Thread: Very simple Question About Pointer Passing in C

Originally Posted by sorayaj

why should I write "(int *)" for passing the matrix to the readMatrix function? Is it passing the pointer to the location of the matrix[4][4] in memorry?

When an array is passed as an argument, it is converted to a pointer to its first element. matrix is an int[4][4], so when passed as an argument an int(*)[4] is passed instead, but this is not compatible with the int* parameter, hence the cast to silence the likely compiler warning.

Originally Posted by sorayaj

can I pass it using ampersand like &matrix?

Yes, and I feel that it would be better style:

Code:

readMatrix("matrix1.txt", &matrix[0][0]);

On the other hand, readMatrix assumes a 4 by 4 matrix, so I'd argue that even better yet would be to change readMatrix:

Code:

void readMatrix(char *filename, int (*matrix)[4])
{
    FILE *fp = fopen(filename, "r");
    if (fp)
    {
        int i, j, val;

        for (i = 0; i < 4; i++)
        {
            for (j = 0; j < 4; j++)
            {
                if (fscanf(fp, "%d", &val) == 1)
                {
                    matrix[i][j] = val;
                }
                else
                {
                    /* record a special error value? report a read error? */
                }
            }
        }
        fclose(fp);
    }
    else
    {
        /* report a file open error? */
    }
}

Now you can write:

Code:

void p1(void)
{
    int matrix[4][4];
    readMatrix("matrix1.txt", matrix);
}

EDIT:
Incidentally, filename should be a const char* rather than just a char* because you clearly do not intend to modify the string.