without any loss of information.

And better for code maintenance:

paiArray=malloc(x * sizeof *paiArray);

If the type of paiArray changes (say to pointer to array 15 of int), you only have to make changes at the point of the definition of paiArray.

Russ
bobbitts@hotmail.com

http://home.earthlink.net/~bobbitts

 

[Lim]

But in future you probably will want to use this code in C++.
For example we are use both code in our programm C and C++ and sometime you need to call C++ from C so you just compile it as C++. In this case (void*) will not be cast by default to (int(*)[3]) at least in gcc, you will have a compilation error. So it is never hurt to use a cast.
Of cause you can skip this cast for your homework.
Also this type int(*)[3] can help you to pass this array as an argument in some function
int func(int(*)[3] in_paiArray)
{
}
I have spent some time to find the right type for this argument.

But second advice about using paiArray to get it size is very wise

 

[rbobbitt]

That's a good point and this is an important difference between C and C++. C++, indeed, doesn't allow such conversions to occur implicitly so the cast is required if you were using Lim's example in C++ code.

Russ
bobbitts@hotmail.com

http://home.earthlink.net/~bobbitts