使队列数据结构作为数组实现的问题

Question

this code to make simple queue data structure as array implementation

#include <stdio.h>
#define Q_MAX_SIZE 255
#include <stdbool.h>
struct queue
{
    int* pointer;
    int* currentValue;
    int max, count, theQueue[Q_MAX_SIZE];
};

void initQueue(struct queue*);
bool pushQueue(struct queue*, int);
int* popQueue(struct queue*);

int main(void)
{
    int i, j, num = 0;
    struct queue obj[5];

    for(i=0; i<5; i++)
    {
        initQueue(&obj[i]);

        for(j = 0; j<3; j++)
        {
            num++;
            pushQueue(&obj[i], num);
        }
        num = 0;
    }

    for(i=0; i<5; i++)
    {
        printf("Queue[%d]:\n", i);
        int* inputobj;
        inputobj = popQueue(&obj[i]);

        while(inputobj != NULL)
        {
            printf("Queue[No.%d] = %d\n", i, *inputobj);
            inputobj = popQueue(&obj[i]);
        }
        putchar('\n');
    }

    puts("done..!");

    return 0;
}

//@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

void initQueue(struct queue *Q)
{
    Q->pointer = Q->theQueue;
    Q->max = Q_MAX_SIZE;
    Q->count = 0;
}

bool pushQueue(struct queue *Q, int input)
{
    if(Q->count < Q->max)
    {
        *Q->pointer = input;
        Q->pointer++;
        Q->count++;
        return 1;
    }
    else
        return 0;
}

//@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

int* popQueue(struct queue *Q)
{
    int i;
    if(Q->count > 0)
    {
        Q->currentValue = Q->theQueue;
        Q->pointer--;
        Q->count--;

        for(i=0; i < Q->count; i++)
        {
            int* currentPtr = Q->theQueue + i;
            int* nextPtr = currentPtr + 1;
            *currentPtr = *nextPtr;
        }
        return Q->currentValue;
    }

    else
        NULL;
}

there is a problem with the code in the function popQueue() in this line:

Q->currentValue = Q->theQueue;

it is work put the output is not correct

output:
Queue[0]:
Queue[No.0] = 2
Queue[No.0] = 3
Queue[No.0] = 3

Queue[1]:
Queue[No.1] = 2
Queue[No.1] = 3
Queue[No.1] = 3

Queue[2]:
Queue[No.2] = 2
Queue[No.2] = 3
Queue[No.2] = 3

Queue[3]:
Queue[No.3] = 2
Queue[No.3] = 3
Queue[No.3] = 3

Queue[4]:
Queue[No.4] = 2
Queue[No.4] = 3
Queue[No.4] = 3

done..!

but after i change the pointer ( currentValue ) in the queue struct to make it of type integer and edit some lines in the function popQueue() every thing work fine.

--here is the function after editing:

int* popQueue(struct queue *Q)
{
  int i;
  if(Q->count > 0)
  {
    Q->currentValue = Q->theQueue[0];
    Q->pointer--;
    Q->count--;

    for(i=0; i < Q->count; i++)
    {
      int* currentPtr = Q->theQueue + i;
      int* nextPtr = currentPtr + 1;
      *currentPtr = *nextPtr;
    }
    return &Q->currentValue;
  }

-- and this is the correct output:

Queue[0]:
Queue[No.0] = 1
Queue[No.0] = 2
Queue[No.0] = 3

Queue[1]:
Queue[No.1] = 1
Queue[No.1] = 2
Queue[No.1] = 3

Queue[2]:
Queue[No.2] = 1
Queue[No.2] = 2
Queue[No.2] = 3

Queue[3]:
Queue[No.3] = 1
Queue[No.3] = 2
Queue[No.3] = 3

Queue[4]:
Queue[No.4] = 1
Queue[No.4] = 2
Queue[No.4] = 3

The Question is: what makes the first code provide wrong output?

Answer 1

What gave you the wrong output in the first case was the fact that the pointer Q->currentValue never changed its value (the address which it was holding). It was always pointing to the first element in the queue.

Say the queue contained {1, 2, 3 |,<garbage>} .

That means, that after the first pop, the queue became:

{2, 3 |, 3, <garbage>}

and currentValue still held the address of the first element in the array, which is 2 .

After the second pop:

{3 |, 3, 3, <garbage>}

and currentValue points to the first element, which value is 3 ,

The last time, the array is unchanged (as Q->count-- changes Q->count 's value to 0), so the conent is

{| 3, 3, 3, <garbage>} {| 3, 3, 3, <garbage>} and currentValue is still pointing to a 3 .

I assume that you changed the second example to make Queue->currentValue an int .

This way, it retains the original first element (which is popped).

That makes your prints work properly in your test case.

However,

• Your implementation will fail if you had a 0 in your queue.
• Your implementation adds needless complexity to the pop operation (O(n)). It would be much better to implement a cyclic queue, with head and tail .
• Retaining a copy of the popped element in order to return it would not be my first choice. I would suggest implementing an isEmpty() method, check its result in the while loop and while the queue is not empty, simply have pop() return the queue advance its head and return the previous head element.