Using qsort on arrays of sequential data

An introduction to a useful function

Introduction

qsort is part of the C and C++ language. It is a quick sort algorithm that is both fast and easy to implement. It is a recursive algorithm, but it is surprising fast. (30,000 items in about a fourth of a second on a PII-333 machine.)

This 'How To' demonstrates the use of qsort on a CArray.
However, it would work just as well on any other sequential data structure. Static arrays, CStringArray, CPtrArray or something of your own creation.

The demo project is written in VC 5.0. It has not been plagiarized from any other source. There are no restrictions on the source.

Sorting

Suppose you wanted to store and sort number of items of type CArrayClass, defined as follows:

class CArrayClass  
{
public:
  CArrayClass();
  virtual ~CArrayClass();
  
  WORD    m_wMsgId;
  CString m_strMsgType;
};

Using the CArray template class we can store the CArrayClass objects using the following array declaration

typedef CArray<CArrayClass, CArrayClass&> Type_aCArrayClass;
Type_aCArrayClass m_aCArrayClass;

To use qsort you need to declare a callback function whose address is known at compile time - meaning it must either be a global or static function. The qsort callback function must be defined as:

int (__cdecl *compare )(const void *elem1, const void *elem2 )

Items elem1 and elem2 are pointers to two items in the array, and must be cast to the appropriate type and compared. The return value of the function is as follows:

Return Value	Description
< 0	elem1 less than elem2
0	elem1 equivalent to elem2
> 0	elem1 greater than elem2

In our case the callback function would look like the following:

// This function can be global or it can be a static member of your class,
// but it's address must be know at compile time.
int CCArrayExampleDlg::SortTheArray(void* p1,void* p2)
{
  short n=0;
  CArrayClass* a1 = (CArrayClass*)p1;
  CArrayClass* a2 =  (CArrayClass*)p2; // If we were sorting a CPtrArray then we 
                                       // would have to do a second level 
                                       // of casting to get to the data keys that we 
                                       // would be sorting.
  
  if (a1->m_strMsgType  <a2->m_strMsgType)    //Ascending
      n = -1; 
  elseif (a1->m_strMsgType  >a2->m_strMsgType)
      n = 1; 
  elseif(a1->m_wMsgId  <a2->m_wMsgId)    //Ascending
      n = -1; 
  elseif(a1->m_wMsgId  >a2->m_wMsgId)
      n = 1;
  
  return n;
}

To use this sort callback we need to fill the array, call qsort and then display the results.

void CCArrayExampleDlg::OnSort() 
{ 
  // Fill the array m_aCArrayClass with items of type CArrayClass 
  ...
  
  // Sort the array 
  if(m_aCArrayClass.GetSize() >  0) 
  {
      qsort((void*)&m_aCArrayClass[0],
            (size_t)m_aCArrayClass.GetSize(), 
            sizeof(CArrayClass),
            (int(*)(const void*, const void*))CCArrayExampleDlg::SortTheArray);
  }
  
  // Display the newly sorted array
  ...
}