Thread: Dynamic memory in C

In this case, the only function I see that needs malloc() is the InputRecord function, I took temp out of global and added that to ResetList and changed from using if to switch statements in main. To give an update on what my whole code looks like now this is it:

Code:

#include <stdlib.h>
#include <stdio.h>

struct point
{
	int x; //point x
	int y; //point y
	char label[21]; //label of 2 points
	struct point *ptrNext;	// self referential pointer
};

struct point *ptrFirst = NULL;
struct point *ptrLast = NULL;
struct point *ptrNew = NULL;

int isEmptyList(struct point *ptrF);
void PrintList(struct point *ptrF);
void ResetList(struct point *ptrF, struct point *ptrL);
void AddToBeginning(struct point *ptrF, struct point *ptrL);
void AddToEnd(struct point *ptrF, struct point *ptrL);
void InputRecord(struct point *ptrNew); // used by Add to interactively get the values from the user
int isEmptyList(struct point *ptrF)//emptylist will determine if the list is empty
{
   {
		if(ptrF==NULL)
		{
			return 0;
		}
		else
			return 1;
   }
}
//print list will print out everything
void PrintList(struct point *ptrF)
{
	struct point *pC = ptrF;
	while(pC != NULL)
   {
		printf("Label: %s\nx: %d y:%d\n", pC->label, pC->x, pC->y);
		pC = pC->ptrNext;  // this is how a list is traversed
   }	
}
//reset list will make everything equal to NULL
void ResetList(struct point *ptrF, struct point *ptrL)
{
	struct point *temp=NULL;
	while(ptrF!=NULL)
	{
		temp=ptrF->ptrNext;
		free(ptrF);
		ptrF=temp;
	}
}
//addtobeginning will add a number to the beginning
void AddToBeginning(struct point *ptrF, struct point *ptrL)
{
	if(ptrF==NULL)
         ptrF=ptrNew;
	InputRecord(ptrNew);
	ptrFirst->ptrNext=NULL;
}
//addtoend will add a number to the end
void AddToEnd(struct point *ptrF, struct point *ptrL)
{
	if(ptrF==NULL)
		ptrF = ptrNew;
	else
	{
		InputRecord(ptrNew);
		ptrL=ptrNew;
	}	
}
//reads in input
void InputRecord(struct point *ptrNew)
{
	ptrNew=(struct point *)(malloc(sizeof(struct point)));
	printf("Please enter the 2 Data points(x and y):\n");
	ptrFirst=ptrNew;
	if(ptrNew==NULL)
		printf("No more memory.\n");
	else
	{
		scanf("%d%d",&ptrFirst->x, &ptrFirst->y);	
		printf("Please enter the label:\n");
		scanf("%s", ptrFirst->label);
	}
}

void main()
{
	int empty=0;//helps determine if list is empty
	int input=1;//reads in user input
	while(input!=0)
	{
		printf(
		"1. Add a point at the END of the list.\n"
		"2. Add a point at the BEGINNING of the list.\n"
		"3. Is the list empty?\n"
		"4. Erase all points from the list (reset).\n"
		"5. Display the list.\n"
		"6. Save the list to a sequential file (reset/replace file contents)\n"
		"7. Read the list back from a sequential file\n" 
		"(reset/replace current memory content)\n"
		"0. Exit\n\n"
		);
		scanf("%d", &input);
		switch(input)
		{
		case 1:
		{
			AddToEnd(ptrFirst,ptrLast);
		}	
		case 2:
		{
			AddToBeginning(ptrFirst,ptrLast);
		}
		case 3:
		{
			empty=isEmptyList(ptrFirst);
			if(empty==1)
			{
				printf("The list is not empty\n\n");
			}
			else
			{
				printf("The list is empty\n\n");
			}
		}
		case 4:
		{
			ResetList(ptrFirst,ptrLast);
		}
		case 5:
		{
			PrintList(ptrFirst);
		}
		case 6:
		{
		}
		case 7:
		{
		}
		default:
		{
			printf("Invalid entry, try again\n\n");
		}
                }
	}
}

Originally Posted by CommonTater

Similarly when adding to the end of the list you need to traverse the list, find the item with the null pointer (marking end of list) and set it's value to point to your new record. The pointer to the next item in your record then becomes NULL signalling the new end of list

He's already keeping track of the last node. So all he has to do is make that node point to his new one, then set ptrLast to his new one.


Quzah.

In InputRecord you should not set ptrFirst... Stay on task, create the new record and return a pointer to it.

Code:

struct point *InputRecord ( void )
{ struct point *ptrN= malloc(sizeof(struct point));
   if(ptrN == NULL) 
      { printf("No more memory.\n");
         exit(1);  }                           // this is a fatal error so exit if it happens

     // get new information 
     printf("Please enter the 2 Data points(x and y):\n");
     scanf("%d%d",&ptrN->x, &ptrN->y);	
     printf("Please enter the label:\n");
     scanf("%s", ptrN->label);
     return ptrN; }                         // return pointer to new struct

In AddToBeginning again stay on task, here you only manipulate the pointers...

Code:

void AddToBeginning(struct point *ptrF)
{ struct point *temp = ptrf;                   // save pointer to first record
   ptrF = InputRecord();                     // get pointer to new first record
   ptrF->ptrNext = temp;  }                   // next in new record points to saved

It's late here, I'm sure there's a minor error or two in there, but this should give you some idea.

Got addtobeginning and input record set now, and I think reset list is working, just gotta work on printlist

Code:

void PrintList(struct point *ptrF)
{
	struct point *pC = ptrF;
	while(pC != NULL)
   {
		printf("Label: %s\nx: %d y:%d\n\n", pC->label, pC->x, pC->y);
		pC = pC->ptrNext;  // this is how a list is traversed
                PrintList(struct point *pC)
   }	
}

How does this not work, it has a new pointer which points to the first and while it isn't null it prints out and then points to the next, anyone see what's wrong here?

Why are you recursively calling PrintList? Just call it once, and use a loop. If you are using recusion, you don't need the while loop. You just need an if check. Recursion is just another way to loop. Use one or the other here, not both.

Quzah.