在 C++ 中使用向量设置开关

Question

我正在尝试学习 C++，但我有点卡住了。

我正在使用Geany 和 CentOS 创建一个基本的成绩簿应用程序。

Geany 在应用程序的 for 和 cout 行上显示红色卷线。我觉得一切都在正确的语法，但我得到“编译失败”。

我的来源有什么问题？

#include <vector>
#include <string>
#include <iostream>

using namespace std;

int main(void){

    // Declaration of local variables for gradebook
    vector<int> gradeVector;
    char choice;
    bool menu = true;

    // User Menu
    while (menu){
        cout << "This is the gradebook menu\n";
        cout << "Please enter (a) to Show All Grades\n";
        cout << "Please enter (l) to Show Last Grade\n";
        cout << "Please enter (g) to Add Grade\n";
        cout << "Please type 0 to Exit.\n";
        cin >> choice;

        switch(choice){
            case 'a':
                for (i=0;i <= gradeVector.size(); i++)
                cout << "These are the grades entered: \n"<<gradeVector.at(i);
            break;
            case 'l':
                cout << "This was the last grade entered: \n" << gradeVector;
            break;
            case 'g':
                cout << "Please add a grade to the gradebook: \n" << cin >> gradeVector.push_back();
            break;
            case 0:
                menu = false;
            break;
            default:
                cout << "Please ONLY use lower-case a, l, g, or 0 as your choice.\n"
        }
    }
    return (0);
}

Answer 1

您的代码中有多个错误。首先你忘了声明整数i。你试图输出一个向量，从你的文字中我猜你只想要最后一个成绩。输入应使用 push_back 进行，但应首先将其读入临时变量。更正这一点并更改一些通用代码将使您的代码成为您的代码：

#include <vector>
#include <string>
#include <iostream>

using namespace std;

int main(void){

    // Declaration of local variables for gradebook
    vector<int> gradeVector;
    char choice;
    bool menu = true;

    // User Menu
    while (menu){
        cout << "This is the gradebook menu\n";
        cout << "Please enter (a) to Show All Grades\n";
        cout << "Please enter (l) to Show Last Grade\n";
        cout << "Please enter (g) to Add Grade\n";
        cout << "Please type 0 to Exit.\n";
        cin >> choice;

        switch(choice){
            case 'a':
                cout << "These are the grades entered: \n"
                for (int i=0 ;i < gradeVector.size(); i++)
                    cout << gradeVector[i] << " ";
            break;
            case 'l':
                cout << "This was the last grade entered: \n" << gradeVector[gradeVector.size()-1];
            break;
            case 'g':
                cout << "Please add a grade to the gradebook: \n";
                int input;
                cin >> input;
                gradeVector.push_back(input);
            break;
            case 0:
                menu = false;
            break;
            default:
                cout << "Please ONLY use lower-case a, l, g, or 0 as your choice.\n";
        }

     }

    return (0);
}

Answer 2

首先，你的 for 循环语法是错误的。

for (i=0;i <= gradeVector.size(); i++)

那每次都会给你一个segmentation fault，应该是

for (i=0;i < gradeVector.size(); i++)

因此您不会访问超出范围的内存。请记住，计数从 0 开始，您不能访问超过分配的内存。

你的编译错误在这里：

cout << "This was the last grade entered: \n" << gradeVector;

你无法计算出这样的向量；你必须找出一个特定的元素。如果要计算向量中的所有元素，请使用 for 循环。