ilan343@violet.berkeley.edu (11/21/89)
I am trying to use the standard C qsort routine to rank an array of
floats. I don't want to disturb the original data, so I tried the
following:
a) Copy the addresses of each array element into an array of pointers.
b) Invoke qsort on the array of pointers.
c) Print ranks by subtracting pointers
Well, it didn't work.
If you are in "good samaritan" mode you can have look at the code in the
next page, and tell me where I went wrong.
Thank for your help,
Geraldo Veiga
This is the output produced by the code below:
Before sort After sort
Data Rank Data Rank
0 0.427055 0 0 0.427055 0
1 0.230254 1 1 0.230254 1
2 0.020845 2 2 0.020845 5
3 0.052413 3 3 0.052413 6
4 0.052413 4 4 0.052413 4
5 0.122733 5 5 0.122733 3
6 0.094287 6 6 0.094287 2
They should be in descending order. The exact same behavior was
replicated in every compiler I tried (Turbo-C, BSD 4.3, pcc (on a
3B2)), so I can only assume I messed up somewhere.
Here is the code:
/* Test qsort routine */
#include <stdio.h>
main ()
{
float data[7], *rank[7];
int n, i, compare ();
data[0] = 0.427055; data[1] = 0.230254; data[2] = 0.020845;
data[3] = 0.052413; data[4] = 0.052413; data[5] = 0.122733;
data[6] = 0.094287;
/* Collect addresses */
for (i = 0; i < 7; i++)
rank[i] = &data[i];
/* Sort */
printf ("Before sort\n");
for (i = 0; i < 7; ++i)
printf ("%d %f %d \n", i, data[i], rank[i] - data);
(void) qsort ((void *) rank, 7, sizeof (float *), compare);
printf ("After sort\n");
for (i = 0; i < 7; ++i)
printf ("%d %f %d \n", i, data[i], rank[i] - data);
}
int
compare (p1, p2)
float **p1, **p2;
{
if (**p1 < **p2)
return(1);
else if (**p1 == **p2)
return(0);
else
return(-1);
}aic@mentor.cc.purdue.edu (George A. Basar) (11/21/89)
In article <1989Nov20.213702.20821@agate.berkeley.edu>, ilan343@violet.berkeley.edu writes: > I am trying to use the standard C qsort routine to rank an array of > floats. I don't want to disturb the original data, so I tried the > following: > > a) Copy the addresses of each array element into an array of pointers. > b) Invoke qsort on the array of pointers. > c) Print ranks by subtracting pointers No need for step c, the entries in rank are pointers which had been sorted based upon the values in data. > printf ("%d %f %d \n", i, data[i], rank[i] - data); This should be printf ("%d %f %f \n", i, data[i], *rank[i]); Since your compare checks the value in data, qsort swapped pointers in rank. Printing out the values referenced by the elements in rank in order, as above, gives a sorted list. * George A. Basar (317)742-8799 (home) * aic@mentor.cc.purdue.edu BASAR@PURCCVM.BITNET | General Consultant (317)494-1787 (work) | Purdue University Computing Center Disclaimer: My opinions are just that, mine.