Thread: Beginner C issue ( matrix 90*)

So I have this program I made which makes a matrix rotate by 90 degrees to the left. The problem is after I run it, it does show me rotated matrix but it crashes giving me this : Gyazo - 1645b2f0f3328381d8cd773c6c96e843.png
Gyazo - f7f3a9280469291ef1331ed119d83821.png
Gyazo - 30de4561f483bb50dfc8fd40c2d1ba04.png

Code :

Code:

#include <stdlib.h>
#include <stdio.h>




int *read_matrix(int *dim)
{
    int i,j;
    int *A;
    printf("Enter the matrix dimension ");
    scanf("%d",dim);
    
    A=(int *)malloc((*dim)*sizeof(int));
    if (!A)
        {
            printf("Memory couldn't be allocated'");
            exit(1);
        }
    printf("Memory allocated succesfully \n");
    
    for(i=0;i<*dim;i++)
            for(j=0;j<*dim;j++)
                {
                    printf("A[%d][%d] = ",i+1,j+1);
                    scanf("%d",A+*dim*i+j);
                }
        
    return A;
}




int *rotate_matrix(int *A,int dim)
{
    int i,j;
    int temp;
    
    for ( i=0; i< dim/2 ;i++ )
    {


        for ( j=i;j<dim-i-1;j++ )
            {


                    temp = *(A+dim*i+j);
 
                *(A+dim*i+j) = *(A+dim*j+dim-1-i); 
 
                *(A+dim*j+dim-1-i) = *(A+dim*(dim-1-i)+dim-1-j); 


                *(A+dim*(dim-1-i)+dim-1-j) = *(A+dim*(dim-1-j)+i); 
 
                *(A+dim*(dim-1-j)+i) = temp;
               }
    }
    
 
    return A;
}






void show_matrix(int *A,int dim)
{
    int i,j;
    for(i=0;i<dim;i++)
        {
            for(j=0;j<dim;j++)
                {
                    printf("%d \t",*(A+dim*i+j));
                }
            printf("\n");
        }
}


void free_m(int *A)
    {
        if (A!=NULL)
            free(A);
    }


int main()
{
    int i,j;
    int *A;
    int dim=0;




    A=read_matrix(&dim);
    
    printf("Matrix is  : \n");
    show_matrix(A,dim);
    
    A=rotate_matrix(A,dim);
    
    printf(" The rotated matrix is :  \n");
    show_matrix(A,dim);
    
    free_m(A);
    
     system("PAUSE");
    return 0;
}

You're not allocating enough memory. Instead of malloc(*dim * sizeof(int)) it should be malloc(*dim * *dim * *sizeof(int))

Thank you, it worked.

Originally Posted by MarkussR

Thank you, it worked.

No problem. Another point is that rotate_matrix doesn't need to return the matrix since it's rotating the input matrix in place. And it can be made a little cleaner looking:

Code:

void rotate_matrix(int *A, int dim) {
    for (int row = 0; row < dim / 2; row++) {
        int row_end = dim - row - 1;

        for (int col = row; col < row_end; col++) {
            int col_end = dim - col - 1;

            int *p = A + dim * row     + col;
            int *q = A + dim * col     + row_end;
            int *r = A + dim * row_end + col_end;
            int *s = A + dim * col_end + row;

            int temp = *p;
            *p = *q;
            *q = *r;
            *r = *s;
            *s = temp;
        }
    }
}