C++ Variables Losing Assignment Inside "if" Loop - c++

Question

So this is a very simple problem I'm sure, but I'm just starting out.
In the program, there is simple input validation. If inputed is entered properly, no issues.
The problem is, when testing the program with an error, like entering a zero or negative number, all the variables are blank (i.e., strings become blank and numbers become zero) in the output.
Thanks ahead of time for the help and insight.
// 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

An if statement is a simple branch, not a loop. At the end of the if, execution continues past the end of the block.
if (menu_selection >= 1 && menu_selection <= 3)
This, when false, will skip the meat of your program and jump to your code that handles invalid input.
else
{
cout << "\nMenu selection is not 1, 2, or 3.\n\nPlease correctly enter a number 1 through 3: ";
cin >> menu_selection;
}
After you input menu_selection again, control flows to
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;
The new input is never acted on, and the untouched values are printed. Replace the initial if with a do {...} while(condition); loop that wraps the user input. Once the input is satisfactory, you can then proceed to the core of the program.
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);

Answer 2

You can handle zero, negative number, or all possible inputs that are not defined in your case block by adding default block to your switch statement. Then your code will look like something like this.
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;
default:
// handle zero, negative numbers and so on.
break;
}
Reference: http://www.tutorialspoint.com/cplusplus/cpp_switch_statement.htm