I am writing a program in C and have the code written but this error has me stum
ID: 3758080 • Letter: I
Question
I am writing a program in C and have the code written but this error has me stumped. Firstly, here is the assignment:
Modify the linked list example in the book so that it is a doubly linked list. Prove that your program works properly by implementing a print backwards function.
Here is the output I need:
Enter your choice:
1 to insert an element into the list.
2 to delete an element from the list.
3 to end.
? 1
Enter a character: a
The list is:
a --> NULL
The list in reverse is:
a --> NULL
? 1
Enter a character: z
The list is:
a --> z --> NULL
The list in reverse is:
z --> a --> NULL
? 1
Enter a character: n
The list is:
a --> n --> z --> NULL
The list in reverse is:
z --> n --> a --> NULL
? 1 Enter a character: d The list is: a --> d --> n --> z --> NULL The list in reverse is: z --> n --> d --> a --> NULL ? 2 Enter character to be deleted: x x not found. ? 2 Enter character to be deleted: n n deleted. The list is: a --> d --> z --> NULL The list in reverse is: z --> d --> a --> NULL ? 2 Enter character to be deleted: a a deleted. The list is: d --> z --> NULL The list in reverse is: z --> d --> NULL ? 2 Enter character to be deleted: z z deleted. The list is: d --> NULL The list in reverse is: d --> NULL ? 2 Enter character to be deleted: d d deleted. List is empty. ? 1 Enter a character: s The list is: s --> NULL The list in reverse is: s --> NULL ? 1 Enter a character: t The list is: s --> t --> NULL The list in reverse is: t --> s --> NULL ? 3 End of run. Press any key to continue . . .
Here is my code:
#include <stdio.h>
#pragma warning(disable: 4996)
#include<string>
#include<stdlib.h>
#include<time.h>
/* self-referential structure */
struct listNode {
char data; /* each listNode contains a character */
struct listNode *nextPtr; /* pointer to next node*/
struct listNode *prevPtr; /* pointer to previous node*/
}; /* end structure listNode */
typedef struct listNode ListNode; /* synonym for struct listNode */
typedef ListNode *ListNodePtr; /* synonym for ListNode* */
/* prototypes */
void insert(ListNodePtr *sPtr, char value);
char delete (ListNodePtr *sPtr, char value);
int isEmpty(ListNodePtr sPtr);
void printList(ListNodePtr currentPtr);
void printListBackwards(ListNodePtr currentPtr);
void instructions(void);
int main(void)
{
ListNodePtr startPtr = NULL; /* initially there are no nodes */
int choice; /* user's choice */
char item; /* char entered by user */
instructions(); /* display the menu */
printf("? ");
scanf("%d", &choice);
/* loop while user does not choose 3 */
while (choice != 3) {
switch (choice) {
case 1:
printf("Enter a character: ");
scanf(" %c", &item);
insert(&startPtr, item); /* insert item in list */
printList(startPtr);
break;
case 2:
/* if list is not empty */
if (!isEmpty(startPtr)) {
printf("Enter character to be deleted: ");
scanf(" %c", &item);
/* if character is found, remove it */
if (delete(&startPtr, item)) { /* remove item */
printf("%c deleted. ", item);
printList(startPtr);
} /* end if */
else {
printf("%c not found. ", item);
} /* end else */
} /* end if */
else {
printf("List is empty. ");
} /* end else */
break;
default:
printf("Invalid choice. ");
instructions();
break;
} /* end switch */
printf("? ");
scanf("%d", &choice);
} /* end while */
printf("End of run. ");
return 0; /* indicates successful termination */
} /* end main */
/* display program instructions to user */
void instructions(void)
{
printf("Enter your choice: "
" 1 to insert an element into the list. "
" 2 to delete an element from the list. "
" 3 to end. ");
} /* end function instructions */
/* Insert a new value into the list in sorted order */
void insert(ListNodePtr *sPtr, char value)
{
ListNodePtr newPtr; /* pointer to new node */
ListNodePtr previousPtr; /* pointer to previous node in list */
ListNodePtr currentPtr; /* pointer to current node in list */
newPtr = malloc(sizeof(ListNode)); /* create node */ <----- Error
if (newPtr != NULL) { /* is space available */
newPtr->data = value; /* place value in node */
newPtr->nextPtr = NULL; /* node does not link to another node */
newPtr->prevPtr = NULL; /* node does not link to previous node */
previousPtr = NULL;
currentPtr = *sPtr;
/* loop to find the correct location in the list */
while (currentPtr != NULL && value > currentPtr->data) {
previousPtr = currentPtr; /* walk to ... */
currentPtr = currentPtr->nextPtr; /* ... next node */
} /* end while */
/* insert new node at beginning of list */
if (previousPtr == NULL) {
newPtr->nextPtr = *sPtr;
*sPtr = newPtr;
} /* end if */
else { /* insert new node between previousPtr and currentPtr */
previousPtr->nextPtr = newPtr;
newPtr->nextPtr = currentPtr;
newPtr->prevPtr = previousPtr;
if (currentPtr != NULL)
currentPtr->prevPtr = newPtr;
} /* end else */
} /* end if */
else {
printf("%c not inserted. No memory available. ", value);
} /* end else */
} /* end function insert */
/* Delete a list element */
char delete(ListNodePtr *sPtr, char value)
{
ListNodePtr previousPtr; /* pointer to previous node in list */
ListNodePtr currentPtr; /* pointer to current node in list */
ListNodePtr tempPtr; /* temporary node pointer */
/* delete first node */
if (value == (*sPtr)->data) {
tempPtr = *sPtr; /* hold onto node being removed */
*sPtr = (*sPtr)->nextPtr; /* de-thread the node */
if ((*sPtr) != NULL)
(*sPtr)->prevPtr = NULL; /* nullify previous node */
free(tempPtr); /* free the de-threaded node */
return value;
} /* end if */
else {
previousPtr = *sPtr;
currentPtr = (*sPtr)->nextPtr;
/* loop to find the correct location in the list */
while (currentPtr != NULL && currentPtr->data != value) {
previousPtr = currentPtr; /* walk to ... */
currentPtr = currentPtr->nextPtr; /* ... next node */
} /* end while */
/* delete node at currentPtr */
if (currentPtr != NULL) {
tempPtr = currentPtr;
previousPtr->nextPtr = currentPtr->nextPtr;
if (currentPtr->nextPtr != NULL)
currentPtr->nextPtr->prevPtr = previousPtr;
free(tempPtr);
return value;
} /* end if */
} /* end else */
return '';
} /* end function delete */
/* Return 1 if the list is empty, 0 otherwise */
int isEmpty(ListNodePtr sPtr)
{
return sPtr == NULL;
} /* end function isEmpty */
/* Print the list then call printListBackwards*/
void printList(ListNodePtr currentPtr)
{
ListNodePtr previousPtr; /* pointer to previous node in list */
/* if list is empty */
if (currentPtr == NULL) {
printf("List is empty. ");
} /* end if */
else {
printf("The list is: ");
/* while not the end of the list */
while (currentPtr != NULL) {
printf("%c --> ", currentPtr->data);
previousPtr = currentPtr;
currentPtr = currentPtr->nextPtr;
} /* end while */
printf("NULL ");
printListBackwards(previousPtr);
} /* end else */
} /* end function printList */
/* Print the list Backwards*/
void printListBackwards(ListNodePtr currentPtr)
{
printf("The list in reverse is: ");
/* while not the end of the list */
while (currentPtr != NULL) {
printf("%c --> ", currentPtr->data);
currentPtr = currentPtr->prevPtr;
} /* end while */
printf("NULL ");
} /* end function printListBackwards */
Error is marked with bold, but to specify, we have:
newPtr = malloc(sizeof(ListNode)); <---a value of type "void" cannot be assigned to an entity type of "ListNodePTR".
Explanation / Answer
You are highlighted below code is an error.
newPtr = malloc(sizeof(ListNode)); /* create node */ <----- Error
error is displayed as shown in below.
a value of type "void" cannot be assigned to an entity
For this problem
step1: check whether which extension you are maintain .c or .cpp
Step2 : If the code in C++
newPtr = malloc(sizeof(ListNode)); // not worked please replace insted of this as
newPtr=new ListNode; // allocation
delete newPtr; //free the node
Step 3: If the code in C
newPtr = malloc(sizeof(ListNode)); // allocate it
free(new ptr);
Step4 :So check whether which extension ,c or .cpp you are maintain
To implementing a print backwards function:
void printListBackwards(ListNodePtr currentPtr);
{
if(currentPtr==NULL)
{
printf("The List is empty. ");
} /* end the if */
else{
printf("the list is in reverse is: "); // print the values
while(currentPtr !=NULL){ // condition
printf("%c --> ", currentPtr->data ); // print current ptr
currentPtr=currentPtr->prevPtr;
} /* end else statement */
printf("NULL ");
} /* end the function printBack */
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