How can I iterate a 2 dimensional array via pointers?

Question

I have a 20x20 matrix. I want to extract chunks of data from the matrix. I have

int theMatrix[20][20] = {};//Full code initializes with #'s
int *theMatrixPointer = &theMatrix;

But then I get a compiler warning saying

warning: initialization from incompatible pointer type

I went ahead and ran the code and it looks to be moving across the matrix from left to right. At least in the short run. The implementation:

//left to right with pointer;
while(theMatrixPointer)
{
    int one = 0;
    int two = 0;
    int three = 0;
    int four = 0;
    double currentProduct = 0;
    //int stop = 0;
    for(int i = 0; i < depth; i++)
    {
        /*if(!theMatrixPointer)
        {
            stop = 1;
            break;
        }*/
        int currentValue = *theMatrixPointer++;
        printf("%d\n",currentValue);
        if(i == 0)
        {
            one = currentValue;
        }
        else if (i == 1)
        {
            two = currentValue;
        }
        else if (i == 2)
        {
            three = currentValue;
        }
        else if (i == 3)
        {
            four = currentValue;
        }
    }
    /*if(stop)
        break;*/
    currentProduct = one * (double)two * three * four;
    printf("PRODUCT: %f\n",currentProduct);
    startPoint++;
    theMatrixPointer = startPoint;
}

...breaks over time as the data is garbage (big ints that are not in the matrix). So, how can I properly iterate this matrix with a pointer?

Oliver Charlesworth · Accepted Answer · 2011-04-28 20:32:32Z

1

Firstly, the reason you get the warning is because &theMatrix is of type int(*)[20][20], whereas theMatrixPointer is of type int *. You want this:

int *theMatrixPointer = &theMatrix[0][0];

Secondly, the reason you are getting garbage is because you're going past the end of the array. while (theMatrixPointer) will iterate until theMatrixPointer == 0. But remember that theMatrixPointer is an address. This won't be 0 until you've iterated over the entire address space and wrapped back round!

You are probably best off doing this:

int i, j;
for (i = 0; i < 20; i++)
{
    for (j = 0; j < 20; j++)
    {
        // matrixPointer[i][j] is the current element
    }
}

answered Apr 28, 2011 at 20:32

Oliver Charlesworth

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2 Comments

P.Brian.Mackey Over a year ago

The problem with going the route of i,j index iteration is that the algorithm I planned out doesn't work. I need to pull blocks from the matrix 4 at a time. After each pull the next index needs to go up by one with rinse-repeat. On a similar, but different problem I solved this cleanly with a string and pointers, but this matrix is proving far more challenging. I may need to rethink my approach.

P.Brian.Mackey Over a year ago

@ssegvic Thanks guys I now know what it takes to do this with pointers.

Community · Accepted Answer · 2017-05-23 11:55:50Z

1

Check my answer to a similar question here. Basically, I think that dealing with theMatrix[20*20] is a wiser default approach than dealing with theMatrix[20][20].

edited May 23, 2017 at 11:55

CommunityBot

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answered Apr 28, 2011 at 20:36

ssegvic

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