我的搜索功能无法正常工作，因为我的if语句出了点问题，并且我正在使用链表

Question

#include <stdio.h>
#include <conio.h>
#include <stdlib.h>
#include <string.h>

#define TRUE 1
#define FALSE 0

void new_contact();//add contact using linked list
void list_contact();
void delete_contact();

//int sorting_contact();//sort contact name using sorting
void search_contact();//search for contact phone number using searching

struct contact
{
    char phone_number[20];
    char contact_name[70];
    char address [80];
    struct contact *ptrnext;
};

struct contact *headptr, *newptr, *currentptr, *previousptr;

int main()
{
    system("color B");
    int key;
    char ch;
    int choice=TRUE;
    headptr=(struct contact *)NULL;

    while(choice==TRUE)
    {

        printf("\n\t\tWelcome to Smart Phone Book System");
        printf("\n\nAdd a contact :A\nList a contact :L\nDelete a contact :D"
               "\nSearch for contact :S\nSort phonebook :T\nExit :x\n");
        printf("\nPlease enter the mode you want:");
        scanf(" %c%*c", &ch);
        switch(ch)
        {
            case 'A': new_contact();system("CLS");break;

            case 'L': system("CLS");list_contact();break;

            case 'D': system("CLS");delete_contact();break;

            case 'S': search_contact();break;
            //case 'T': sorting_contact(); break;

            case 'X': choice=FALSE; break;

            default: system("CLS");printf("\nPlease enter a proper mode");
        }
    }
}

void new_contact()//add data
{
    newptr=(struct contact *)malloc(sizeof (struct contact)); // pointer to a new
                    //  memory allocation

    if (headptr==(struct contact *)NULL)//node is empty?
    {
         headptr=newptr; //first pointer point to first node
         newptr->ptrnext=(struct contact *)NULL; //first node pointer point to null
    }
    else
    {
         newptr->ptrnext=headptr;// new node pointer point to previous first node
         headptr=newptr; // head point to new node,new node become first node
    }
    printf("\nEnter a new name : ");
    fgets( newptr->contact_name,sizeof(newptr->contact_name),stdin);
    printf("\nEnter a number : ");
    fgets(newptr->phone_number,sizeof(newptr->phone_number),stdin);
    //scanf(" %d", &newptr->phone_number);
    //getchar();
    printf("\nEnter address : ");
    fgets(newptr->address,sizeof(newptr->address),stdin);
}

void list_contact()
{
    int i=1;
    if (headptr==(struct contact*)NULL) //empty list
    {
        printf("\nEmpty list");
        return;
    }

    currentptr=headptr;
    do
    {
        printf("\nContact  number %d",i);
        printf("\nName : %s", currentptr->contact_name);
        printf("\nPhone number : %s", currentptr->phone_number);
        printf("\n\nAddress : %s", currentptr->address);

        i++;
        printf("\n");
        currentptr=currentptr->ptrnext; //point to next node
    }

    while(currentptr !=(struct contact *)NULL);
}

void delete_contact()
{
    char contact_name_delete[20];

    if (headptr==(struct contact *)NULL)//node is empty?
    {
        printf("\n\nThe list is empty. Cannot delete!!!\n");
        //inform the user that the list is empty
    }
    else
    {
        printf("\nDelete list by name.");
        printf("\nEnter contact name to delete: ");
        fgets(contact_name_delete,sizeof(contact_name_delete),stdin);

        currentptr=headptr;

        while(currentptr ->ptrnext!=(struct contact *)NULL)
        {

            if (strncmp(currentptr->contact_name,contact_name_delete,6)==0) 
            //found the location
            {
                break;
            }
            else
            {
                previousptr=currentptr;//save the previous current address
                currentptr=currentptr->ptrnext; 
                //point to next node
            }
        }

        if (strncmp(currentptr->contact_name,contact_name_delete,6)==0)
        {
            if ( currentptr==headptr) //number is the first and only node in list
            {
                headptr=currentptr ->ptrnext; //head point to NULL
                free(currentptr);
            }
            else //delete at the middle of link list
            {
                previousptr->ptrnext=currentptr->ptrnext;
                free(currentptr);//destroy  node, free the memory.
            }
            printf("\nPhone number were deleted!\n");
        }
        else
            printf("\nNumber to be deleted is not in the list!!! ");
    }
}

My problem is in the search_contact() function:

void  search_contact()//The function for searching the contact based on name
{
    currentptr=headptr;
    char contact_name_add[30];
    if (headptr ==(struct contact *)NULL)
    {
        printf("The phonebook is empty..");
    }
    else
    {

        printf("Please enter the name of your contact for searching : ");//user inputs name for searching
        fgets(contact_name_add,sizeof(contact_name_add),stdin);//uses linear search concept

        printf("\n\nThe computer will now search for your contact");

        do
        {
            if (strncmp(currentptr->contact_name,contact_name_add,5)==0)
            {
                printf("\nName :         %s", currentptr->contact_name);
                printf("\nPhone Number : %s",currentptr->phone_number);
                printf("\nAddress :      %s", currentptr->address);
                currentptr=currentptr->ptrnext;
            }
            else
            {
                currentptr=currentptr->ptrnext;

I've found out that the line below is my problem, but I don't know why

                if (currentptr->ptrnext=NULL
                    && strncmp(currentptr->contact_name,contact_name_add,5)!=0)
                {
                    printf("\nThe name you seek is not in the phonebook~~");
                }
            }
        } while(currentptr!=(struct contact *)NULL);
    }
}

I can search my contact that I've inserted just fine, but the problem exists when I have 2 or more contacts. The search function is quite similar to my delete function, but I've coded it to display more that 1 contacts (in the case of similar contact names), but when it reaches NULL, it gives out an error, and I could not find it on Google (my bad on that part).

Answer 1

尝试使用以下IF：

if(currentptr->ptrnext==NULL && strncmp(currentptr->contact_name,contact_name_add,5)!=0)

Answer 2

Don't do assignments in your if statements! The code reading:

if (currentptr->ptrnext=NULL

should be:

if (currentptr->ptrnext == NULL

With the assignment (single = ), the result of the assignment is false, so the second half of the conditional — the strncmp() — is never executed.

If you use GCC, you should be using options so that it warns you about such problems — -Wall would be sufficient for that warning. I routinely use this (or -std=c11 and the other options):

gcc -g -O3 -std=c99 -Wall -Wextra -Wmissing-prototypes -Wstrict-prototypes \
    -Wold-style-definition -Wold-style-declaration -Werror ...

Your code would not compile under those options.

---

Given the claim made in comments that the assignment is a typo, then the rest of the problem is the logic. The loop as currently written is, when presented more legibly:

    do
    {
        if (strncmp(currentptr->contact_name, contact_name_add, 5) == 0)
        {
            printf("\nName :         %s", currentptr->contact_name);
            printf("\nPhone Number : %s", currentptr->phone_number);
            printf("\nAddress :      %s", currentptr->address);

            currentptr = currentptr->ptrnext;
        }
        else
        {
            currentptr = currentptr->ptrnext;

            if (currentptr->ptrnext == NULL &&
                strncmp(currentptr->contact_name, contact_name_add, 5) != 0)
            {
                printf("\nThe name you seek is not in the phonebook~~");
            }
        }
    } while (currentptr != (struct contact *)NULL);

I don't trust bottom-testing loops; there are occasions when you need to go at least once through a loop, but lists can be empty and then you don't want to go through the body of the loop at all.

I'd probably use:

    while (currentptr != NULL)
    {
        if (strncmp(currentptr->contact_name, contact_name_add, 5) == 0)
        {
            printf("Name :         %s\n", currentptr->contact_name);
            printf("Phone Number : %s\n", currentptr->phone_number);
            printf("Address :      %s\n", currentptr->address);
            break;
        }
        currentptr = currentptr->ptrnext;
    }
    if (currentptr == NULL)
        printf("The name you seek (%s) is not in the phonebook\n", contact_name_add);

You might note that I end lines of output with a newline; I don't usually begin lines of output with a newline unless I want double spacing. Your input code needs to be fixed to remove the newlines that fgets() keeps. Write a function to call fgets() and strip the newline from the input.

[Can you] modify the loop and give the search result in case of multiple same contact name?

Of course. It is software; you can always change the spec and then change the code. You lose the break in the loop and count the matches and only report 'no match' if the count is zero after the loop.

To print all matches:

    int counter = 0;
    while (currentptr != NULL)
    {
        if (strncmp(currentptr->contact_name, contact_name_add, 5) == 0)
        {
            printf("Name :         %s\n", currentptr->contact_name);
            printf("Phone Number : %s\n", currentptr->phone_number);
            printf("Address :      %s\n", currentptr->address);
            ++counter;
        }
        currentptr = currentptr->ptrnext;
    }
    if (counter == 0)
        printf("The name you seek (%s) is not in the phonebook\n", contact_name_add);