Bumping my object on a linked list

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Bumping my object on a linked list

 
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  #1
Nov 28th, 2005
I have a simple plane object with public datamembers of a flight number and a pointer. I am successfully adding "planes" and deleting "planes," however when I try to move one up a spot, though it compiles well, I seem to get memory crashes.

Here is my bump function that passes the lead pointer for my linked list. I put cout statements in the function to see if i could pin point where it was crashing and it seems to be in my while loop. I'm sort of a newb really so im sure im doing something fairly obvious that is incorrect.


C++ Syntax (Toggle Plain Text) 
  1. void bumpPlane  (plane*& pstart)
  2. {
  3. 	int flight;
  4. 	cout << " Enter the flight number you would like to move ahead" << endl;
  5. 	cin >> flight;
  6. 	plane* ptarget = NULL;
  7. 	plane* pcurrent;
  8. 	pcurrent = pstart;
  9. 	plane* pbefore = NULL;
  10.  
  11. 	if(pstart == NULL)
  12. 		cout << "No planes on the queue" << endl;
  13.  
  14.  
  15. 	while (pcurrent != NULL)
  16. 	{
  17. 		if(pcurrent->flightnum == flight)
  18. 			ptarget = pcurrent;
  19.  
  20. 		else
  21. 		{
  22. 			pbefore = pcurrent;
  23. 			pcurrent = pcurrent->pnext;
  24.  
  25. 		}
  26. 	}
  27. 	if (ptarget = NULL)
  28. 	{
  29. 		cout << "Could not find flight number in the queue" << endl;
  30. 		return;
  31. 	}
  32.  
  33. 	plane* psave = NULL;
  34.  
  35. 	psave = pcurrent->pnext;
  36.  
  37. 	pcurrent->pnext = pbefore;
  38.  
  39. 	pbefore = psave;
  40.  
  41. }



ps. My first post here, though im a lurker and like to browse around here to pump me up to do my homework.
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Re: Bumping my object on a linked list

 
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  #2
Nov 28th, 2005 
void bumpPlane  (plane*& pstart)
{
	int flight;
	cout << " Enter the flight number you would like to move ahead" << endl;
	cin >> flight;
	plane* ptarget = NULL;
	plane* pcurrent;
	pcurrent = pstart;
	plane* pbefore = NULL;

	if(pstart == NULL)
	{		// better to get used to using braces even if there is only one statement after the if condition
		cout << "No planes on the queue" << endl;
		 return; 		 // thought you might want to return if the list is empty 
	}


	while (pcurrent != NULL)
	{
		if(pcurrent->flightnum == flight)
		{
			ptarget = pcurrent;
			break;	// thought you might want to break the loop if a match is found
		}
		else
		{
			pbefore = pcurrent;
			pcurrent = pcurrent->pnext;

		}
	}
	if (ptarget ==  NULL)  // == not = ( this is one of the places where most newcomers get it wrong.  
	{
		cout << "Could not find flight number in the queue" << endl;
		return;
	}

	 // there is no error in here. But is this what you want to do?
	// At start
	//		---->A-----> B------> C 
	plane* psave = NULL;

	psave = pcurrent->pnext;

	pcurrent->pnext = pbefore;

	pbefore = psave;
	 // After the above code it becomes
	//	---->A-------------->C
	//	     |
	//	     B
	// You will not be able to reach B after this. Is that ok with your objective? 

}
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Re: Bumping my object on a linked list

 
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  #3
Nov 29th, 2005
[edit] - I think instead of swirling around a ton of pointers im just going to swap the data members in the objects not the objects themselves.



Actualy, losing 'b' is exactly my problem. What I would like it to do if for excample 'c' was the target, would be.

a--->c--->b.

C++ Syntax (Toggle Plain Text) 
  1. void bumpPlane  (plane*& pstart)
  2. {
  3. 	int flight;
  4. 	cout << " Enter the flight number you would like to move ahead" << endl;
  5. 	cin >> flight;
  6. 	plane* ptarget = NULL;
  7. 	plane* pcurrent;
  8. 	pcurrent = pstart;
  9. 	plane* pbefore = pstart;
  10. 	plane* pbefore2 = pstart;
  11. 	plane* pafter = pstart;
  12.  
  13. 	if(pstart == NULL)
  14. 		cout << "No planes on the queue" << endl;
  15.  
  16.  
  17. 	while (pcurrent != NULL)
  18. 	{
  19. 		if(pcurrent->flightnum == flight)
  20. 		{	
  21. 			ptarget = pcurrent;
  22. 			pafter = pcurrent->pnext;
  23. 			cout << "target found" << endl;
  24.  
  25. 			pcurrent = NULL;
  26. 		}
  27. 		else
  28. 		{
  29. 			pbefore2 = pbefore;
  30. 			pbefore = pcurrent;
  31. 			pcurrent = pcurrent->pnext;
  32. 			cout << "target not found, moving pointers"<< endl;
  33.  
  34.  
  35.  
  36.  
  37. 		}
  38. 	}
  39. 	if (ptarget = NULL)
  40. 	{
  41. 		cout << "Could not find flight number in the queue" << endl;
  42. 		return;
  43. 		cout << "ok inside next iff" << endl;
  44. 	}
  45.  
  46.  
  47. 	plane* psave = NULL;
  48.  
  49. 	psave = pbefore2->pnext;
  50.  
  51. 	pbefore2->pnext = pbefore->pnext;
  52.  
  53. 	pbefore->pnext = pafter;
  54. 	ptarget->pnext = psave;
  55.  
  56. }

This program crashes from the very last line in the program. (Even if I move it up a line or two.)

If I remove the last line and run the program I loose the object before my target.
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Re: Bumping my object on a linked list

 
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  #4
Nov 29th, 2005
Try this
C++ Syntax (Toggle Plain Text) 
  1.         plane* psave = NULL;
  2.         psave = pbefore2->pnext;
  3.  
  4. 	pbefore2->pnext = pbefore->pnext;
  5. 	psave->pnext = ptarget->pnext;
  6.  
  7. 	ptarget->pnext = psave;

Correct the other mistakes I showed you in my previous reply and the one below.

	if (ptarget = NULL)
	{
		cout << "Could not find flight number in the queue" << endl;
		return;
		cout << "ok inside next iff" << endl; //You wont reach this line also
	}

Check what happens if the target is within the first 2 nodes of the list. Wont it crash in that case? Run it and see.
Also try to use the STL list next time if possible.
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