malloc problem

[rakeshkool27]

Hello everyone,I am now stucked to a program now which is just about adding two tables like matrices.This program must be a simple one for you but its a complicated one for me.I think everything in this program is fine except for the implementation of malloc function. each time i compile it shows an error in the malloc function line.Please look into it if all my declaration of the 2D arrays are correct.The program code is:

#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
#define COL 20
void readinput(int (*x)[COL],int,int);//function prototypes
void computesum(int (*x)[COL],int (*y)[COL],int (*z)[COL],int,int);
void printout(int (*x)[COL],int,int);
int main()
{
	int row,nor,noc;//nor=no of rows noc=no of columns
	int (*a)[COL],(*b)[COL],(*c)[COL];//THIS STATEMENT DECLARES THAT a,b,c are pointers to dimensional arrays
	printf("\nHOW MANY ROWS?  ");
	scanf("%d",&nor);
	printf("\nHOW MANY COLUMNS?  ");
	scanf("%d",&noc);
	for(row=0;row<nor;row++)
	{
		(*a)[row]=(int *) malloc (noc * sizeof(int));//dynamic memory allocation for arrays a,b,c
		(*b)[row]=(int *) malloc (noc * sizeof(int));
		(*c)[row]=(int *) malloc (noc * sizeof(int));
	}
	printf("\nREAD INPUT OF FIRST TABLE:\n");
	readinput(a,nor,noc);//inputting first table(function call)
	printf("\nREAD INPUT OF SECOND TABLE:");
	readinput(b,nor,noc);//inputting second table(function call)
	computesum(a,b,c,nor,noc);//computing the sum of the tables(function call)
	printf("\nPRINTING OUTPUT\n");
	printout(c,nor,noc);//showing the result(function call)
	getch();
	return 0;
}
void readinput(int (*x)[COL],int r,int c)
{
	int i,j;
	for(i=0;i<r;i++)
	{
		for(j=0;j<c;j++)
		{
			scanf("%d",*(x+i)+j);//reading each element of the table
		}
	}
	return;
}
void computesum(int (*x)[COL],int (*y)[COL],int (*z)[COL],int r,int c)
{
	int i,j;
	for(i=0;i<r;i++)
	{
		for(j=0;j<c;j++)
		{
			*(*(z+i)+j) = *(*(x+i)+j) + *(*(y+i)+j);//adding corresponding elements
		}
	}
}
void printout(int (*x)[COL],int r,int c)
{
	int i,j;
	for(i=0;i<r;i++)
	{
		for(j=0;j<c;j++)
		{
			printf("%-5d",*(*(x+i)+j));//printing out the numbers
			if(j==c-1)
			printf("\n");
		}
	}
}

Please post reply if you can.......thanks

[quzah]

int (*a)[COL],(*b)[COL],(*c)[COL];//THIS STATEMENT DECLARES THAT a,b,c are pointers to dimensional arrays

Your comment is wrong. That statement declares that they are pointers to an array of integers that is COL elements in size. That is to say, it points to a 1D array of COL integers.

Easy dynamic "2D arrays":

int **twodee;
size_t rows, cols, x;

... fill out rows and cols some place ...

twodee = malloc( rows * sizeof *twodee ); /* allocate rows number of pointers to integers */
if( twodee )
{
    for( x = 0; x < rows; x++ )
        twodee[ x ] = malloc( cols * sizeof **twodee ); /* allocate a row, cols elements in size */
}

Freeing works the same way. Free the rows first, then free the whole thing.

for( x = 0; x < rows; x++ )
    free( twodee[ x ] );
free( twodee );

Quzah.

[rakeshkool27]

Easy dynamic "2D arrays":

int **twodee;
size_t rows, cols, x;

... fill out rows and cols some place ...

twodee = malloc( rows * sizeof *twodee ); /* allocate rows number of pointers to integers */
if( twodee )
{
    for( x = 0; x < rows; x++ )
        twodee[ x ] = malloc( cols * sizeof **twodee ); /* allocate a row, cols elements in size */
}

Freeing works the same way. Free the rows first, then free the whole thing.

for( x = 0; x < rows; x++ )
    free( twodee[ x ] );
free( twodee );

Quzah.[/QUOTE]
Thanks quzah, for your reply and pointing out the correct statement.
quzah, actually i am new to C programming, and dont want to go to more complex programming now or any alternative way.But i want to know what was wrong with my codes in the malloc allocation.I want to first clear out my concept in that way first.Please execute and run the codes to see the error.I have been trying a lot and stuck to this..................Please help me out..............Yours every timely help is appreciated

[MK27]

You are doing something in a very unusual way, due I think to our discussion the other day of pointers to arrays.

But you don't want pointers to arrays here, you want arrays of pointers:

	int *a[nor], *b[nor], *c[nor], i;
	for (i=0; i<nor; i++) {
		a[i] = malloc(noc*sizeof(int));
		b[i] = malloc(noc*sizeof(int));
		c[i] = malloc(noc*sizeof(int));
	}

Notice you do not have to (cast)malloc in C.

If you do not want to declare the size of the array initially you can use a pointer to a pointer (slightly counter-intuitive, but there you go):

	int **a, i;
	a = malloc(nor*sizeof(int*));
	for (i=0;i<nor;i++) a[i] = malloc(noc*sizeof(int));

Notice "a" is malloc'd an array of pointers, not an array of ints, then each pointer is malloc'd it's row.

[rakeshkool27]

hi mk27,you are doing a great job always and always ready to help.Thanks....

Anyway, in this problem i want pointer to arrays only.Here the expression
int (*a)[COL],(*b)[COL],(*c)[COL];
is declaring a,b,c as pointer to 2 dimensional arrays with their 2nd dimension as COL=20.

actually i have been totally confused with pointers and multidimensional arrays.
As it is given in book, to use a two dimensional array as pointer we should declare as

data_type (*p)[2nd Dimension];//p being a pointer to 2D array

which is equivalent to

data_type x[1st dimension][2nd dimension];//x being a 2D array

So i was trying to implement it in the program i had shown before.Now i am confused whether this pointer declaration of 2D arrays as shown is declaring a pointer to an array with a fixed space of 20 memory locations or is the memory space variable and later can be changed to more than 20, since we are using malloc to allocate memory space after it.
Please reply.......

[MK27]

Anyway, in this problem i want pointer to arrays only.

You can want to use whatever you like -- cheese donuts from outer space, etc -- that does not mean it will work.

AFAIK you have two choices: you can do it the way I demonstrated in post #4, or the way quzah demonstrated in post #2.

Let me try and explain better why

you don't want pointers to arrays here, you want arrays of pointers

int (*a)[5] is a pointer to an array of 5 ints. That means (*a)[0], (*a)[1], etc. are ints. Not pointers. So you cannot go:

(*a)[row]=(int *) malloc (noc * sizeof(int));

So three more times:
you don't want pointers to arrays here, you want arrays of pointers
you don't want pointers to arrays here, you want arrays of pointers
you don't want pointers to arrays here, you want arrays of pointers

data_type (*p)[2nd Dimension];//p being a pointer to 2D array

which is equivalent to

data_type x[1st dimension][2nd dimension];//x being a 2D array

This is NOT TRUE. I suggest you not worry about (*a)[x] for now.

[rakeshkool27]

yep MK27,whatever you told is true.here we require arrays of pointers not pointers to arrays.So I am finding more about pointers in the book and trying to solve the problem..............Thanks

[rakeshkool27]

MK27, finally it was solved using arrays of pointers.So i understood to represent 2D arrays we can use arrays of pointers.I actually thought it was applicable to strings only not number arrays.I got it now.
but you said that we dont have to (cast) malloc in C, but without casting it shows compile error,so what is it then????

[MK27]

MK27, finally it was solved using arrays of pointers.So i understood to represent 2D arrays we can use arrays of pointers.I actually thought it was applicable to strings only not number arrays.I got it now.
but you said that we dont have to (cast) malloc in C, but without casting it shows compile error,so what is it then????

You mean you get an error for not including the red bit?

x = (int*)malloc(500);

Then you are using your compiler in C++ mode. It is an error in C++, but not in C.

A void pointer (the return value of malloc) can be any other kind of pointer in C, so you never have to cast them. It is not an error to do so, just it is not necessary.

[Salem]

> is declaring a,b,c as pointer to 2 dimensional arrays with their 2nd dimension as COL=20.
In which case, the single malloc to initialise each one of them reads

a = malloc ( nor * sizeof *a );
// this gets you a[nor][20]; in effect

The value of noc doesn't matter to the declaration of your pointers. You only need check that it is <= 20 for everything to work as expected.

If you really want a variable number of columns, then you need to start with
int **a;
and the appropriate 2-step malloc sequence written out several times above.

[rakeshkool27]

> is declaring a,b,c as pointer to 2 dimensional arrays with their 2nd dimension as COL=20.
In which case, the single malloc to initialise each one of them reads

a = malloc ( nor * sizeof *a );
// this gets you a[nor][20]; in effect

The value of noc doesn't matter to the declaration of your pointers. You only need check that it is <= 20 for everything to work as expected.

If you really want a variable number of columns, then you need to start with
int **a;
and the appropriate 2-step malloc sequence written out several times above.

Yes salem,,,,i understood a bit of it.
Since i am a beginner in pointer i am really stucked up and confused with these array and pointer stuffs.Do every beginner face problem initially with pointer or is it bcoz i have little concept? I dont know how will I master C......there is a long way to go........i dont even have the pointer concept clearly

[MK27]

A lot of beginners are confused by pointers. The best thing to do is study and experiment with them, which is what you are doing.

[rakeshkool27]

Anyway MK27,which compiler do you use and which one is the best to use?
I only write,compile and execute the programs and dont know anything much about the compilers.Is there anything like updating the library function or like that.
I have downloaded some of the game programming codes which i have been not able to compile due to some errors.is this the problem with the compiler or the missing library functions?Can you advice.............If anyone knows please let me know.................

[MK27]

It could be a lot of things not least of which is the code itself. Not every example or demo someone has placed on the web is guaranteed to be free from errors.

I use gcc, but I don't program on windows.