Programming

//THIS FUNCTION ADDS A NEW ENTRY TO THE phonebook_list
void insert(void){
    //variables
    int flag=0, length=0, menu=0, i=0, check=0;
    NODE *p,*q, *temp;

    //USER MUST ENTER FIRST NAME ONLY FOR ADDITION TO PHONEBOOK
    printf("\nPlease insert the <First Name> of the entry you would like to add not separated by a space.\n");
    if((temp=(NODE *)malloc(sizeof(NODE))) == (NODE *)NULL)
    {
        printf("Memory could not be allocated! Returning to menu...\n\n");
        return;
    }
    scanf("%s", temp->name);
    //check if name is too long for the array IF IT IS PROMPTS USER TO RE ENTER SHORTER NAME
    while(flag==0)
    {                                    
        length=strlen(temp->name);
        if(length<50){
            for(i=0;i<length;i++)
                temp->name[i] = toupper(temp->name[i]);
            flag=1;
        }else{                    
            printf("\nName is too long! Please re-enter (less than 50 characters).\n");
            scanf("%s", temp->name);
            length=strlen(temp->name);
        }
    }
    //prompt user for telephone number
    printf("%s",temp->number);
    printf("\nPlease insert the telephone number in the following format XXXXXXXXXX.\n");
    scanf("%s", temp->number);
    flag=0;

    //check if number has the correct number of chars
    while(flag==0){
        length=strlen(temp->number);
        if(length==10){
            flag=1;
        }else{
            printf("\nThis is an invalid number. Please re-enter.\n");
            scanf("%s", temp->number);
            length=strlen(temp->number);
        }
    }
    printf("%s", head->name);
    //IF PHONEBOOK IS EMPTY THEN THE HEAD WILL BE A NULL POINTER AND TEMP NODE WILL BE ADDED
    if(head==NULL)
        {
        if((head=(NODE *)malloc(sizeof(NODE))) == (NODE*)NULL)
        {
            printf("Memory could not be allocated!\nReturning to menu...\n\n");
            return;
        }else{    
            strcpy(head->number,temp->number);
            strcpy(head->name,temp->name);
            head->next=tail;
            free(temp);
            printf("Entry successfully added.\n\n");
            return;
        }
    }else{
        //IF NEW NODE IS NOT THE FIRST ENTRY(NODE) IN PHONEBOOK(LIST) WILL LOOP THROUGH PHONEBOOK TO FIND CORRECT SPOT TO ADD
        p=head;
        printf("%s", p->name);
        while(p->next!=NULL)
        {
            //IF NAME IS ALREADY IN PHONEBOOK ERROR IS GIVEN AND PROGRAM RETURNS USER TO MAIN MENU
            if(strcmp(p->name,temp->name)==0)
            {
                printf("This name already exists in phonebook!\nReturning to menu...\n\n");
                return;
            //OTHERWISE THE WHILE LOOP WILL CONTINUE THROUGH LIST UNTIL IT FIND THE ADDITION THAT IS ALPHABETICALLY HIGHER
            //AND WILL ADD THAT NODE IN ITS CORRECT SPOT
            }else if(strcmp(p->name,temp->name)<0)
            {
                printf("\nTRUE\n");
                temp->next=p;
                temp->prev=p->prev;
                p->prev->next=temp;
                p->prev=temp;
                printf("Entry successfully added.\n\n");
                return;
            }
            p=p->next;
        }
        //IF NO NAME IS ALPHABETICALLY HIGHER, THAN IT SHOULD REACH THE LAST NODE AND WILL BE ADDED TO THE END OF THE PHONEBOOK(LIST)
        if(p->next==NULL)
        {
            if((temp=(NODE *)malloc(sizeof(NODE))) == (NODE *)NULL)
            {
                printf("Memory could not be allocated!\nReturning to menu...\n\n");
                return;
            }
            temp->next=tail;
            temp->prev=q;
            q->next=temp;
            free(temp);
            printf("Entry successfully added.\n\n");
            return;
        }
    }
}

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define NODE struct node

void insert(void);
/*
int delete(void);
int list(void);
*/

NODE *head = NULL;
NODE *tail = NULL;

struct node{
	char name[51];
	char number[11];
	char *next;
	char *prev;
	
};


int main(void){
	//variables
	int menu=0, flag=0;


		//INSTRUCTIONS for user in main program
	printf("Welcome to our phone book.\n\n");

	while(flag==0){
		printf("Press (1) to add a new name and number.\n");
		printf("Press (2) to delete an entry.\n");
		printf("Press (3) to list all entries.\n");
		printf("Press (4) to quit.\n");
		scanf("%d", &menu);	
	

	//switch statement for menu selection -- calls functions to main that perform the functions described in INSTRUCTIONS above
		switch(menu){
	
			case 1:
				insert();
				break;
			
			case 2:
				counter = delete();
				break;
	
			case 3:
				list();
				break;
			
			case 4:
				flag=1;
				break;
			
			default:
				break;
		}
	
	}	
	return 0;
}

NODE *create_node(const char *name, const char *num, NODE *prev, NODE *next)
{
        int n;

        NODE *mem=malloc(sizeof(NODE));
        if(mem == NULL) return(NULL);

        strncpy(mem->number, num, sizeof(mem->number));
        mem->number[sizeof(mem->number)-1]='\0';

        strncpy(mem->name, name, sizeof(mem->name));
        mem->name[sizeof(mem->name)-1]='\0';
        for(n=0; mem->name[n] != '\0'; n++)
                mem->name[n]=topper(mem->name[n]);

        mem->next=next;
        mem->prev=prev;
        return(mem);
}

void insert(NODE *node)
{
        node *h=head;

        if(h == NULL) // List is empty, just set it to this node
        {
                head=node;
                return;
        }

        // Loop until the end of the list,
        // but STOP at the last one.  h will not be NULL after this loop.
        while(h -> next != NULL)
        {
                int v=strcmp(node->name, h->name);

                if(v == 0) return; // Reject duplicate
                else if(v < 0)
                {
                        h=h->next;
                        continue;
                }

                // We want to change a <-> b <-> d <-> e into
                // a <-> b <-> c <-> d <-> e
                // We are at node d.

                // point ahead to d
                node->next=h;
                // point back to b
                node->prev=h->prev;

                // make b point ahead to us
                if(h->prev != NULL) h->prev->next=node;
                // make d point back to us
                h->prev=node;

                return;
        }

        if(h->next == NULL) // append to list
        {
                h->next=node;
                node->prev=h;
                return;
        }
}