why does my isFull function is always return 1?-CodePudding

I made a function to check if my circular queue is full, but it always return 1 (aka "true");

this is my Queue setup:

    #define MAX_LENGTH 150

typedef double ElementType;
typedef int bool;

typedef struct
{
    ElementType Elements[MAX_LENGTH];
    int front;
    int rear;
} Queue;

and this is my function:

int isFull(Queue Q)
{
    if (Q.front == Q.rear  )
        return 1;
    if ((Q.rear == MAX_LENGTH) && (Q.front == 0))
        return 1;
    return 0;
}

This function always return 1 mean the queue is full even though i have not put anything in the queue yet.

how do I fix this? Thanks.

Edit: I edited Q.rear to Q.rear 1 Here is how I initialize the Queue.

void resetQueue(Queue *Q)
{
    Q->front = -1;
    Q->rear = -1;
};

I found out that the problem is right at:

if (Q.front == Q.rear 1)
        return 1;

part.

Now I just can put 2 Elements in the queue, if I put the third Elements, it will become full.

CodePudding user response：

As part of your problem, please learn the difference between prefix and postfix (i.e. between Q.rear and Q.rear ).

Since you initialize both front and rear to the same value, the comparison Q.front == Q.rear will be true, because the increment happens after the value of Q.rear is used.

The code

if (Q.front == Q.rear  )
{
    // ...
}

is really more equivalent to

int old_value_of_rear = Q.rear;
Q.rear = Q.rear   1;

if (Q.front == old_value_of_rear)
{
    // ...
}

If you want to compare Q.front with Q.rear 1, use that exact addition:

if (Q.front == Q.rear   1)