C++ - 负数和正数之间的比较返回 false

Question

I am implementing a Queue using Vector in C++

I use sz variable to keep track of how many free elements are in the queue at any insert/delete operation.

I ran into an issue in my implementation where I was comparing:

sz < vec.size() - 1

At some point, this line started to return 0 and the comparison was

-1 < 5

One thing that helped was doing this

int vecsz = vec.size() - 1;
int regsz = sz;

if (regsz < vecsz)
{
    sz++;
}

Even though it helped, I still don't understand why. Can someone shed some light on this please?

queue.h

#include <iostream>
#include <vector>

using namespace std;

template <typename Object>
class queue
{
public:
    queue(int s) 
      : vec(s) 
    {
        sz = s - 1;
    }
    queue () { }
    ~queue() { }

    /**
     * Add element to Q and decrease size 
     */
    void enque(Object obj)
    {
        if (sz >= 0) {
            vec[sz] = obj;
            sz--;
        }
    }

    /**
     * Remove element from Q and increase size 
     */
    Object deque()
    {
        // grab last element
        int last = vec[vec.size() - 1];

        // remove last element from array
        vec.pop_back();

        // insert 0 in the front
        vec.insert(vec.begin(), 0);

        int vecsz = vec.size() - 1;
        int regsz = sz;

        // increase size tracking var
        if (regsz < vecsz)
        {
            sz++;
        }

        return last;
    }

    void print() {
        for (int i = 0; i < vec.size(); ++i) {
            cout << "i: " << vec[i] << endl;
        }
    }

private:
    vector<int> vec;
    int sz;
};

main

int main()
{
    queue<int> Q(5);
    cout << "Empty Q:" << endl; 

    Q.print(); 
    cout << endl;

    Q.enque(1);
    Q.enque(2);
    Q.enque(3);

    cout << "Full Q1:" << endl; 

    Q.print(); 

    cout << "Deque " << endl;
    cout << Q.deque() << " " << Q.deque() << " " << Q.deque() << " " << endl;
    cout << Q.deque() << endl;

    cout << "Print " << endl;
    Q.print(); 

    cout << "Enqueue " << endl;
    Q.enque(10);
    Q.enque(20);
    Q.enque(30);
    Q.enque(40);
    Q.enque(50);

    cout << "\nFull Q2:" << endl; 
    Q.print(); 

    cout << endl;
    cout << Q.deque() << endl;

    Q.enque(100);

    cout << "Full Q3:"; Q.print(); cout << endl;

    cout << Q.deque() << endl;
    cout << Q.deque() << endl;
    cout << Q.deque() << endl;
    cout << Q.deque() << endl;

    return 0;
}

Answer 1

This is why it is important no to mix signed and unsigned types. With

sz < vec.size() - 1

you are comparing an signed integer to an unsigned integer. When you do that, the compiler converts the signed integer to the same type as the unsigned integer. When you do that and the signed integer happens to be negative, then you get a value that is max value of the unsigned type, minus the value of the signed integer So

-1 < 5

becomes

4294967295 (for 32 bits) or 18446744073709551615 (for 64 bits) < 5

which is false.

To fix this, make sz an unsigned type.

Answer 2

vec.size() is an unsigned integer. When you compare an unsigned integer type to a signed integer type of same size or smaller, the signed type will be converted to the same type as the other. Negative numbers are not representable by unsigned types.

In general, you should avoid comparing signed numbers with unsigned numbers, because of the problems that you've observed.