Variable names

[tygerrock]

I am trying to find which is a valid option and am not able to find the answer.
The 2 options are Sum_ and _s_u_m_. I'm thinking it is _Sum but looking for the reason why.

Thanks in advance for your help.

[AbstractionAnon]

What do you mean by "valid option"?
Both are valid variable names.

Variable names that begin with underscore are generally reserved for the implementation.

[tygerrock]

OK thanks. I knew the options to start a variable had to be a letter or (_) underscore, could find any rules about how the variable ended. I was told only one option was invalid. Thanks for your help.

[JLBorges]

In user code:

Sum_ is a valid name everywhere.

_s_u_m_ is not a valid name if it is in the global namespace; anywhere else, it is a valid name.

_Sum is not a valid name anywhere.

Certain sets of names and function signatures are always reserved to the implementation:

— Each name that contains a double underscore __ or begins with an underscore followed by an uppercase letter is reserved to the implementation for any use.

— Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace.