Thread: Dynamic memory in C

Right, I understand that and made this:

Code:

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

and I KNOW it makes it through inputrecord because it scans in those values:

Code:

struct point *InputRecord (void)
{
	struct point *ptrN=malloc(sizeof(struct point));
	if(ptrN==NULL)
	{
		printf("No more memory.\n");
		exit(1);
	}
	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	
}

so it must fail in addtobeginning because when I put a flag in printlist it will not enter because it finds the first to be null:

Code:

void AddToBeginning(struct point *ptrF, struct point *ptrL)
{	
	struct point *temp = ptrFirst;                   // save pointer to first record
    ptrF = InputRecord();                     // get pointer to new first record
	ptrF->ptrNext = temp;
}

If I assume its a problem with the first and last lines, but if thats right what should I change on them?

Attempted to re-write the functions to avoid the whole global problem and using case 1 got:
Declared in main:
struct point *ptr=NULL;

Code:

case 1:
{
	ptr=InputRecord();
	AddToEnd(ptr);
	break;
}

Code:

struct point *InputRecord (void)
{
	struct point *ptrN=malloc(sizeof(struct point));
	if(ptrN==NULL)
	{
		printf("No more memory.\n");
		exit(1);
	}
	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	
}

Code:

void AddToEnd(struct point *ptrN)
{
	if(ptrFirst==NULL)
		ptrFirst=ptrN;
	ptrLast->ptrNext=ptrN;
	ptrN->ptrNext=NULL;
	ptrLast=ptrN;
}

Code:

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

Note I still kept these global but I think it helps avoid the problem, they still don't work, just thought I would try that to get rid of the global thing, and make it easier to work with. I tried anyway..
struct point *ptrFirst = NULL;
struct point *ptrLast = NULL;

Here it is:

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;

int isEmptyList(struct point *ptrF);
void PrintList(struct point *ptrF);
void ResetList(struct point *ptrF, struct point *ptrL);
void AddToBeginning(struct point *ptrF);
void AddToEnd(struct point *ptrN);
struct point *InputRecord( void ); // 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)
{
	while(ptrF != NULL)
   {
		printf("Label: %s\nx: %d y:%d\n\n", ptrF->label, ptrF->x, ptrF->y);
		ptrF = ptrF->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=ptrF;
	while(temp!=NULL)
	{
		ptrF=ptrF->ptrNext;
		free(temp);
		temp=ptrF;
	}
}
//addtobeginning will add a number to the beginning
void AddToBeginning(struct point *ptrF)
{	
	ptrF->ptrNext=ptrFirst;
	ptrFirst=ptrF;
}
//addtoend will add a number to the end
void AddToEnd(struct point *ptrN)
{
	if(ptrFirst==NULL)
		ptrFirst=ptrN;
	ptrLast->ptrNext=ptrN;
	if(ptrLast!=NULL)
	{
		ptrN->ptrNext=NULL;
		ptrLast=ptrN;
	}	
}
//reads in input
struct point *InputRecord (void)
{
	struct point *ptrN=malloc(sizeof(struct point));
	if(ptrN==NULL)
	{
		printf("No more memory.\n");
		exit(1);
	}
	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	
}

int main()
{
	int empty=0;//helps determine if list is empty
	int input=1;//reads in user input
	struct point *ptr=NULL;
	while(1)
	{
		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 0:
			{
				return 0;
				break;
			}
			case 1:
			{
				ptr=InputRecord();
				AddToEnd(ptr);
				break;
			}	
			case 2:
			{
				AddToBeginning(ptrFirst);
				break;
			}
			case 3:
			{
				empty=isEmptyList(ptrFirst);
				if(empty==1)
				{
					printf("The list is not empty\n\n");
				}	
				else
				{
					printf("The list is empty\n\n");
				}
				break;
			}
			case 4:
			{
				ResetList(ptrFirst,ptrLast);
				break;
			}
			case 5:
			{
				PrintList(ptrFirst);
				break;
			}
			case 6:
			{
				break;
			}
			case 7:
			{
				break;
			}
			default:
			{
				printf("Invalid entry, try again\n\n");
				break;
			}
		}
	}
}

Ok, I got it compiled now, it will run and accept 1 list, I added what you mentioned:

Code:

void AddToEnd(struct point *ptrN)
{
	if(ptrFirst==NULL)
		ptrFirst=ptrN;
		ptrLast=ptrN;
	ptrLast->ptrNext=ptrN;
	if(ptrLast!=NULL)
	{
		ptrN->ptrNext=NULL;
		ptrLast=ptrN;
	}	
}

It however will only hold 1 list at a time while printing. I won't be able to check on until tomorrow morning but will work on it again then.