Thread: FIFO Queue in C (unable to detect the error reason)

Hi, I follow one article's help to write a c code that is to make FIFO Circular queue. During Compilation I am getting the following error:
ERROR:
fifo1.c: In function ‘enqueue’:
fifo1.c:43:14: error: ‘rear’ undeclared (first use in this function)
q->items[rear] = x;
^
fifo1.c:43:14: note: each undeclared identifier is reported only once for each function it appears in
fifo1.c: In function ‘dequeue’:
fifo1.c:50:18: error: ‘front’ undeclared (first use in this function)
x = q->items[front];
^
************************************
My code is:
************************************

Code:

#include <stdio.h>
#include <stdlib.h>
#define max 10


typedef struct fifo_q{
   int count;
   int front;
   int rear;
   int items[max];
};


void initialize (struct fifo_q *q);
int isEmpty(struct fifo_q q);
int isFull(struct fifo_q q);
void enqueue(struct fifo_q *q, int x);
int dequeue(struct fifo_q *q);




void initialize (struct fifo_q *q)
{
    q->count = 0;
    q->front = 0;
    q->rear = -1;
}


int isEmpty(struct fifo_q q)
{
    return (q.count == 0);
}


int isFull(struct fifo_q q)
{
    return (q.count == max);
}




void enqueue(struct fifo_q *q, int x){
if (q->count == max){
    printf( "%d is not inserted. Queue is full.\n", x);
}
else{
    q->count = (q->count + 1);
    q->rear = (q->rear + 1) % max;
    q->items[rear] = x;
    }
}


int dequeue(struct fifo_q *q){
    int x;
    q->count = q->count - 1;
    x = q->items[front];
    q->front = (q->front + 1)% max;
    return x;
    }


int main(){
    struct fifo_q fifo1;
    int n;
    initialize(&fifo1);
   // Just to check I am calling the Enqueue function  
    printf("Enter your data \n");
    scanf("%d", &n);
    enqueue(&fifo1, n);
    ......
    ..... 
    return 0;
}

Please anyone give the reason why I am getting this declaration related error.

Look closely at the following snippet:

Code:

void enqueue(struct fifo_q *q, int x)
{
   if (q->count == max){
      printf( "%d is not inserted. Queue is full.\n", x);
   }
   else{
      q->count = (q->count + 1);
       q->rear = (q->rear + 1) % max;
       q->items[rear] = x; // <<<<<<<<<<<<
   }
}

Look at that highlighted line. Where has "rear" been defined?

Also you need to be consistent about your indentation, it'll make reading your program much much easier.


Jim

Hello Jim,
Thanks for the quick help. I am still confused about that code.
Because, as compiler was saying variable is not declared but the variable "rear" is struct data member, so why should I re-declare it again.
And it is a int type so in items[rear], rear will be replaced by its value.
Actually, I changed the code in following ..

Code:

int r;
void enqueue(struct fifo_q *q, int x)
{
     if (q->count == max){
      printf( "%d is not inserted. Queue is full.\n", x);
 }
else{
        q->count = (q->count + 1);
        q->rear = (q->rear + 1) % max;
        r = q->rear;
        q->items[r] = x;
       }
}

Because, as compiler was saying variable is not declared but the variable "rear" is struct data member, so why should I re-declare it again.

Code:

       q->rear = (q->rear + 1) % max; // q->rear is a structure member.
       q->items[rear] = x; // <<<<<<<<<<<< rear is not a structure member.

Because in the second line above rear is not a member of any structure, look at the previous line where rear is a member of a structure, notice the difference.

Code:

void enqueue(struct fifo_q *q, int x)
{
     if (q->count == max){
      printf( "%d is not inserted. Queue is full.\n", x);
 }
else{
        q->count = (q->count + 1);
        q->rear = (q->rear + 1) % max;
        q->items[q->rear] = x;  
       }
}

Jim

Thanks a lot Jim...