Determining row indices in Sorting Comparison function

The Sorting mechanism ( SortItems() ) of CListCtrl, does not provide us with the row indices
inside the Comparison function. Without the row indices, the easiest way to get to our data inside
the function is to store the text data in memory and set it’s pointer as the LPARAM for each row.
If the requirement is to sort the ListCtrl based on the item text alone, we shouldn’t have to waste
this memory. In such cases the following code can be used. It uses the FindItem() to determine the
row index by using the LPARAM passed to the comparison function. Once we have the row index, we can
use the GetItemText() to get the text. The pre-requisite of using this method is to set the LPARAM
of each row to a unique value – a simple serial number will do. Compared to the sorting routines
based on QuickSort provided by many in this site, this method was faster upto 5000 rows. After
this threshold the FindItem() becomes slower and the custom sorting routines were much more faster.

/////////////////////////////////////////////////////////////////////////////
// SortInfo structure definition
typedef struct tagSortInfo
{
CListCtrl * pListControl;
int nColumnNo;
bool nAscendingSortOrder;
}SortInfo;

/////////////////////////////////////////////////////////////////////////////
// CTestDialog dialog

class CTestDialog : public CDialog
{
public:
CTestDialog(CWnd* pParent = NULL); // standard constructor
//Dialog Data
//{{AFX_DATA(CTestDialog)
enum { IDD = IDD_DIALOG1 };
CListCtrl m_ListControl;
//}}AFX_DATA
// Overrides
// ClassWizard generated virtual function overrides
//{{AFX_VIRTUAL(CTestDialog)
protected:
virtual void DoDataExchange(CDataExchange* pDX); // DDX/DDV support
//}}AFX_VIRTUAL

static int CALLBACK SortList(LPARAM lpOne, LPARAM lpTwo, LPARAM lpArg);
// Implementation
SortInfo m_SortInfo;
protected:
// Generated message map functions
//{{AFX_MSG(CTestDialog)
//}}AFX_MSG
afx_msg void OnHeaderClicked(NMHDR* pNMHDR, LRESULT* pResult);

DECLARE_MESSAGE_MAP()
};

/////////////////////////////////////////////////////////////////////////////
//
void CTestDialog::OnHeaderClicked(NMHDR* pNMHDR, LRESULT* pResult)
{
HD_NOTIFY *phdn = (HD_NOTIFY *) pNMHDR;
if(phdn->iButton == 0)
{
if (phdn->iItem == m_SortInfo.nColumnNo)
m_SortInfo.nAscendingSortOrder = !m_SortInfo.nAscendingSortOrder;
else
m_SortInfo.nAscendingSortOrder = TRUE;

m_SortInfo.nColumnNo = phdn->iItem;
m_SortInfo.pListControl = &m_ListControl;

m_ListControl.SortItems( SortList,(DWORD)&m_SortInfo );
}
*pResult = 0;
}

int CALLBACK CTestDialog::SortList(LPARAM lpOne, LPARAM lpTwo, LPARAM lpArg)
{
int nResult = 0;
SortInfo * pSortInfo = (SortInfo *) lpArg;

int nColumnNo = pSortInfo->nColumnNo;
CListCtrl * pListControl = pSortInfo->pListControl;
bool nAscendingSortOrder = pSortInfo->nAscendingSortOrder;

int lFirstData = -1, lSecondData = -1;

LV_FINDINFO FindInfo;
// use LVFI_WRAP for cases where lpTwo represents a row before lpOne
FindInfo.flags = LVFI_PARAM | LVFI_WRAP;
FindInfo.lParam = lpOne;
lFirstData = pListControl->FindItem(&FindInfo);

FindInfo.lParam = lpTwo;
// reduce searching time by setting the start row as lFirstData
lSecondData = pListControl->FindItem(&FindInfo,lFirstData);
// because we are searching for LPARAM sent to us, FindItem() on
// these values should always be successful
ASSERT(lFirstData != -1); ASSERT(lSecondData != -1);

CString FirstText = pListControl->GetItemText(lFirstData,nColumnNo);
CString SecondText = pListControl->GetItemText(lSecondData,nColumnNo);

int nCompareValue = FirstText.Compare(SecondText);
nResult = nCompareValue * ((nAscendingSortOrder)?1:-1);
// or
// your own comparison code

return nResult;
}

Last updated: 5 August 1998

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