Thread: problem in reading value of 2 D array

In the program the 2 D array whose rows and columns are to be given at run time has to be displayed as a spiral i.e. from outer rows and columns to inner ones in clockwise direction .
The code is not working for rows > 4 and columns > 4 .

The scanf function is not reading correct value in the pointer.

Logic of program ( for clarity of source ):
1) store value in 2D array using pointer
2) display top row
3) storing its remaining values in another 2D array ( strored such that it spins as well )
4) displaying top row of this array
5) continuing till columns are more than 1.

Code:

#include<stdio.h>
#include<conio.h>

void main()
{
	int **ptr,**ptr1,i,j,row1,row,col;
	ptr1=NULL;

	clrscr();

	printf("\nenter no of rows ");
	scanf("&#37;d",&row);

	printf("\nenter no of columns ");
	scanf("%d",&col);

	printf("\nenter matrix\n");

	for(i=0;i<row;i++)
	{
		for(j=0;j<col;j++)
		{
			scanf("%d", &ptr[i][j] );  /* not reading correct value of input */
		}
	}

	while(row>0 && col>0)
	{
		for(j=0;j<col;j++)
			printf("%d   ",ptr[0][j]);

		if(row>1)
		{
			for(i=0;i<col;i++)
			{
				for(j=0;j<row-1;j++)
				{
					ptr1[i][j]=ptr[j+1][(col-1)-i];
				}
			}
		}
		row1=row;
		row=col;
		col=row1-1;
		ptr=ptr1;
	}
	getch();
}

Well, that allocates enough memory. But the way a pointer to pointer works, if you do ptr[i][j] or some such, it will go look for a pointer at location ptr+i, and that's most likely not a valid pointer to the location you want.

As suggested, go look for an example of 2D array allocation.

Also, in C, you should NOT cast the result of malloc(), as it can hide hard to find bugs.

--
Mats

A bit off topic but casting of the return value of malloc() is quite normal. It prevents compiler warnings about mismatched data types especially at level-4 (highest persnickety setting).

As long as the appropriate headers are included (function prototypes) such that physically the proper sized value is returned.

You can not expect malloc()'s result to match the data type for your needs. It's not clairvoyant.

You tell malloc how many bytes you'd like, what you do with the memory is upto you. implicit void promotion is legal, hell that's one reason why void exists.

> mismatched data types especially at level-4 (highest persnickety setting).
Between void and x...?

Originally Posted by nonoob

A bit off topic but casting of the return value of malloc() is quite normal. It prevents compiler warnings about mismatched data types especially at level-4 (highest persnickety setting).

Not in C. If it does, then it's a broken compiler/warning.
Implicit conversion from void* to T* is perfectly valid in C (though not C++, where a cast is required).

use this,

Code:

  ptr = (int**)malloc(row*sizeof(int*));// allocates memory for rows

  for(count=0;count<row;count++)
      ptr[count]= (int*)malloc(column*sizeof(int)); //allocates memory for columns at each row

now you can use ptr[][] just as a 2D array.

Originally Posted by san_crazy

use this,

Code:

  ptr = (int**)malloc(row*sizeof(int*));// allocates memory for rows

  for(count=0;count<row;count++)
      ptr[count]= (int*)malloc(column*sizeof(int)); //allocates memory for columns at each row

now you can use ptr[][] just as a 2D array.

http://faq.cprogramming.com/cgi-bin/...&id=1043284351