Calculators for gas law equations

I've made a C++ program that calculates a missing variable in an equation for one of the five following laws of gas:

• Boyle's Law

• Charles' Law

• Gay-Lussac's Law

• Avogadro's Principle

• Ideal Gas Law

The way I made it includes a lot of repetitive if statement checking and different functions with small variations. I feel like there is a cleaner and more efficient way of doing this, but I'm not sure on how to go about making that happen.

Here's the code:

```
#include
#include

using namespace std;

string x;
bool invalid = false;

double boyOperation();
double charOperation();
double lusOperation();
double avogOperation();
double idealOperation();

int selectLaw() {
system("clear");
cout > x;
system("clear");
invalid = false;
if (x == "1") {
cout > variableStr;
system("clear");
if (variableStr == "P1") {
cout > P2;
cout > V1;
cout > V2;
P1 = (P2 * V2)/V1;
return P1;
} else if (variableStr == "P2") {
cout > P1;
cout > V1;
cout > V2;
P2 = (P1 * V1)/V2;
return P2;
} else if (variableStr == "V1") {
cout > P1;
cout > P2;
cout > V2;
V1 = (P2 * V2)/P1;
return V1;
} else if (variableStr == "V2") {
cout > P1;
cout > P2;
cout > V1;
V2 = (P1 * V1)/P2;
return V2;
} else {
cout > variableStr;
system("clear");
if (variableStr == "V1") {
cout > T1;
cout > V2;
cout > T2;
V1 = (V2/T2)*T1;
return V1;
} else if (variableStr == "V2") {
cout > T1;
cout > T2;
cout > V1;
V2 = (V1/T1)*T2;
return V2;
} else if (variableStr == "T1") {
cout > T2;
cout > V1;
cout > V2;
T1 = V1/(V2/T2);
return T1;
} else if (variableStr == "T2") {
cout > T1;
cout > V1;
cout > V2;
T2 = V2/(V1/T1);
return T2

First things first:

-
Do not use namespace std; This is bad style.

-
Create a class to properly encapsulate your code and simplify passing of variables.

-
I would suggest, that you use enumerations to simplify your code. Basically, if you have 4 possible inputs and you only want 3 of them, create a for loop where you split the appropriate input.

for (size_t input = 0; input < 4; ++input) {
    if (input == YOUR_ENUM_COICE) {
        continue;
    }
    GetInput(input);
}

-
Obviously, you would need a suitable class structure so that you can set the other variables easily.

YourClass::GetInput(const size_t input) {
    switch(input) {
    case (ENUM_ALTERNATIVE1):
        std::cout > P;
        break;
    case (ENUM_ALTERNATIVE2):
        ...
    }
}

-
You should also perform real error checking, aka "what happens when the user enters a letter?"

StackExchange Code Review Q#156741, answer score: 8