Solved

problem in using qsort

Posted on 2001-06-12
4
160 Views
Last Modified: 2010-04-02
Hi,

   I am writing a C++ program with the use of qsort to sort a double array.

   part of the code is as follow:

int CGetAvgTopInfoGain::compare(const void * a, const void * b) {

        double *x, *y;
        x=(double*)a;
        y=(double*)b;

        if (&x<&y)
                return -1;
        else if (&x==&y)
                return 0;
        else if (&x>&y)
                return 1;
}                                      

feature is a double array with size 500;
size is an integer which equals 20;
when I try to call the following method
  qsort (feature_IG, size, SIZE_DOUBLE, compare);

CGetAvgTopInfoGain.cc:74: argument passing to `void *' from `double'
is always resulted. Could you help me to solve this problem?

0
Comment
Question by:godman_polly
4 Comments
 
LVL 2

Accepted Solution

by:
GloriousRain earned 50 total points
ID: 6184517
try leave compare function alone, not in CGetAvgTopInfoGain class, godman_polly.
0
 
LVL 14

Expert Comment

by:AlexVirochovsky
ID: 6184523
   if (&x<&y) (and same) -> error!
must be
   if (*x < *y)
0
 

Author Comment

by:godman_polly
ID: 6184565
It seems that the problem is not as you suggest.
0
 
LVL 9

Expert Comment

by:jasonclarke
ID: 6185426
There is a much better way of sorting stuff than using qsort in modern C++.  You could get rid of your compare function altogether add an include:

#include <algorithm>

and then just call:

std::sort(feature_IG, feature_IG+size);

apart from being far simpler to work with, this will result in *quicker* sorting in most cases.

If you insist on using quicksort, then both above comments appear to be correct.  Your compare function cannot be a member function and you need to use the * operator rather than & to get the actual values of the doubles.


0

Featured Post

Does Powershell have you tied up in knots?

Managing Active Directory does not always have to be complicated.  If you are spending more time trying instead of doing, then it's time to look at something else. For nearly 20 years, AD admins around the world have used one tool for day-to-day AD management: Hyena. Discover why

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

What is C++ STL?: STL stands for Standard Template Library and is a part of standard C++ libraries. It contains many useful data structures (containers) and algorithms, which can spare you a lot of the time. Today we will look at the STL Vector. …
Go is an acronym of golang, is a programming language developed Google in 2007. Go is a new language that is mostly in the C family, with significant input from Pascal/Modula/Oberon family. Hence Go arisen as low-level language with fast compilation…
The viewer will learn how to user default arguments when defining functions. This method of defining functions will be contrasted with the non-default-argument of defining functions.
The viewer will learn additional member functions of the vector class. Specifically, the capacity and swap member functions will be introduced.

778 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question