Assume a structure called patient and necessary data members.

• Write function to read patient name and mobile number

• Write function to issue token. If token is not available, it should display appropriate message

• Write a function to display patient name and mobile number given token number

C Program to Implement Token System for Doctor’s Clinic

• Below C program implement Token System to book an appointment at doctor's clinic.
• Everyday Doctor can check only 20 patients. Therefore, the below C program is implemented in such a way that it can book 20 appointments per day.
• To do this 20 tokens numbered from 1 to 20 are used in the program.
• This C program follows a bottom-up approach.
• In the program Structure named patient is implemented that stores the data of patients and related necessary information.
• The program contains two functions PatientInformation() and Display().
• The PatientInformation() function performs tasks like taking the information of the patient's name, mobile numbers, and allocating the appropriate token number to the patients.
• The Display() function shows the complete appointment list for the day that includes token number, patient's name, and mobile number.

C Program Source Code:

/*C Program to implement Token System for Doctor’s Clinic*/

/*Importing C Libraries*/
#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
#include<string.h>

/*Create Structure named patient that contains information of token numbers, patient name, mobile no. and pointer next*/

struct patient
{
int tnumber;
char pname[30];
char pmobile[11];
struct patient *next;
};
typedef struct patient patient;
int token = 20;                          /*Total 20 Tokans*/
patient *head;

/*User-defined function PatientInformation that takes input data of patients like name & mobile no and assign a token number (1-20) after appointment booked successfully*/

void PatientInformation(patient *pointer)
{
    int tnumber;
    char key;
    /*First check token is available or not. If all 20 appointment are booked then token == 0 and display following message*/  
    if(token==0)
    {
        printf("\n\n\n Sorry! Today's all Appointments are full.\n\n Try to book an appointment for Tomorrow Early Morning at 8 AM\n\n\n");
    }
    /*Otherwise ask to enter the patient data like name and mobile number*/
    else
    {
        if(pointer==NULL)
        {
            head = (patient*)malloc(sizeof(patient));
            pointer = head;
            pointer->next = NULL;  
            read:                                         /*Lable read*/
            printf("\n Name of the Patient: ");
            getchar();
            gets(pointer->pname);                     /*Take patient name as input string*/
            mobile_no:
            printf("\n Mobile Number of the Patient: ");
            gets(pointer->pmobile);                  /*Take patient mobile number as input*/
            int length = strlen(pointer->pmobile);     
                                      /*Store the lengthe of mobile no. to variable length*/
            if(length != 10)                          
         /*Check the length of mobile no. is 10 or not. If not 10 then display error message*/    
            {
               printf("Invalid Mobile Number. Please, give valid mobile number\n");
               goto mobile_no;
            }
           int i;
           for(i = 0; pointer -> pmobile[i]!=NULL; i++)
           {
            if(pointer->pmobile[i]<'0' || pointer->pmobile[i]>'9')
            {
                 printf("Invalid Mobile Number. Please, give valid mobile number\n");
                 goto mobile_no;
            }
           }
        token--;
        tnumber = 20 - token;
        pointer->tnumber = tnumber;
        printf("\n\n Your Appointment with Doctor is Booked Successfully for Today.\n"); 
                       /*Display message if appointment with doctor is booked successfully*/ 
        printf("\n Your Token Number is %d.\n\n\n", pointer->tnumber);                  
                                           /*Display assigned token number of appointment*/
        printf("\n\n Press any key to continue.....");
        scanf("%c", &key);                           
        if(key >= 0)
        return;                             /*Press any key to return back to main menu*/
        }
        while(pointer->next != NULL)                          /*Pointer moves to next*/  
        {
            pointer = pointer->next;
        } 
        pointer->next = (patient*)malloc(sizeof(patient));
        pointer = pointer->next;
        pointer->next = NULL; 
        goto read;                 /*Jump to lable read to avoid repeatation of code*/
    }
}

/*User-defined function Display. This function displays the appointment list with all appointments booked for today*/

void Display(patient *pointer)
{
    printf("\n############################ Today's Appointment List ############################\n\n");
    /*If no appintment is booked till time then it display following message*/
    if(pointer == NULL)
    {
        printf("\n\n\n No Appointment Booked for Today Yet!!!\n\n If you want to book Today's appointment with Doctor please Proceed..... \n\n\n");
    }
    /*If some appontments are booked then display all appointments with thire token numbers, patient name and mobile numbers*/ 
    while(pointer != NULL)
    {
        printf("\n\nToken Number - %d: ", pointer->tnumber);
        printf("%s (Mobile No.: %s)\n", pointer->pname, pointer->pmobile);
        pointer = pointer->next;
    }
}
int main()                      /*Main Function of C*/
{
    head = NULL;               /*Initilly assign head as NULL*/ 
    char n;                /*char n to take input whether option 1 or option2 from the user*/ 
    while(1)        /*By default control comes back here after completion  of any operation*/   
    {
      main_menu:               /*Lable main_menu*/
        printf("\n############################ Main Menu ################################\n\n");
        printf("Press 1 to Book a Doctor's Appointment \n\nPress 2 to Display Today's Appointment List\n\n\n");
        scanf(" %c", &n);     /*Take input from the user*/  
            if(n == '1')
            {
              PatientInformation(head);    
                           /*If option 1 is selected call function PatientInformation(head)*/
            }
            else if(n == '2') 
            { 
              Display(head);               
                          /*If option 2 is selected call function Display(head)*/ 
            } 
            else       /*If other than options 1 & 2 selected the following error message displayed and jump to label main_menu*/ 
            {
              printf("\nPressed Wrong Option Key!!!!\n\nPlease, Select Right Option\n\n");
              goto main_menu;
            }
    }
return 0;
}

Source Code Image of the above C Program:

OUTPUT of the above C Program:

1] Initial Output Screen looks as follows:

2] To check how many appointments are booked for today press Option 2 at first. It will initially show you below Output Screen because no appointments are booked yet:

3] To book an appointment today with the doctor press option 1 and fill in the details like patient name and mobile number as shown below Output Screen:

This will display a message your appointment with the doctor is booked successfully for today along with an assigned token number between 1 to 20.

4] If all 20 appointments are booked for today and you will try to book an appointment $21^{st}$ of time then it will show the following message on the Output Screen:

5] If you want to see a list of today’s all 20 appointments then press option 2. It will show you the following Output Screen:

Display Today’s Appointment List with Token Number, Patient Name, and Mobile Number

6] If you enter an invalid mobile number like 9 digits or 11 digits mobile number, do not fill any mobile number made field vacant or else try to enter alphabets instead of numbers then it will show you the following error message and ask to give a valid mobile number. No one book appointment with an invalid mobile number.

7] If select a wrong option other than Option 1 or 2 then it will show the following error message and again ask to select the right option among 1 or 2: