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.