Insert and Delete element on Circular Queue

Question

I'm studying about circular queue in data structure . As you can see from the code below, I try to delete a specific data and insert data on Circular queue. However, when I try to run it there's a problem when deleting data and insert a new one. I had no clue about it. I was trying to solve this for many hours but I can't find anything. Any help would be appreciated.

#include <stdio.h>
#define SIZE 3

typedef struct queue{
    int val[SIZE]={NULL};
    int front;
    int rear;
}queue;
void display(struct queue *q);
void enqueue(struct queue *q){
    int ins,i=1;
    if((q->rear == SIZE-1 && q->front == 0) || (q->rear == q->front-1)){
        printf("Queue is full!\n");
    }
    else if (q->front == -1)
    { 
        printf("Enqueue data : ");
        scanf("%d",&ins);
        q->front = q->rear = 0; 
        q->val[q->rear] = ins; 
    }
    else if (q->rear == SIZE-1) 
    { 
        printf("Enqueue data : ");
        scanf("%d",&ins);
        q->rear = 0; 
        q->val[q->rear] = ins; 
    }   
    else
    {   
        printf("Enqueue data : ");
        scanf("%d",&ins);
        q->rear++; 
        q->val[q->rear] = ins; 
    } 
    display(q);
};

void dequeue(struct queue *q);
int main(){
    queue *q= new queue;
    q->front = -1;
    q->rear = -1;
    char select;
flag1:
    printf("\n------- Please Select Operations ---------\n");
    printf("Press e: Enqueue\n"); 
    printf("Press d: Dequeue\n");   
    printf("Press x: Exit Program\n");                   
    printf("------------------------------------------\n");
    printf("Select Menu : ");
    scanf(" %c",&select);
    switch(select){
        case 'e' : enqueue(q); goto flag1;
        case 'd' : dequeue(q); goto flag1;
        case 'x' : break;
    }
    
 return 0;  
}

void dequeue(struct queue *q){
    int deq,hold;
    if (q->front == -1) 
    { 
        printf("Queue is Empty"); 
    } 
    else
    {
     printf("Dequeue data : ");
     scanf("%d",&deq);
     for(int i=0;i<SIZE;i++){
        if(deq==q->val[i]){
           if(i==q->front){
            q->val[q->front]=NULL;
            q->front = i;
            q->rear=q->rear+1;
            if(q->rear=SIZE-1){
                q->rear=0;
               }
           }
           else
            q->val[q->front]=NULL;
        }   
    }
    display(q);
}
};

void display(struct queue *q){
    int i;
    printf("Queue : |");
    for (i= 0; i<SIZE; i++){
        if(q->val[i]==NULL){
        printf(" |");   
        }
        else
        printf("%d|", q->val[i]);
    }
};

Answer 1

GIGO!
Your code is overly complex.
Complex code requires complex testing and debugging.

Try the following code:

void enqueue( struct queue *q, int v) {
    int r = (q.rear + 1) % SIZE
    if(( r == q.front) {
        printf( "Queue is full!\n");
    } else {
        q.val[ q.rear] = v;
        q.rear = r;
    }
};

int dequeue( struct queue *q) {
    if( q.front == q.rear) {
        printf( "Queue is Empty");
        v = NULL; # or whatever (required as a return value)
    } else {
        v = q.val[ q.front];
        q.front = ( q.front + 1) % SIZE;
    }
    return v;
};

int main() {
    queue *q = new queue;
    q->front = q->rear = 0;
    ...
};

To summarize:

• rear is index of the youngest element
• front is the index of the oldest element
• % (the modulus operator) take care of the index overwrapping.
• (front == rear) means empty buffer
• ((rear +1) % SIZE == front) means full buffer.

Please note that this simple algorithm always leave an unused element in the buffer. This is required to distinguish "full" from "empty".