C structs and linked lists struggle - c

Question

So, we just started learning linked lists and data structures in my online class, but I have been really struggling with this topic. I have a general understanding of pointers but when you combine everything, I get completely lost.
So for this program, I am supposed to process employee data and access it through the use of structs and pointers. However, I think I am not passing the correct values into the functions because the data is not being saved and I can't access it. Any help would be appreciated.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
struct address {
char street[49];
char city[29];
char state[2];
char zip[5];
};
struct data{
char name[39];
struct address addy;
float salary;
struct data *next;
} emp;
void LIST(struct data* head);
void INSERT(struct data* head, char* name, struct address addr, float salary);
void DELETE(char* name, struct data* head);
void EDIT(struct data* head, char* name, struct address addr, float salary);
void CALCPAYROLL(struct data* head);
void HELP();
int main()
{
char input[15];
int quitter = 1;
struct data *head = NULL;
head = malloc(sizeof(emp));
if (head == NULL){
return 1;
}
while(quitter == 1)
{
printf(" enter command: \n");
fgets(input,15, stdin);
input[strlen(input) -1] = '\0';
if((strcmp(input, "List") == 0) || (strcmp(input, "list") == 0))
{
LIST(head);
}
if((strcmp(input, "Insert") == 0) || (strcmp(input, "insert") == 0))
{
scanf("%s-%s-%s-%s-%s-%f", head->name, head->addy.street, head->addy.city, head->addy.state, head->addy.zip, &head->salary);
INSERT(head, head->name, head->addy, head->salary);
}
if ((strcmp(input, "Delete") == 0) || (strcmp(input, "delete") == 0))
{
DELETE(head->name, head);
}
if ((strcmp(input, "Edit") == 0) || (strcmp(input, "edit") == 0))
{
EDIT(head, head->name, head->addy, head->salary);
}
if ((strcmp(input, "Payroll") == 0) || (strcmp(input, "payroll") == 0))
{
CALCPAYROLL(head);
}
if ((strcmp(input, "Help") == 0) || (strcmp(input, "help") == 0))
{
HELP();
}
if ((strcmp(input, "Quit") == 0) || (strcmp(input, "quit") == 0))
{
printf("============\nGood bye!\n");
quitter = 0;
}
}
return 0;
}
void LIST(struct data* head)
{
struct data* temp = head;
while (temp) {
printf("%s\n%s\n%s, %2s %5s\n%9.2f\n-----\n", temp->name, temp->addy.street, temp->addy.city, temp->addy.state, temp->addy.zip, temp->salary);
temp = temp->next;
printf("%s\n%s\n%s, %2s %5s\n%9.2f\n-----\n", temp->name, temp->addy.street, temp->addy.city, temp->addy.state, temp->addy.zip, temp->salary);
temp = temp->next;
}
}
void INSERT(struct data* head, char* name, struct address addr, float salary)
{
struct data* newEmployee = NULL;
newEmployee = (struct data*)malloc(sizeof(emp));
strcpy(newEmployee->name, head->name);
newEmployee->salary = head->salary;
strcpy(newEmployee->addy.street, head->addy.street);
strcpy(newEmployee->addy.city, head->addy.city);
strcpy(newEmployee->addy.state, head->addy.state);
strcpy(newEmployee->addy.zip, head->addy.zip);
struct data* temp = head;
while(temp->next && temp->next->name > newEmployee->name)
{
temp = temp->next;
}
newEmployee->next = temp->next;
temp->next = newEmployee;
}
void DELETE(char* name, struct data* head)
{
char del[39];
scanf("%39s", del);
struct data * toBeDeleted = NULL;
struct data * temp = head;
while (strcmp(del, temp->name) == 0)
{
strcpy(temp->next->name, temp->name);
temp->addy = temp->next->addy;
temp->salary = temp->next->salary;
toBeDeleted = temp->next;
temp->next = temp->next->next;
free(toBeDeleted);
printf("RECORD DELETED\n");
}
temp = temp->next;
printf("RECORD NOT FOUND\n");
}
void EDIT(struct data* head, char* name, struct address addr, float salary)
{
char edit[39];
scanf("%39s", edit);
struct data* temp = head;
while (strcmp(edit, temp->name) == 0)
{
temp->addy = addr;
temp->salary = salary;
printf("RECORD EDITED\n");
return;
}
temp = temp->next;
printf("RECORD NOT FOUND\n");
}
void CALCPAYROLL(struct data* head)
{
struct data* temp = head;
float total;
while (temp)
{
total += temp->salary;
temp = temp->next;
}
printf("total payroll: %f", total);
}
void HELP()
{
printf("commands:\n");
printf("List - shows the list of employees\n");
printf("Insert - Creates a new employee record\n");
printf("Delete - Deletes an existing employee record\n");
printf("Edit - Modifies the contents of an employee record\n");
printf("Payroll - Calculates and displays the total payroll\n");
printf("Help - Displays the set of available commands\n");
printf("Quit - Prints the message ""good bye!"" and exits the program" );
}

Answer 1

First of all, I have to commend you for a very neat presentation. The code is readable and easy to follow. The only fault I find with it is that all uppercase names are usually reserved for macros and constants declared with #define.
Here:
struct data{
char name[39];
struct address addy;
float salary;
struct data *next;
} emp; // declaring a variable at the same time as the structure? Not very nice.
I see an issue that happens when you create your first list item, the head. Insert must have a way to modify the head parameter. This needs an extra level of indirection. INSERT should also return an error code, in case malloc fails, a common way to do this is to return a negative value (-1) on error.
void INSERT(struct data* head, char* name, struct address addr, float salary)
// should be
int INSERT(struct data** head, char* name, struct address addr, float salary)
You'll have to change the logic a little bit to set head with the newly allocated buffer when head is NULL on entry. Calling INSERT then becomes.
if (INSERT(&head, name, &addr, salary) < 0)
{
// error !
}
I see a problem in data allocation in INSERT.
struct data* newEmployee = NULL;
//newEmployee = (struct data*)malloc(sizeof(emp)); // ?? you allocate a struct data, what's emp?
// should read
newEmployee = (struct data*)malloc(sizeof(struct data));
DELETE and EDIT do not work. They cannot find the employee as written. You should consider using a find() function, something like this:
data* FindEmployeeData(data* head, const char* name)
{
while (head)
{
if (strcmp(head->name, name) == 0)
break;
head = head->next;
}
return head;
}
This could be a time saver when writing new operations on employees.
In INSERT:
while(temp->next && temp->next->name > newEmployee->name)
I thought strings were supposed to be compared with strcmp()...
In DELETE: same as for INSERT, the function could change the head of the list
int DELETE(data** head, const char* name); // returns -1 on error.
Here:
scanf("%s-%s-%s-%s-%s-%f", head->name, head->addy.street, head->addy.city, head->addy.state, head->addy.zip, &head->salary);
Terrible things will happen when the user enters a name, address, state, street, city or zip that are too long for the space you have allocated.

Answer 2

You can't access to the data because in INSERT function head must be a double pointer, because when you want to insert something in the head of a linked list you have to change the address stored in head. In your code you're modifying a copy of head. remember that in C everything is passed for value. This mean that for modify the address content in a pointer you have to pass the address of the pointer at your function.