Operations on QUEUE :)

#include<stdio.h>
#include<conio.h>
#define MAX 20 //defining size of queue
void insert(int); //function declaration for insertion
int del();        //function declaration for deletion
void display();   //function declaration for displaying queue
int queue[MAX];   //queue array
int front=-1,rear=-1;     /*initialy setting both parameters to -1 position i.e out of the array*/
main()
{
      int choice,num; //variables that we require in the entire program
      while(1) //for repeatingly asking the user whether to continue the program?
      {
              printf("Enter your choice\n");
              printf("1. INSERT \n");
              printf("2. DELETE \n");
              printf("3. DISPLAY \n");
              printf("4. EXIT \n");
              scanf("%d",&choice); //getting the choice of the user
              switch(choice) //using switch statement to select the working of the entered(desired) choice
              {
              case 1: //If user wishes to insert element in the queue
                   printf("Enter number to be inserted in the queue: ");
                   scanf("%d",&num); //getting the element to be inserted from the user
                   insert(num); //calling insert function
                   break; //jumping out of switch statement
                   case 2://If user wishes to delete element from the queue
                        num=delete(); //calling delete function and saving the result in an int variable(num)
                        break;
                        case 3://If user wishes to display the queue
                             display(); //calling display function
                             printf("\n");
                             break;
                             case 4: //If user wishes to exit(stop continuing the program)
                                  exit(1); //end of while loop
                                  default ://If user enters a number other than 1 to 4
                                          printf("INVALID CHOICE \n");
                                          }
                                          }
                                          }
                                          //FUNCTION DEFINATION
      void insert (int x) //value of number to be inserted(num) gets copied in 'x' when function is called
      {
           if(rear==MAX-1) //rear is at the last position in queue array
           {
                          printf("Queue is full (OVERFLOW)\n");
                          return; //returning control back to calling function
                          }
                          if (front==-1)//only if we wish to insert 1st element in the queue
                          {
                                       front=0;//front is set to 1st position i.e queue[0] and here the first insertion will take place
                                       }
                                       rear=rear+1; //insertion take place at REAR
                                       queue[rear]=x;//element to be inserted is placed at the rear position of queue
                                       }//for eg: for 1st insertion: front is made 0 and rear is also made 0(-1+1)
                                       //and so the insertion will take place at queue[0]
                                       //for next insertion rear will be 1(0+1) and front will remain 0 as insertion occurs at REAR
        int delete () //for deleting element from queue
        {
            int x;
            if(front==-1 || front==rear+1) //front==-1 means front is yet not incremented or set to 0 for first insertion
                     //front==rear+1 : if for eg front=0,rear=2
                     //for first deletion front=1,rear=2;for second deletion front=2,rear=2;now if front=3(rear+1),there will be no element in the queue
                        {              
                        printf("Queue is empty (UNDERFLOW) \n");
                        }
                        x=queue[front];//saving value of element at FRONT(to be deleted) in a variable
                        front=front+1;//deletion occurs at FRONT,and so we increment it for each deletion
                        printf("%d has been deleted \n",x);
                        return x;
                        }
       void display() //for displaying queue
       {
            int i;
            if(front==-1 || front==rear+1) //checking if queue is empty
            {
                        printf("Queue is EMPTY \n");
                        }
                        //if queue is not empty:
                        for(i=front;i<=rear;i++)//start printing elements of queue from FRONT to REAR
                        {
                                printf("%d ",queue[i]);//displaying QUEUE
                                }
                                }