Thread: Need help with if statements and switch statments

I'm working on a program for my class which requires us to create an Ideal Gas Law solver using if and switch commands. The program should ask the user which variable they want to solve for: pressure, temperature or volume. After picking which one, the program should ask for the other two in appropriate units. I've continued working on the program when I ran into an issue. I've also included a few default gasses with their corresponding molar masses and a 6th option for the user to input their own molar mass but rather than redirecting a user to input their own molar mass it goes straight to asking for the variable input and I can't figure out how to get around this issue. I'm fairly new to C++ and stumbled on this forum for help. I'm not asking for anyone to complete my program for me, but rather point out what I can include to make it suit what I'm looking for. Any additional help on how what is wrong in my program would be very very appreciated. Again, I know that I need to know how to do this to pass my class so all I ask is guidance. Thank you so much!

Code:

#include <iostream>
#include <cmath>
using namespace std;
int main()
{
//Declare and initialize variables and constants
//p = Pressure (kPa), V = Volume (m^3), m = mass (kg), R = Gas Constant (0.0281), T = Temperature (C) and (K)
//MM = other molar mass


int p, V, m, R, T, K, MM, code ;
R = 0.287 ;
K = T + 273.15 ;
//**Print**//
//Introduction
cout << "Welcome to the My Ideal Gas Law Solver! \n" <<endl;
cout << "This program solves p*V=m*R*T for p, V or T given" <<endl;
cout << "the other two along with mass and the type of gas. \n" <<endl;
cout << "Select a gas by entering the corresponding number:" <<endl;
cout << "(1) Steam M = 18.02" <<endl;
cout << "(2) Nitrogen M = 28.01" <<endl;
cout << "(3) Air M = 28.97" <<endl;
cout << "(4) Oxygen M = 32.00" <<endl;
cout << "(5) Carbon Dioxide M = 44.01" <<endl;
cout << "(6) Other \n" <<endl;
//User inputs
switch (code)
{
case 1:
cout << "You have chosen Steam." <<endl;
break;
case 2:
cout << "You have chosen Nitrogen." <<endl;
break;
case 3:
cout << "You have chosen Air." <<endl;
break;
case 4:
cout << "You have chosen Oxygen." <<endl;
break;
case 5:
cout << "You have chosen Carbon Dioxide." <<endl;
break;
case 6:
cout << "Please enter a Molar Mass for the gas you wish to calculate in kg:" <<endl;
break;
default:
cout << "Error with selection. Please pick a number between 1 and 6 next time." <<endl;
break;
}


int input = 0;
while (((input<1 || input>5) && input!=6))
{ cout << "Please enter an integer between 1 and 6: ";
  cin >> input;
}


if (code == 6)
{
    cin >> MM ;
cout << "You have chosen " << MM <<endl;
}


cout << "Select a variable to solve for by entering the corresponding number: " <<endl;
cout << "(1) Pressure" <<endl;
cout << "(2) Volume" <<endl;
cout << "(3) Temperature" <<endl;
cin >> code;
switch (code)
{
case 1:
cout << "Please enter the volume in cubic meters: " <<endl;
cin >> V;
cout << "Please enter the temperature in degrees celsius: " <<endl;
cin >> T;
break;
case 2:
cout << "Please enter the pressure in kiloPascals: " <<endl;
cin >> p;
cout << "Please enter the temperature in degrees celsius: " <<endl;
cin >> T;
break;
case 3:
cout << "Please enter the pressure in kiloPascals: " <<endl;
cin >> p;
cout << "Please enter the volume in cubic meters: " <<endl;
cin >> V;
break;


//Compute Ideal Gas Law equation
if (code == 1)
{
    p = (m * R * T)/V ;
}
if (code == 2)
{
    V = (m * R * T)/p ;
}
if (code == 3)
{
    T = (p * V)/(m * R) ;
}


//Compute Temperature conversion
cout << "Ideal Gas Law computation results:" << endl;
cout << "Gas name: " << endl;
cout << "Unknown value: " << endl;
cout << "Molar weight: " << endl;
cout << "Gas constant: " << R << " kJ/kg-K" << endl;
cout << "Mass: " << endl;
cout << "Pressure: " << p << " kPa" << endl;
cout << "Temperature: " << T << " C" << endl;
cout << "Volume: " << V << " m^3" << endl;
return 0;
}
}

In lines 60-64 the if code is meant to redirect the user to an option where they input their own molar mass. Otherwise, options 1-5 on lines 27-50 with the preset molar masses are meant to directly take the user to the variable selection screen (i.e. volume, pressure, temperature). I'm sorry, I should have elaborated my question a little better.

I've been working on this program some more and have this:

Code:

#include <iostream>
#include <cmath>
using namespace std;
int main()
{
//Declare and initialize variables and constants
//p = Pressure (kPa), V = Volume (m^3), m = mass (kg), R = Gas Constant (0.0281), T = Temperature (C) and (K)
//MM = other molar mass
int p, V, m, R, T, K, MM, code;
//**Print**//
//Introduction
cout << "Welcome to the My Ideal Gas Law Solver! \n" <<endl;
cout << "This program solves p*V=m*R*T for p, V or T given" <<endl;
cout << "the other two along with mass and the type of gas. \n" <<endl;
cout << "Select a gas by entering the corresponding number:" <<endl;
cout << "(1) Steam M = 18.02" <<endl;
cout << "(2) Nitrogen M = 28.01" <<endl;
cout << "(3) Air M = 28.97" <<endl;
cout << "(4) Oxygen M = 32.00" <<endl;
cout << "(5) Carbon Dioxide M = 44.01" <<endl;
cout << "(6) Other \n" <<endl;
//User inputs
int input = 0;
while (input<1 || (input>5 && input!=6))
{ cout << "Please enter a number between 1 and 6: ";
  cin >> input;
  code = input;
}
switch (code)
{
case 1:
cout << "You have chosen Steam." <<endl;
m=18.02;
break;
case 2:
cout << "You have chosen Nitrogen." <<endl;
m=28.01;
break;
case 3:
cout << "You have chosen Air." <<endl;
m=28.97;
break;
case 4:
cout << "You have chosen Oxygen." <<endl;
m=32.00;
break;
case 5:
cout << "You have chosen Carbon Dioxide." <<endl;
m=44.01;
break;
case 6:
cout << "Please enter a Molar Mass for the gas you wish to calculate in kg:" <<endl;
cin>>MM;
cout << "You have chosen " << MM <<endl;
m=MM;
break;
}
cout << "Select a variable to solve for by entering the corresponding number: " <<endl;
cout << "(1) Pressure" <<endl;
cout << "(2) Volume" <<endl;
cout << "(3) Temperature" <<endl;
cin >> code;
switch (code)
{
case 1:
cout << "Please enter the volume in cubic meters: " <<endl;
cin >> V;
cout << "Please enter the temperature in degrees celsius: \n" <<endl;
cin >> T;
break;
case 2:
cout << "Please enter the pressure in kiloPascals: " <<endl;
cin >> p;
cout << "Please enter the temperature in degrees celsius: \n" <<endl;
cin >> T;
break;
case 3:
cout << "Please enter the pressure in kiloPascals: " <<endl;
cin >> p;
cout << "Please enter the volume in cubic meters: \n" <<endl;
cin >> V;
break;}
//Compute Ideal Gas Law equation
R = 0.287 ;
K = T + 273.15 ;
if (code == 1)
{
    p = (m * R * K)/V ;
}
if (code == 2)
{
    V = (m * R * K)/p ;
}
if (code == 3)
{
    K = (p * V)/(m * R) ;
}
//Compute Temperature conversion
cout << "Ideal Gas Law computation results!" << endl;
cout << "Gas name: " << endl;
cout << "Unknown value: " << endl;
cout << "Molar weight: " << endl;
cout << "Gas constant: " << R << " kJ/kg-K" << endl;
cout << "Mass: " << endl;
cout << "Pressure: " << p << " kPa" << endl;
cout << "Temperature: " << T << " C" << endl;
cout << "Volume: " << V << " m^3" << endl;
return 0;
}

It somehow keeps getting messed up every time I paste it here. I've got the indents in my program.

does this help some?

Code:

#include <iostream>
#include <cmath>
using namespace std;
int main()


{
//Declare and initialize variables and constants
//p = Pressure (kPa), V = Volume (m^3), m = mass (kg), R = Gas Constant (0.0281), T = Temperature (C) and (K)
//MM = other molar mass


int p, V, m, R, T, K, MM, code;


//**Print**//


//Introduction
    cout << "Welcome to the My Ideal Gas Law Solver! \n" <<endl;
    cout << "This program solves p*V=m*R*T for p, V or T given" <<endl;
    cout << "the other two along with mass and the type of gas. \n" <<endl;
    cout << "Select a gas by entering the corresponding number:" <<endl;
    cout << "(1) Steam M = 18.02" <<endl;
    cout << "(2) Nitrogen M = 28.01" <<endl;
    cout << "(3) Air M = 28.97" <<endl;
    cout << "(4) Oxygen M = 32.00" <<endl;
    cout << "(5) Carbon Dioxide M = 44.01" <<endl;
    cout << "(6) Other \n" <<endl;
//User inputs
int input = 0;
while (input<1 || (input>5 && input!=6))
{ cout << "Please enter a number between 1 and 6: ";
  cin >> input;
  code = input;
}
switch (code)
{
    case 1:
        cout << "You have chosen Steam." <<endl;
    m=18.02;
        break;
    case 2:
        cout << "You have chosen Nitrogen." <<endl;
    m=28.01;
        break;
    case 3:
        cout << "You have chosen Air." <<endl;
    m=28.97;
        break;
    case 4:
        cout << "You have chosen Oxygen." <<endl;
    m=32.00;
        break;
    case 5:
        cout << "You have chosen Carbon Dioxide." <<endl;
    m=44.01;
        break;
    case 6:
        cout << "Please enter a Molar Mass for the gas you wish to calculate in kg:" <<endl;
        cin >> MM;
        cout << "You have chosen " << MM <<endl;
        m=MM;
        break;
}


    cout << "Select a variable to solve for by entering the corresponding number: " <<endl;
    cout << "(1) Pressure" <<endl;
    cout << "(2) Volume" <<endl;
    cout << "(3) Temperature" <<endl;
    cin >> code;
    switch (code)
{
    case 1:
        cout << "Please enter the volume in cubic meters: " <<endl;
        cin >> V;
        cout << "Please enter the temperature in degrees celsius: \n" <<endl;
        cin >> T;
    break;
    case 2:
        cout << "Please enter the pressure in kiloPascals: " <<endl;
        cin >> p;
        cout << "Please enter the temperature in degrees celsius: \n" <<endl;
        cin >> T;
    break;
    case 3:
        cout << "Please enter the pressure in kiloPascals: " <<endl;
        cin >> p;
        cout << "Please enter the volume in cubic meters: \n" <<endl;
        cin >> V;
    break;}
//Compute Ideal Gas Law equation
    R = 0.287 ;


    K = T + 273.15 ;


if (code == 1)
{
    p = (m * R * K)/V ;
}
if (code == 2)
{
    V = (m * R * K)/p ;
}
if (code == 3)
{
    K = (p * V)/(m * R);
    T = K - 273.15;
}
//Compute Temperature conversion
    cout << "Ideal Gas Law computation results!" << endl;
    cout << "Gas name: " << endl;
    cout << "Unknown value: " << endl;
    cout << "Molar weight: " << endl;
    cout << "Gas constant: " << R << " kJ/kg-K" << endl;
    cout << "Mass: " << endl;
    cout << "Pressure: " << p << " kPa" << endl;
    cout << "Temperature: " << T << " C" << endl;
    cout << "Volume: " << V << " m^3" << endl;
    return 0;
}

For some reason, all of the variables in your program are integers, but you want to use floating point math.
After all, you can't properly convert to kelvin without adding 273.15!

So either you are entering in decimals anyway and ignoring the type of your variables, which will make your cin statements break, or you really are entering in whole numbers and then your calculations break because all of the nice floating point constants you use get implicitly converted to integers. You really ought to use double for any variables involved in calculations, and this does NOT mean that you go to the 1 line that has all of your variables declared and change int to double! It's an extremely bad habit to declare variables that way.

For 1) C++ variable types can often have multiple parts separated by whitespace (such as &, *, < and >) that only lead to type related errors if you aren't careful.
For 2) it's messy.
For 3) you seem to think that there is a magic type that all of your variables can be, and this is not true -- programs can depend on hundreds of types -- so declaring everything on one big line is not plausible.

Originally Posted by whiteflags

It's not unique to C++ at all, though.

I edited my post before you replied (sorry). I think however it is unique to C++... it's a moving target that nobody in their right mind can possibly hope to keep up with. Or, for that matter, want to keep up with.

Thank you so much guys on the suggestions! It's slowly morphing more and more into what I am looking for. Is this what you guys meant about changing the variable so they're not only integers?

Code:

#include <iostream>
#include <cmath>
using namespace std;
int main()
{
//Declare and initialize variables and constants
//p = Pressure (kPa), V = Volume (m^3), m = mass (kg), R = Gas Constant (0.0281), T = Temperature (C) and (K)
//MM = other molar mass
float p, V, m, R, T, K, MM;
int code;
std::string gN;
//**Print**//
//Introduction
    cout << "Welcome to the My Ideal Gas Law Solver! \n" <<endl;
    cout << "This program solves p*V=m*R*T for p, V or T given" <<endl;
    cout << "the other two along with mass and the type of gas. \n" <<endl;
    cout << "Select a gas by entering the corresponding number:" <<endl;
    cout << "(1) Steam M = 18.02" <<endl;
    cout << "(2) Nitrogen M = 28.01" <<endl;
    cout << "(3) Air M = 28.97" <<endl;
    cout << "(4) Oxygen M = 32.00" <<endl;
    cout << "(5) Carbon Dioxide M = 44.01" <<endl;
    cout << "(6) Other \n" <<endl;
//User inputs
int input = 0;
while (input<1 || (input>5 && input!=6))
{ cout << "Please enter a number between 1 and 6: ";
  cin >> input;
  code = input;
}
switch (code)
{
    case 1:
        cout << "You have chosen Steam." <<endl;
    m=18.02;
    gN="Steam";
    R=8.314;
        break;
    case 2:
        cout << "You have chosen Nitrogen." <<endl;
    m=28.01;
    gN="Nitrogen";
    R=8.314;
        break;
    case 3:
        cout << "You have chosen Air." <<endl;
    m=28.97;
    gN="Air";
    R=0.287;
        break;
    case 4:
        cout << "You have chosen Oxygen." <<endl;
    m=32.00;
    gN="Oxygen";
    R=8.314;
        break;
    case 5:
        cout << "You have chosen Carbon Dioxide." <<endl;
    m=44.01;
    gN="Dioxide";
    R=8.314;
        break;
    case 6:
    cout << "Please enter the name of the Gas you are calculating:"<<endl;
    cin >> gN;
    cout << "You have chosen the name" << gN << endl;
    cout << "Please enter a Molar Mass for the gas you wish to calculate in kg:" <<endl;
    cin >> MM;
    cout << "You have chosen " << MM <<endl;
    m=MM;
    R=8.314;
        break;
}
    cout << "Select a variable to solve for by entering the corresponding number: " <<endl;
    cout << "(1) Pressure" <<endl;
    cout << "(2) Volume" <<endl;
    cout << "(3) Temperature" <<endl;
    cin >> code;
switch (code)
{
    case 1:
        cout << "Please enter the volume in cubic meters: " <<endl;
        cin >> V;
        cout << "Please enter the temperature in degrees celsius: \n" <<endl;
        cin >> T;
    break;
    case 2:
        cout << "Please enter the pressure in kiloPascals: " <<endl;
        cin >> p;
        cout << "Please enter the temperature in degrees celsius: \n" <<endl;
        cin >> T;
    break;
    case 3:
        cout << "Please enter the pressure in kiloPascals: " <<endl;
        cin >> p;
        cout << "Please enter the volume in cubic meters: \n" <<endl;
        cin >> V;
    break;}
//Compute Ideal Gas Law equation
K = T + 273.15 ;
//pV = m(8.314/M)T
if (code == 1)
{
    p = (m * R * K)/V ;
}
else if (code == 2)
{


    V = (m * R * K)/p ;
}
else if (code == 3)
{
    K = (p * V)/(m * R) ;
}
//Compute Temperature conversion
    cout << "Ideal Gas Law computation results:" << endl;
    cout << "Gas name: " << gN << endl;
    cout << "Molar weight: " << endl;
    cout << "Mass: " << endl;
    cout << "Pressure: " << p << " kPa" << endl;
    cout << "Temperature: " << T << " C" << endl;
    cout << "Volume: " << V << "m^3" << endl;
return 0;
}