Access violation reading location in dimensional matrix

Question

I would like to get your help to understand and finish my program.

This is what I have to do:

"You must exercise program that: First. An absorbing two - dimensional integer arr [M] [N]. M - number of rows N - number of columns. (Matrix size was received from the user) Two. The program uses auxiliary functions "shift" moves the values of the matrix to the right one place, as shown in the picture (2 entered instead of 1, 3 instead of 2, 4 instead of 3, ... 20 instead of 19, first place 20). Shift have to write a function and call her three times in the sample matrix loop .."

Example picture: enter image description here

error msg: enter image description here

After trying to solve the problem i gave up, i would like to get your help to understand what is wrong in my code. Is it about the memory? This is my code:

#include "stdio.h"
#include "conio.h"
#include "stdlib.h"
#include "malloc.h"

void shift (int **arr,int rows,int cols);
void freemalloc ( int **arr,int rows);

void main()
{
    int **arr,cols,rows;
    int i,j;
    printf("please insert rows and columns of the matrix: ");
    scanf_s("%d%d",&rows,&cols);
    arr=(int **)malloc(rows*sizeof(int *));
    for(i=0; i<rows; i++)
        arr[i]=(int *)malloc(cols*sizeof(int));
    for (i=0; i<rows; i++)
        for (j=0; j<cols; j++)
        {
            printf("rows %d , cols %d :\n", i, j);
            scanf_s("%d", &arr[i][j]);
        }
    shift (arr,rows,cols);
    freemalloc (arr,rows);
    system("pause");
    return ;
}

void shift (int **arr,int rows,int cols)
{
    int i,j,temp=0;
    for(i=0; i<rows ; i++ )
        for( j=0 ; j<cols ; j++);
    {
        temp=arr[i][j];
        arr[i][j]=arr[i][cols-1];
        arr[i][cols-1]=temp;
    }
    for(i=0; i<rows ; i++)
    {
        for(j=0; j<cols ; j++)
        {
            printf("%d ",arr[i][j]);
        }
        printf("\n");
    }
}

void freemalloc ( int **arr,int rows)
{
    int i;
    for (i=0 ; i<rows ; i++)
    {
        free(arr[i]);
    }
    free(arr);
}

If you include standard header files, like `stdio.h` use `` instead. Also, you should not cast the return value of functions returning `void *`, like `malloc`. — Some programmer dude, Sep 16 '13 at 15:38
As for your problem, run the program in a debugger, it will stop at the location of the crash, and you can then examine the function call stack and local variables and arguments. — Some programmer dude, Sep 16 '13 at 15:40
You will want some type of delimiter when reading the values, otherwise scanf won't be able to tell them apart and you will likely end up using an uninitialized `cols` value: `scanf_s("%d %d",&rows,&cols);` — josh poley, Sep 16 '13 at 15:46
You seem to be writing in C. Your tag says "c++". C and C++ are very different languages. — n. m. could be an AI, Sep 16 '13 at 15:47
Mark,I have to post the whole code to get help. How i should do it? — Sagi Binder, Sep 16 '13 at 15:47
I ran the debugger and i don't have the knowledge of finding what's wrong. i am newbie in this business.please point me what is wrong — Sagi Binder, Sep 16 '13 at 15:50
Does the debugger catch the access violation? Does it give you a call stack? Which line of your code causes this? — doctorlove, Sep 16 '13 at 15:54
Doctor love, it's send me to this line. in the fucntion temp=arr[i][j]; — Sagi Binder, Sep 16 '13 at 16:01
@n.m.: I've edited the tags, if you have enough reputation you can do so as well. — user7116, Sep 16 '13 at 16:52

WhozCraig · Answer 1 · 2013-09-16T16:59:21.153

I noted this in general-comment, and I'll amplify it here. This is wrong, and wiil result in undefined behavior:

for(i=0; i<rows ; i++ )
    for( j=0 ; j<cols ; j++); // <<== this is wrong.
{
    temp=arr[i][j];
    arr[i][j]=arr[i][cols-1];
    arr[i][cols-1]=temp;
}

That trailing semi-colon is outright wrong. With it, the code literally becomes this:

// A useless nested for loop that runs `i` to `rows` and `j to `cols`
for(i=0; i<rows ; i++ )
    for( j=0 ; j<cols ; j++);

// Now `i=rows` and `j=cols`. then you do this
temp=arr[i][j];
arr[i][j]=arr[i][cols-1];
arr[i][cols-1]=temp;

You're accessing memory you don't own, as arr is only indexible to [rows-1][cols-1] Anything outside that is undefined behavior.

Correction

for(i=0; i<rows ; i++ )
    for( j=0 ; j<cols ; j++) // note: no semi-colon.
    {
        temp=arr[i][j];
        arr[i][j]=arr[i][cols-1];
        arr[i][cols-1]=temp;
    }

Fix your code by removing that semi-colon, and at least that part of your problem is addressed. I cannot speak for other issues.

Finally, enable your compiler warnings to pedantic levels. Any reasonable compiler configured with properly heightened warnings would catch this issue.

score 2 · Answer 2 · edited May 23 '17 at 11:53

2

The error has been already pointed out in another answer, but let me give you another piece of good advice: do not use pointers-to-pointers to represent multidimensional arrays.

Also, do NOT cast the return value of malloc().

This is what you should do (should have done):

int (*mat)[width] = malloc(height * sizeof(*mat));

edited May 23 '17 at 11:53

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answered Sep 16 '13 at 17:04

Access violation reading location in dimensional matrix

2 Answers2