Thread: Recursion in place of looping

Hi there,
I am a new commer to the programming world. And I am finding it difficulties in the 2 questions :

* What are the "difference between Recursion and Looping"?

* What are the "conditions for which recursion can be used in place of looping"?

If any body know about this, please help?

Originally Posted by Babkockdood

goto statements, maybe? I don't know, recursion and looping sound like the same thing to me.

They're not... a loop is a group of instructions repeated several times, recursion is when a function calls itself.

Example for looping: test reading a file.

Code:

// single file test
void TestFile(PVOID FName)
  { DWORD BytesRead = 1;
    if (!Scanning)
      return;
    ++ FileCount;
    HANDLE fh = CreateFile(FName,FILE_READ_DATA,FILE_SHARE_READ,NULL,
                            OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL | 
                            FILE_FLAG_NO_BUFFERING |
                            FILE_FLAG_SEQUENTIAL_SCAN,NULL);
    if (fh == INVALID_HANDLE_VALUE)
      { AddError(GetLastError(),FName);
        return; }
    while ((BytesRead > 0) && Scanning)
      { if (!ReadFile(fh,FileBuffer,BufferSize,&BytesRead,NULL))
          { PostMessage(WHandle[0],UM_ADDERR,GetLastError(),(LPARAM)FName);
            break; }
        ByteCount += BytesRead; }
    CloseHandle(fh); }

Example for recursion: Tunneling down subdirectories in a file system.

Code:

// recursive directory search
void TestDir(PVOID SPath)  
  { HANDLE HDir;
    ++ FolderCount;
    if (!SetCurrentDirectory(SPath))
      { PostMessage(WHandle[0],UM_ADDERR,GetLastError(),(LPARAM)SPath);
        return; }
    HDir = FindFirstFile("*.*",&DirData);
    if (HDir == INVALID_HANDLE_VALUE)
      { PostMessage(WHandle[0],UM_ADDERR,GetLastError(),0);
        return; }
    do
      { if (DirData.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY)
          {if (Recurse && (DirData.cFileName[0] != '.'))
             TestDir(DirData.cFileName); } 
        else
          TestFile(DirData.cFileName); }
    while (FindNextFile(HDir,&DirData) && Scanning);
    FindClose(HDir);
    SetCurrentDirectory(".."); }

Is there ever a time when a recursive solution cannot be replaced by one using loops, or v.v? Certainly some problems will intuitively favor one over the other, but you can always use either, no?

Originally Posted by CommonTater

They're not... a loop is a group of instructions repeated several times, recursion is when a function calls itself.

If you're bored, google stackless recursion. You'll get something like this:

Stack Free Recursion


Quzah.

Originally Posted by KBriggs

Is there ever a time when a recursive solution cannot be replaced by one using loops, or v.v?

No, it is always possible to rewrite the code from one to the other.

Originally Posted by KBriggs

Certainly some problems will intuitively favor one over the other, but you can always use either, no?

Yes. I'd say that CommonTater misread this: we're not saying that looping and recursion is the same thing, but that they can be used to achieve the same net effect (assuming no stack overflow and such).

Originally Posted by dnayak382

Specify the conditions for which recursion can be used in place of looping?

I can think of two conditions:
1. The original poster (dnayak382) shows an attempt to solve the problem by him or herself, and stops disguising an imperative as though it were a question.

2. The other condition I wont share just yet, though another poster has essentially mentioned the condition.