如果选择<5，C ++如何让菜单选项返回顶部

Question

我在第一个菜单中使用了 for 循环，但我决定添加一个二级菜单选项以消除混乱。

当我添加第二个菜单时，for 循环搞砸了一切，所以我将其删除。我只是希望程序能够在用户输入菜单列表中不存在的数字时说“数字无效，请输入正确的数字”。

代码如下...

#include <iostream>
#include <cmath>

using namespace std;

int main ()
{

    double mi, ki, F, C;
    double lbs, kg, ft, m;
    int choice, choice2 = 0;


    cout<<"              MENU ONE               " << endl;
    cout<<"*************************************" << endl;
    cout<<"|1.) Convert miles to kilometers    |" << endl;
    cout<<"|2.) Convert kilometers to miles    |" << endl;
    cout<<"|3.) Convert Fahrenheit to Celsius  |" << endl;
    cout<<"|4.) Convert Celsius to Fahrenheit  |" << endl;
    cout<<"|5.)      FOR MORE CONVERSIONS      |" << endl;
    cout<<"*************************************" << endl;
    cin>> choice;

    if (choice >= 1 && choice < 5){
    switch (choice)
    {
        case 1:
            cout<< "Now enter the amount of miles you wish to convert: " <<endl;
            cin>> mi;
            cout<< "The answer is: " << mi / 0.62137 <<" Kilometers" <<endl;
            break;

        case 2:
            cout<<"Now enter the amount of Kilometers you want to convert: " << endl;
            cin>> ki;
            cout<< "The answer is: " << ki * .62137 <<" Miles " <<endl;
            break;

        case 3:
            cout<<"Now enter the degree in Fahrenheit: " << endl;
            cin>>F;
            cout<< "The answer is: " << (F-32)/1.8 <<" degrees Celsius "<< endl;
            break;

        case 4:
            cout<<"Now enter the degree in Celsius: " << endl;
            cin>>C;
            cout<< "The answer is: " << (C*1.8)+32 <<" degress Fahrenheit "  << endl;
            break;


        }
    }

//********************************************************************************************************//

    else if (choice == 5)
    {
        cout<<"             MENU TWO            " << endl;
        cout<<"*********************************" << endl;
        cout<<"|1.) Convert Pounds to Kilograms|" << endl;
        cout<<"|2.) Convert Kilograms to Pounds|" << endl;
        cout<<"|3.) Convert Feet to Meters     |" << endl;
        cout<<"|4.) Convert Meters to Feet     |" << endl;
        cout<<"|********************************" << endl;
        cin>>choice2;
    }
        if (choice2 >= 1 && choice2 < 5){
        switch (choice2)
        {


        case 1:
            cout<<"Now enter the weight in pounds: " <<endl;
            cin>>lbs;
            cout<<"The answer is: " << lbs / 2.2046 <<" kilograms " << endl;
            break;

        case 2:
            cout<<"Now enter the weight in kilograms: " << endl;
            cin>>kg;
            cout<< "The answer is: " << kg * 2.2046 <<" pounds "<< endl;
            break;

        case 3:
            cout<<"Now enter the number of feet to be converted to 'M': " <<endl;
            cin>>ft;
            cout<<"The answer is: " << (ft / 3.20808) << " meters "<< endl;
            break;

        case 4:
            cout<<"Now enter the number of meters to be converted to 'FT': " <<endl;
            cin>>m;
            cout<<"The answer is: " << (m * 3.20808) << " feet "<< endl;
            break;

            return 0;

            }

        }


    return 0;
}

Answer 1

为两个开关盒添加默认值：

default:
  cout << "Invalid input..." ;
  break ;

Answer 2

您可能希望将switch (choice2) 放在else if 块中（因为choice2 是该块中的局部变量 - 您无法在该语句块之外访问它；它也会遵循案例 1 到 4，这将表示第二个菜单在未被要求时显示）。然后，您可以在 if 和 else if 语句之后使用 else 块打印出错误消息。

if (choice >=1 && choice < 5)
{
    switch (choice)
    // the switch
}
else if (choice == 5)
{
    // the second menu
    switch (choice2)
    {
        // switch...
    }
else {
    // print error message
}

或者您可以将所有内容都放在一个 switch 中。

然后，将所有内容放回一个循环中（或使用 goto），并在选择 == 5 的情况下添加一个类似的循环（如果您想确保用户在第二个菜单上输入正确的数字）。对循环的条件使用布尔值；当用户输入错误时将其设置为 true，否则设置为 false。

bool repeat = false;
while (repeat)
{
    // ask the user to input a number
    switch (choice)
    {
    // cases 1 through 4 (omitted)
    // don't forget to set repeat to false for these cases
    case 5:
    // print second menu and input choice2
        switch (choice2)
        {
            // prompt the user to enter another number
            bool repeat2 = false;
            while (repeat2)
            {
            // cases 1 through 4
            default: // use the default case label for numbers that are not 1 to 4
            // print error message
            repeat2 = true;
            }
        }
        repeat = false;
    break;
    default: // use the default case label for numbers that are not 1 to 5
        // print error message
        repeat = true;
    }
}