c++ - 如何仅使用Iterator显示Vector的前10个元素？

Question

我正在尝试使用displayVectorVer2()使其仅显示前10个元素，但是我不知道如何使用迭代器。我确实尝试了一些愚蠢的事情，只是为了看看会发生什么:我在for循环中将迭代器与displayLimit进行了比较。因为我的教授只让我使用15个元素，所以我通过减去vobj.end()-5来玩耍，但是我完全知道这不是一个好主意。

#include <iostream>
#include <vector>
#include <ctime>

template <class T>
void fillVector(std::vector<T>& vobj, int n);

template <class T>
void displayVectorVer2(std::vector<T>& vobj, typename std::vector<T>::iterator ptr);

template <class T>
void fillVector(std::vector<T>& vobj, int n)
{
    srand((unsigned int)time(NULL));
    for (int i=0; i<n; ++i)
    {
        vobj.push_back(rand()%99999+1);
    }
}

template <class T>
void displayVectorVer2(std::vector<T>& vobj, typename std::vector<T>::iterator ptr)
{
    std::cout << "Vector object contains " << vobj.size() << " values which are" << std::endl;

    const unsigned displayLimit = 10;
    if (vobj.size()>displayLimit)
    {
        for (ptr=vobj.begin(); ptr<vobj.end(); ++ptr)
        {
            std::cout << "  " << *ptr;
        }
        std::cout << "  ..." << std::endl;
    }
    else
    {
        for (ptr=vobj.begin(); ptr<vobj.end(); ++ptr)
        {
            std::cout << "  " << *ptr;
        }
        std::cout << std::endl;
    }
}

int main()
{
    std::vector<int> vobj;
    std::cout << "Before calling fillVector(...): vobj contains " 
              << vobj.size() << " values." << std::endl;

    std::cout << "\nEnter # of random values you'd like to store in vobj: ";
    int n;
    std::cin >> n;

    std::cout << "\n*** Calling fillVector(...) ***" << std::endl;
    fillVector(vobj, n);

    std::cout << "\n*** Calling displayVectorVer2(...) ***" << std::endl;
    std::vector<int>::iterator ptr;
    displayVectorVer2(vobj,ptr);
}

Answer 1

也许我想的太简单了，但是那个解决了您的问题:

I'm trying to use displayVectorVer2() to have it only display the first 10 elements

不知道您是否进行充分的运动，那将是我的答案:

...
    const unsigned displayLimit = 10;
    if (vobj.size()>displayLimit)
    {
        for (ptr=vobj.begin(); ptr<vobj.begin()+displayLimit; ++ptr) 
        {
            std::cout << "  " << *ptr;
        }
        std::cout << "  ..." << std::endl;
    }
    else
...

编辑:

That worked, but why does it work? I remember adding to vobj.begin() and getting extra empty elements appended to the original vector.

不知道您到底做了什么，但是也许可以帮助您理解代码:

...
   const unsigned displayLimit = 10;
    if (vobj.size()>displayLimit)
    {
        //Init ptr outside the for loop
        ptr = vobj.begin();

        //What the for loop is seeing with a more familiar syntax: 
        //for( ; i < 0 +displayLimit; ++i)
        //what you are seeing
        for (/*ptr init*/; ptr < vobj.begin() +displayLimit; ++ptr)
        {
            std::cout << "  " << *ptr;
        }
        std::cout << "  ..." << std::endl;
    }
...

迭代器只为您提供int值，并且您可以将其与曾经“吃掉” int值的东西一起使用。在您的情况下，for循环。

如果告诉程序使用迭代器，则告诉程序:“只要给我Vector开头的数字并加10”即可。

在您的情况下，0“...并添加10”

您还可以编写类似的代码，并将n传递给函数
能够使用.end-input + 10来显示10行:

...
template <class T>
void displayVectorVer2(std::vector<T>& vobj, typename std::vector<T>::iterator ptr,int n)
{
    std::cout << "Vector object contains " << vobj.size() << " values which are" << std::endl;

    const unsigned displayLimit = 10;
    if (vobj.size()>displayLimit)
    {
       ptr=vobj.begin();
        for (; ptr<vobj.end() -n +displayLimit; ++ptr)
        {
            std::cout << "  " << *ptr;
        }
        std::cout << "  ..." << std::endl;
    }
    else
    {
        for (ptr=vobj.begin(); ptr<vobj.end(); ++ptr)
        {
            std::cout << "  " << *ptr;
        }
        std::cout << std::endl;
    }
}

int main()
{
    std::vector<int> vobj;
    std::cout << "Before calling fillVector(...): vobj contains "
              << vobj.size() << " values." << std::endl;

    std::cout << "\nEnter # of random values you'd like to store in vobj: ";
    int n;
    std::cin >> n;

    std::cout << "\n*** Calling fillVector(...) ***" << std::endl;
    fillVector(vobj, n);

    std::cout << "\n*** Calling displayVectorVer2(...) ***" << std::endl;
    std::vector<int>::iterator ptr;
    displayVectorVer2(vobj,ptr,n);
}
...

您也不应再在现代代码中使用srand，因为它已使用了10年以上since c++11 introduced <random>
冒昧可能会损害您的程序。如果用于生成敏感代码的种子。 srand也不提供应提供的“随机性” srand generates some numbers more often than others-这不是随机的。