qsort question

Hello all -

Is there any way to pass a parameter to the comparison function with qsort without using a global variable?

My goal is to sort the rows of a 2-D matrix based on a particular column. I have a working version that does use globals:

Code:

---

static int nSortColumn = 0; /* global column index used by sortrows */
void sortrows(double** pMatrix,int nStartRow, int nStopRow, int nColumn)
{
        nSortColumn = nColumn;
        qsort((void*) (pMatrix+nStartRow),nStopRow-nStartRow+1,sizeof(double*),&sortrowsCompareFcn);
}

int sortrowsCompareFcn(const void * a,const void * b)
{
        double** val1 = (double**) a;
        double** val2 = (double**) b;

        if(val1[0][nSortColumn] < val2[0][nSortColumn]  )
        {
                return -1;
        }
        else if(val1[0][nSortColumn] > val2[0][nSortColumn]  )
        {
                return 1;
        }
        else
        {
                return 0;
        }
}

---

Is a better way to tell the comparison function which column to sort by?

Thanks!

Quote:

---

Originally Posted by Hops

Is [there] a better way to tell the comparison function which column to sort by?

---

In C? Nope not really, assuming you're not willing to make one comparison function for each possible sort column, or store a pointer to the sort column in the matrix itself.

For small numbers of sorting, it could be done pretty easily this way:

Code:

---

inline int sortrowsCompareFcn(const void * a,const void * b, int nSortColumn)
{
        double** val1 = (double**) a;
        double** val2 = (double**) b;

        if(val1[0][nSortColumn] < val2[0][nSortColumn]  )
        {
                return -1;
        }
        else if(val1[0][nSortColumn] > val2[0][nSortColumn]  )
        {
                return 1;
        }
        else
        {
                return 0;
        }
}

#define SORTNAME(n) sortrowsCompareFcn##n
#define SORTN(n) int SORTNAME(n)(const void * a,const void * b) { sortrowsCompareFcn(a, b, n); }

SORTN(0)
SORTN(1)
SORTN(2)
SORTN(3)
SORTN(4)
SORTN(5)
SORTN(6)
SORTN(7)
SORTN(8)
SORTN(9)
SORTN(10)

typedef int (*SortFunc)(void *a, void *b);
SortFunc sortFunctions[11] = {
  SORTNAME(0),
  SORTNAME(1),
  SORTNAME(2),
  SORTNAME(3),
  SORTNAME(4),
  SORTNAME(5),
  SORTNAME(6),
  SORTNAME(7),
  SORTNAME(8),
  SORTNAME(9),
  SORTNAME(10),
}

void sortrows(double** pMatrix,int nStartRow, int nStopRow, int nColumn)
{
        qsort((void*) (pMatrix+nStartRow),nStopRow-nStartRow+1,sizeof(double*), sortFunctions[n]);
}

---

--
Mats

Quote:

---

Originally Posted by iMalc

In C? Nope not really, assuming you're not willing to make one comparison function for each possible sort column, or store a pointer to the sort column in the matrix itself.

---

How would one store a pointer in the matrix itself? Does that mean that I would need a copy of the pointer for each row of the matrix?

Quote:

---

Originally Posted by Hops

How would one store a pointer in the matrix itself? Does that mean that I would need a copy of the pointer for each row of the matrix?

---

I don't know what iMalc was trying to say, but whatever it was, I'm sure it would require some change to your datastructure itself.

--
Mats

Quote:

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Originally Posted by matsp

I don't know what iMalc was trying to say, but whatever it was, I'm sure it would require some change to your datastructure itself.

--
Mats

---

Yes, the data structure would have to be changed to some struct instead of just a double**. Of course being C you lose the ability to use the [] operator then. Or you could be really evil and cast the pointer to a double or something horrid.

I had thought of the array of function pointers, but hadn't thought of the macro trickery to make it easier to produce the array of them.

By the way, I just thought of another trick that will do it:

Code:

---

void sortrows(double** pMatrix,int nStartRow, int nStopRow, int nColumn)
{
        sortrowsCompareFcn(NULL, &nColumn);  // Set up our sort column.
        qsort((void*) (pMatrix+nStartRow), nStopRow-nStartRow+1, sizeof(double*), &sortrowsCompareFcn);
}

int sortrowsCompareFcn(const void * a, const void * b)
{
        static int nSortColumn= 0;
        if (a == NULL)
        {
                nSortColumn = *(int*)b;
                return 0;
        }

        double** val1 = (double**) a;
        double** val2 = (double**) b;

        if (val1[0][nSortColumn] < val2[0][nSortColumn])
                return -1;
        if (val1[0][nSortColumn] > val2[0][nSortColumn])
                return 1;
        return 0;
}

---

static != global :)
This works since the params can never be NULL during the sort.

So yeah I take back what I said about it not being possible.