Functions

Mathematical functions are similar, but not quite the same as functions in C++.
The mathematical function

y = f(x)

where

f(x) = x²

will produce a value if we give a value for x. For example,
```
   y = f(3) = 9
   y = f(4) = 16
   y = f(5) = 25
   y = f(3) = 9 
```
If the same value for x is given to a mathematical function, it always gives the same result.
C++ functions can, but don't have to produce any value at all.
Some C++ functions produce a result even if we give no value to it. E.g.,

y = f();
For some C++ functions, if the same value for x is given, these functions don't always give the same result.