matrix rotation..for loop changes the value of pointer - c

Question

In the final value of array only first element becomes zero and that too when it again goes to the for loop(checked using gdb)..i have mentioned the problem using comments at the bottom of code.Help me out.. I have no clue of what is going wrong.
#include<stdio.h>
#include<stdlib.h>
int main()
{
int a, b, c;
printf("enter the size of matrix");
scanf("%d%d",&a,&b);
printf("enter the number of rotations");
scanf("%d",&c);
int *arr = malloc (sizeof(int) * a * b);
int x = (a >= b)? a : b;
printf("enter the values of matrix");
// scanning the values
for(int i = 0; i < a; i++)
{
for(int j = 0; j < b; j++)
{
scanf("%d",(arr + i * b + j));
}
printf("\n");
}
// main code starts
for(int y = 0; y < c; y++)
{
// declared a new array
int *arr1 = malloc (sizeof(int) * a * b);
for(int k = 0; k < x / 2; k++)
{
for(int i = k; i < a - k; i++)
{
for(int j = k; j < b - k; j++)
{
if (i == k && j > k)
{
*(arr1 + i * b + j - 1) = *(arr + i * b + j);
}
else if (i == a - k - 1 && j < b - k - 1)
{
*(arr1 + i * b + j + 1) = *(arr + i * b + j);
}
else if (j == k && i < a - k - 1)
{
*(arr1 + i * b + j + b) = *(arr + i * b + j);
}
else if (j == b - k - 1 && i > k)
{
*(arr1 + i * b + j - b) = *(arr + i * b + j);
}
}
}
if (x % 2 != 0 && a == b)
*(arr1 + x / 2 * b + (b / 2)) = *(arr + x / 2 * b + (b / 2));
}
// changing the old array to new array
arr = arr1;
// first value is getting printed correctly here
printf("%d\n",*(arr));
printf("%p\n",&(*arr));
free(arr1);
}
// printing the output
for(int i = 0; i < a; i++)
{
for(int j = 0; j < b; j++)
{
printf("%d ",*(arr + i * b + j));
}
printf("\n");
}
// first value is getting printed incorrectly here, outside the loop
printf("\n%d\n",*(arr));
printf("%p",&(*arr));
}

Answer 1

C doesn't support array assignment. You have:
int *arr = malloc (sizeof(int) * a * b);
…
int *arr1 = malloc (sizeof(int) * a * b);
…
arr = arr1;
…
free(arr1);
The assignment means you've lost your original array (memory leak) and you then invalidate your new array with the free().
Array copying requires more code — usually a function call such as memmove() or memcpy(), possibly wrapped in a function.
For example, add #include <string.h> and use this in place of the arr = arr1; assignment:
memmove(arr, arr1, sizeof(int) * a * b);
free(arr1); // No longer needed
Alternatively:
free(arr);
arr = arr1;
This code runs cleanly under valgrind on Mac OS X 10.11.5 with GCC 6.1.0 with the 'Either' or the 'Or' options for handling the array assignments.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
static void dump_matrix(const char *tag, int *arr, int a, int b)
{
printf("Matrix: %s\n", tag);
for (int i = 0; i < a; i++)
{
for (int j = 0; j < b; j++)
printf(" %3d", arr[i * b + j]);
putchar('\n');
}
}
int main(void)
{
int a, b, c;
printf("enter the size of matrix: ");
scanf("%d%d", &a, &b);
printf("enter the number of rotations: ");
scanf("%d", &c);
int *arr = malloc(sizeof(int) * a * b);
int x = (a >= b) ? a : b;
printf("enter the values of matrix: ");
// scanning the values
for (int i = 0; i < a; i++)
{
for (int j = 0; j < b; j++)
{
if (scanf("%d", (arr + i * b + j)) != 1)
{
fprintf(stderr, "failed to read value arr[%d][%d]\n", i, j);
return EXIT_FAILURE;
}
}
printf("\n");
}
dump_matrix("Initial input", arr, a, b);
// main code starts
for (int y = 0; y < c; y++)
{
// declared a new array
int *arr1 = malloc(sizeof(int) * a * b);
for (int k = 0; k < x / 2; k++)
{
for (int i = k; i < a - k; i++)
{
for (int j = k; j < b - k; j++)
{
if (i == k && j > k)
{
*(arr1 + i * b + j - 1) = *(arr + i * b + j);
}
else if (i == a - k - 1 && j < b - k - 1)
{
*(arr1 + i * b + j + 1) = *(arr + i * b + j);
}
else if (j == k && i < a - k - 1)
{
*(arr1 + i * b + j + b) = *(arr + i * b + j);
}
else if (j == b - k - 1 && i > k)
{
*(arr1 + i * b + j - b) = *(arr + i * b + j);
}
}
}
if (x % 2 != 0 && a == b)
*(arr1 + x / 2 * b + (b / 2)) = *(arr + x / 2 * b + (b / 2));
}
// Changing the old array to new array
// Either:
// memmove(arr, arr1, sizeof(int) * a * b);
// free(arr1);
// Or:
free(arr);
arr = arr1;
dump_matrix("After rotation", arr, a, b);
}
dump_matrix("Finished", arr, a, b);
free(arr);
return 0;
}
Note the use of the dump_matrix() function. Writing such a function once means it can be used multiple places in the code. The tag argument simplifies the use. The 'commercial grade' variant takes a FILE *fp argument too and writes to the specified file stream.
Note the error checking on the main input loop scanf(). I should also have checked the two other scanf() statements. Errors are reported on standard error, of course.
Example run:
$ ./mat31
enter the size of matrix: 3 4
enter the number of rotations: 2
enter the values of matrix: 1 2 3 4 10 11 12 13 99 98 97 96
Matrix: Initial input
1 2 3 4
10 11 12 13
99 98 97 96
Matrix: After rotation
2 3 4 13
1 12 11 96
10 99 98 97
Matrix: After rotation
3 4 13 96
2 11 12 97
1 10 99 98
Matrix: Finished
3 4 13 96
2 11 12 97
1 10 99 98
$
Whether the output is what you intended is a wholly separate discussion. This is simply not abusing the memory.