Solved

problem in using qsort

Posted on 2001-06-12
4
163 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

Free Tool: Path Explorer

An intuitive utility to help find the CSS path to UI elements on a webpage. These paths are used frequently in a variety of front-end development and QA automation tasks.

One of a set of tools we're offering as a way of saying thank you for being a part of the community.

Question has a verified solution.

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

Suggested Solutions

Title # Comments Views Activity
C language IDE – Compilers installation 14 75
Getting IP address 8 89
Move constructor only called if marked noexcept? 6 108
sorting efficency of sorting algorithm 30 121
Errors will happen. It is a fact of life for the programmer. How and when errors are detected have a great impact on quality and cost of a product. It is better to detect errors at compile time, when possible and practical. Errors that make their wa…
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 pass data into a function in C++. This is one step further in using functions. Instead of only printing text onto the console, the function will be able to perform calculations with argumentents given by the user.
The viewer will be introduced to the technique of using vectors in C++. The video will cover how to define a vector, store values in the vector and retrieve data from the values stored in the vector.

830 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