我对队列中的这个算法不太了解

Question

Here's the queue using array implementation in Robert Sedgewick's book:

class QUEUE {
private:
    int* q;
    int N;
    int head;
    int tail;
public:
    QUEUE(int maxN) {
        q = new int[maxN + 1];
        N = maxN + 1; head = N; tail = 0;
    }
    int empty() const {
        return head % N == tail;
    }
    void put(int item) {
        q[tail++] = item; tail = tail % N;
    }
    int get() {
        head = head % N; return q[head++];
    }
};

My concern is about the get() method. Consider the size to be 10; According to the code, initially, the index of head = 11 and index of tail = 0.

Now, add an element, say 1. So, 1 is placed in the index position 0 and the tail position is incremented to 1. Now add another say 5. 5 is at index pos 1 and the tail now is index 2.

Now use the get() function, which does: return q[head++]; which is to return the element at head and then increment the head but BOOOOOM!!!!! The head is index no 11 as we have just seen and it has no value stored in it and this must be a huge error. But, this code is correct as it's Robert Sedgewick and we are the one's mistaken. What's going on guys? :(

Answer 1

You are missing out the `

head = head % n; // this mean head is now the remainder of the division 
                 // between head and N`

It means head is no longer 10;

head = 11 % 11 
which is 0 as the remainder of this division is zero.
Therefore, head = 0;

Then when you return q[head++], you are returning q[0] and then you are setting head to 1.

Answer 2

If you create a queue with size 10 then head equals 10. In get we use

head = head % N;
head = 10   % 10
head = 0

So head is 0 and then we increment it to 1.