pointer initialization

[chachi]

In the following code, why is the line highlighted in red set to null?

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

struct listNode {
	char data;
	struct listNode *nextPtr;
};

typedef struct listNode LISTNODE;
typedef LISTNODE *LISTNODEPTR;

void insert(LISTNODEPTR *, char);
char delete_me(LISTNODEPTR *, char);
int isEmpty(LISTNODEPTR);
void printList(LISTNODEPTR);
void instructions(void);

int main(int argc, char* argv[])
{

LISTNODEPTR startPtr = NULL;

	int choice;
	char item;

	instructions(); /*display the menu*/
	printf("? ");
	scanf("%d", &choice);

	while (choice != 3) {
		switch (choice) {
		case 1:
			printf("Enter a character ");
			scanf("\n%c", &item);
			insert(&startPtr, item);
			printList(startPtr);
			break;
		case 2:
			if (!isEmpty(startPtr)) {
				printf("Enter character to be deleted: ");
				scanf("\n%c", &item);
				if (delete_me(&startPtr, item)) {
					printf("%c deleted.\n", item);
					printList(startPtr);
				}
				else
					printf("%c not found. \n\n", item);
			}
			else
				printf("List is empty.\n\n");
			break;
		default:
			printf("Invalid choice.\n\n");
			instructions();
			break;
		}
		printf("? ");
		scanf("%d", &choice);
	}
	printf("End of run.\n");

	return 0;
}

/*Print the instructions*/
void instructions(void)
{
	printf("Enter your choice:\n"
		"   1 to insert an element into the list.\n"
		"   2 to delete an element from the list.\n"
		"   3 to end.\n");
}

/*Insert a new value into the list in sorted order*/
void insert(LISTNODEPTR *sPtr, char value)
{
	LISTNODEPTR newPtr, previousPtr, currentPtr;
	newPtr = (LISTNODEPTR)malloc(sizeof(LISTNODE));
	if (newPtr != NULL) {
		newPtr->data = value;
		newPtr->nextPtr = NULL;

		previousPtr = NULL;
		currentPtr = *sPtr;
		
		while (currentPtr != NULL && value > currentPtr->data) {
			previousPtr = currentPtr;  /*walk to...*/
			currentPtr = currentPtr->nextPtr;/*...next node*/
		}

		if (previousPtr == NULL) {
			newPtr->nextPtr = *sPtr;
			*sPtr = newPtr;
		}
		else {
			previousPtr->nextPtr = newPtr;
			newPtr->nextPtr = currentPtr;
		}
	}
	else
		printf("%c not inserted. No memory available.\n", value);
}

 

[jtm111]

It is usually good programming strategy to initialize a pointer to NULL. That way you can check if it has a value (if ptr != NULL) before you delete it because deleting an uninitialized pointer will crash a program.

 

[samhandle]

just like initializing a local variable at the time of declaration so that it does not contain some junk value , likewise it is good programming practise to initialize pointers , and the correct / safe initial value a pointer could have is NULL . does it clear your doubt or is there something else you have in mind

 

[chachi]

Gotcha. Thanks for the input.