C++ 简单菜单程序；难以找到用户输入的中间字符

Question

这只是我第二次在这里发帖，所以我希望我这样做是正确的。 我需要让用户输入任意长度的字符串 (usrinput) 并选择 1 - 4。我已经完成了 2、3 和 4，但我不知道如何进行选择 1。 如果用户输入一个字符串“这是一个测试”并选择选项 1，如果奇怪我需要找到中间字母。如果即使不显示中间的两个字母，我也需要让用户知道没有中间。我觉得我的功能接近正确，但我很难理解它的含义。非常感谢任何帮助，如有必要，我可以尝试进一步详细说明。

#include <iostream>    
#include <iomanip>
#include <string>
using namespace std;

int main()
{
    string usrinput, upper, lower, str;   
    int selection, middle, i;    
    bool menu = true;    
    
    cout << "=============================================================" << endl;
    cout << "Welcome to my program. Enter a sentence and make a selection: " << endl;
    cout << "Enter -999 to exit the program. " << endl;
    cout << "=============================================================" << endl;
    cout << "1. display middle character. " << endl; //if user selects 1, do "..."
    cout << "2. display sentence uppercase " << endl; //if user selects 2, do "..."             
    cout << "3. display sentence lowercase" << endl; //if user selects 3, do "..."
    cout << "4. display sentence backwards" << endl; //if user selects 4, do "..."               

    cout << "Enter a sentence: " << endl;        
    getline(cin, usrinput); //sentence input
    
    
    while (menu == true)
    {
    cout << "Make a selection: " << endl; // if selection is 1 - 4 || -999 (good input) anything <1 or >5 (bad input, loop until selection = 1 - 4
    cin >> selection;

    //Step 1. Input Validation
    while (selection != 1 && selection != 2 && selection != 3 && selection != 4 && selection != -999) //If the selection is not 1 - 4 || -999 loop until selection is valid. 
    {
        cout << "Invalid Entry. Please make another selection: " << endl; 
        cin >> selection;
    }

    if (selection == 1) // if the user enters 1: show middle character if there is one / let the user know there isn't one.
    {
        cout << "Middle: " << endl;
        cout << "=======" << endl;   


        if (((i = usrinput.length() /2 % 2) == 1))
        {
            cout << usrinput[i];
            cout << endl;
        }

        else if (((i = usrinput.length() / 2) % 2) >= 1)

        {
            cout << "There is no middle";
            cout << endl;
        }           

    }

Answer 1

对于您的选择 1 if 语句，您只需检查字符串是否均匀除以 2 以确定它是偶数还是奇数，无需除以 2。

你也可以在 if 后面加上一个 else 语句来说明偶数。

 if (((i = usrinput.length() % 2) == 1))
 {
      i = usrinput.length()/2;
      cout << usrinput[i];
      cout << endl;
 }
 else
 {
       cout << "There is no middle";
       cout << endl;
 }