#elseif not working

Projects Programming
1

the people.vcu.edu has the following text in a tutorial on #ifdef:
[If an #ifdef is evaluated to be false, then the #elseif directive is processed as if it was an #ifdef directive.

However, I'm getting an error that says (in the code line that says "#elif _RADIANT") "#elif with no expression"

in declarations I have the following:
    // Pick one unit from 6 below (LIGHT units in a separate file)
    // #define _SHOP    // nanoB PCB, this has the only RTC, bus master
    // #define _OFFICE  // _MINI PCB
     #define _RADIANT // _nanoB
    // #define _BATH    // _MINI PCB
    // #define _PAINT   // _MINI PCB
    // #define _WATER   // _MINI PCB
    // #define _GARAGE  // nanoB PCB
some code in setup()
end setup()

Then some code in loop
end loop

then I follow all that with functions, which includes the following directives within a function:
#ifdef _BATH
  code specific to Bath
#elif _GARAGE
  code specific to Garage
#elif _RADIANT
code specific to Radiant
#else  // all units with heat (shop, office, paint)
  alternate code
#endif

I've seen various errors when using the #elif/#elseif directive and sometimes they go away when I use one versus the other.

Previously I have only used the #ifdef/#endif pair of directives and everything seems to work, eg, if I have two blocks of code

#ifdef _PAINT
code
#endif

#ifdef _OFFICE
code
#endif

and I define _PAINT and comment out _OFFICE, I get the paint code and no error regarding the office block. I really don't know what "define" does--does it make the following statement a boolean and define it as true? If so, why doesn't the use of an undefined string later in the code not generate an error for an unknown variable?

3

Yes, I want it to process that line like an #ifdef, per the instructional text quoted above and repeated here:

If an #ifdef is evaluated to be false, then the #elseif directive is processed as if it was an #ifdef directive.

If this reference is incorrect, is there a way to do an if, elseif, elseif, else, endif sequence of directives for the preprocessor? OK, yes, there is a way, but is there a way using "define" to define the test(s)?

I suppose I could do the following:
bool _PAINT = false ;
bool _OFFICE = false ;
bool _GARAGE = false

and then change false for true for the particular compilation/upload that I'm doing? So the code would look something like

#if _PAINT // where _PAINT has been changed to "true" prior to compilation
code
#elif _GARAGE
code
#else // code for all the other units
code
#endif

The full page where I found the tutorial is here.

Thanks.

4
  1. From that same link above:

#define TEST1 100 // define a variable.
#define TEST2

So this throws me. TEST1 is "defined" as a variable, presumably = 100. What is TEST2? A boolean?

  1. If I do the following and comment out the define TEST2:

#define TEST1 100 // define a variable.
// #define TEST2

and I later use:
#elif TEST2

Why doesn't the IDE throw an error?

Thanks.

5

You’re almost there...
In the last example TEST2 is either #defined or NOT (i.e. empty)
Yes - a boolean in effect for the preprocessor.
Your previous example could also be explored...

bool _PAINT = true ;
bool _OFFICE = false ;
bool _GARAGE = false

could be reconsidered as-

#define _PAINT
//#define _OFFICE
//#define _GARAGE]
to get the pre-processordoing what you want.
Remembering of course these are not ‘variables’, but macro definitions which are considered by the compiler, not by the program at runtime.

6

If an #ifdef is evaluated to be false, then the #elseif directive is processed as if it was an #ifdef directive

I don't believe that that's true. The "#elseif" always behaves like "#if", which means that if you want to check whether a symbol has been defined or not, you need:

#elseif defined(_GARAGE)

"#ifdef X" is equivalent to "#if defined(X)", it doesn't have any magical properties on following "#elseif" statements.

"#elseif GARAGE", if GARAGE is not defined, is expanded to just "#elseif ", which is missing an expression, exactly as your error message says...

7

Thanks to both of you. Getting to understand the higher level directives of the compiler is the key for me and it won't happen overnight--I'll get the macros on by one. Both of your examples make sense and won't be a problem to implement in the code I'm working on (about 2500 lines, so far).

I don't know if it's worth making the point to the author of the "instruction", which I was relying on, and getting his/her error corrected.

Dr Quark

8

That tutorial looks like it is for a custom preprocessor that the author had written, NOT for the standard C/C++ preprocessor!

9

Yup. "dcomment".

10

Thanks. Just one more sand trap to look out for.

DQ

11

well, I've gotten back to this piece of code and using defined() doesn't seem to work. Here's the code:

#ifdef _SHOP
char board[] = "def: SHOP     " ;
const byte UID = 0x81 ;
#elseif defined(_OFFICE)
char board[] = "def: OFFICE  " ;
const byte UID = 0x82 ;
#elseif defined(_BATH)
char board[] = "def: BATH     " ;
const byte UID = 0x83 ;
#elseif defined(_PAINT)
char board[] = "def: PAINT     " ;
const byte UID = 0x84 ;
#elseif defined(_RADIANT)
char board[] = "def: RADIANT     " ;
const byte UID = 0x85 ;
#elseif defined(_WATER)
char board[] = "def: WATER     " ;
const byte UID = 0x86 ;
#elseif defined(_GARAGE)
char board[] = "def: GARAGE     " ;
const byte UID = 0x87 ;
#endif

If I define _SHOP I get error messages for all the #elseif's

02-bath_office:57:2: error: invalid preprocessing directive #elseif

 #elseif defined(_OFFICE)

and if I define _PAINT, get "board[]" unknown errors.

12 
invalid preprocessing directive #elseif

Try #elif

13

thanks. I'll give it a try. I was relying on the statement:

The #elif is functionally equivalent to the #elseif directive. These directives are interchangeable.

from the following website:

on edit
!! #elif solved the error. Thanks

14

Dr_Quark:
from the following website:

More dcomment.