C++ 变量在 "if"循环中丢失赋值

Question

所以我确定这是一个非常简单的问题，但我才刚刚开始。

在程序中，有简单的输入验证。如果输入正确，没有问题。

问题是，当测试程序出现错误时，例如输入零或负数，输出中的所有变量都是空白(即字符串变为空白，数字变为零)。

提前感谢您的帮助和见解。

// This menu driven program determines the time sound will take to travel through
// gas, liquid, and solid, given a distance from a user.

#include <iostream>
#include <iomanip>
#include <string>

using namespace std;

int main()
{
// constants
const double    AIR_SPEED_RATE_PER_SECOND_SOUND = 1100.0,       //in feet per second
                WATER_SPEED_RATE_PER_SECOND_SOUND = 4900.0,     // in feet per second
                STEEL_SPEED_RATE_PER_SECOND_SOUND = 16400.0;    // in feet per second

// Program defined variables
double          Time_To_Travel = 0.0;       // in seconds

string          Medium;

// User defined variables
double          distance_of_travel; //in feet

int             menu_selection;

//Display a menu for mediums of sound conduction.
cout << "Sound travels at different speeds through air, water, and steel." << endl;
cout << "\nThis program will calculate the time it takes, in feet per second, for " \
"sound to travel a given distance." << endl;
cout << "Please select a number choice below:\n\n1. Air\n2. Water\n3. Steel " << endl;

//Get input from user.
cout << "\nEnter Selection: ";
cin >> menu_selection;

cout << "\nEnter distance in feet the sound will travel: ";
cin >> distance_of_travel;


// Input validate selection is on the menu
if (menu_selection >= 1 && menu_selection <= 3)
    {
    if (distance_of_travel > 0.0)  // input validation distance is positive
        {
        switch (menu_selection)  // calculate the time of travel based on user input
            {
            case 1: Medium = "air";
                    Time_To_Travel = distance_of_travel / AIR_SPEED_RATE_PER_SECOND_SOUND;
                break;

            case 2: Medium = "water";
                    Time_To_Travel = distance_of_travel / WATER_SPEED_RATE_PER_SECOND_SOUND;
                break;

            case 3: Medium = "steel";
                    Time_To_Travel = distance_of_travel / STEEL_SPEED_RATE_PER_SECOND_SOUND;
                break;
            }
        }
    else
        {
        cout << "\nPlease enter a distance greater than zero: ";
        cin >> distance_of_travel;
        }
    }

else
    {
    cout << "\nMenu selection is not 1, 2, or 3.\n\nPlease correctly enter a number 1 through 3: ";
    cin >> menu_selection;
    }
// Format to four decimal places and display the time sound takes to travel given distance.
cout << fixed << showpoint << setprecision(4);
cout << "Sound would take " << Time_To_Travel << " seconds to travel given distance of " << distance_of_travel << " feet in " << Medium << "." << endl;;

return 0;

Answer 1

if 语句是一个简单的分支，而不是循环。在 if 的末尾，执行会继续到 block 的末尾。

if (menu_selection >= 1 && menu_selection <= 3)

当为 false 时，将跳过程序的核心内容并跳转到处理无效输入的代码。

else
{
    cout << "\nMenu selection is not 1, 2, or 3.\n\nPlease correctly enter a number 1 through 3: ";
    cin >> menu_selection;
}

再次输入menu_selection后，控制流向

cout << fixed << showpoint << setprecision(4);
cout << "Sound would take " << Time_To_Travel << " seconds to travel given distance of " << distance_of_travel << " feet in " << Medium << "." << endl;;
return 0;

新的输入永远不会被执行，并且未被修改的值被打印出来。将初始的 if 替换为包装用户输入的 do {...} while(condition); 循环。一旦输入令人满意，您就可以进入程序的核心部分。

bool is_good;
do
{
    is_good = false;
    cout << "\nEnter Selection: ";
    cin >> menu_selection;
    cout << "\nEnter distance in feet the sound will travel: ";
    cin >> distance_of_travel;
    if (menu_selection < 1 || menu_selection > 3 || distance_of_travel < 0)
        cout << "error message here";
    else
        is_good = true;
} while (!is_good);