Suggestion: More Unix-like

[NoraCodes]

I would suggest that one major improvement to this tool would be to make it more UNIX-ish, i.e., take arguments from the command line rather than interactively. Since all four functions take three values, this could be relatively simple, with an invocation like:

./PerfectGasLaw function value1 value2 value3

Since you've already modularized your code (good job!) all you'd have to do is change the getValuesForX functions and the main function. Here's a hint to get you started: if you declare main like this:

int main(int argc, char** argv)

you can use argc (the number of arguments) and argv (an array of strings containing the arguments) to get whatever is passed in at the command line.