Accessing enumerations by name and by value (C Language)

funkey_monkey

Hi,

If I created an enumerated type in C - say days of week - am I able to reference any enumeration within that type by either its name (e.g. "Monday", "Saturday", ... etc.) and by a numerical represenation (e.g. Mon = 0, Tues = 1, Wed = 2, ...etc.).

In some other languages I've used, it was possible to overlay an alias onto the address of the variable and read out the enumeration literal assigned to it as an integer value.

How does the C compilers store enumerations? Does it vary depending on the compiler used?


Also, if it is possible, can I modify the type definition to start Monday = 1, Tuesday = 2, ... etc.

Thanks.

14ned

funkey_monkey wrote: »

Hi,

If I created an enumerated type in C - say days of week - am I able to reference any enumeration within that type by either its name (e.g. "Monday", "Saturday", ... etc.) and by a numerical represenation (e.g. Mon = 0, Tues = 1, Wed = 2, ...etc.).

For C style enums, the enum will implicitly convert to int on demand. You have to cast to send it the other way though.

For C++11 style enums, you must always cast between the enum type and its underlying type (which could be an int, float, or any constexpr type).

funkey_monkey wrote: »

In some other languages I've used, it was possible to overlay an alias onto the address of the variable and read out the enumeration literal assigned to it as an integer value.

How does the C compilers store enumerations? Does it vary depending on the compiler used?

enums exist purely within the C and C++ type system. They have no representation in the code generated.

All C and C++ compilers start enums at 0 unless told otherwise, so the first item is always 0, the next item is always 1, the next 2 and so on.

funkey_monkey wrote: »

Also, if it is possible, can I modify the type definition to start Monday = 1, Tuesday = 2, ... etc.

Thanks.

Of course. Simply assign any value you want. Be aware non-contiguous values may produce sub-optimal code though.

Niall

funkey_monkey

I'm trying to convert an Integer into a String. However, I've mucked up somewhere and can't figure out what I've done wrong.

Yes - I realise that I can just return the RHS of the statement instead of assigning a variable as an intermediate point. I had it in place for debug reasons.

char Convert(int day){
  
   // Text held within an array
   const char *dayArray[] = {"Text1", "Text2", "Text3", "Text4", "Text5"};

   char dayString = dayArray[(char)day];       

   return dayString; 

...
    // Debug data
    char firstday = Convert(initialDayInt);
    char finalday = Convert(dayCount);

...
    printf("Initial Day: %c\n", firstday);
    printf("Final Day: %c\n", finalday); 
}

The output from the above (no compilation errors or warnings):

printf("Initial Day: %c\n", firstday);
    printf("Final Day: %c\n", finalday);

I'm also wondering if it would be more efficient to use a switch statement instead of the above. Would this be so?

Alternatively, should I be looking to create dayString to be of enumerate type daysOfWeek and then cast the int day onto dayString? If so, how is this done?

My debug is showing that I'm not getting the dayString variable populated correctly.


Thanks.

funkey_monkey

Okay, I've realised that I need to use %s instead of %c. However, I'm still struggling to get it to work.

14ned

funkey_monkey wrote: »

I'm trying to convert an Integer into a String. However, I've mucked up somewhere and can't figure out what I've done wrong.

Yes - I realise that I can just return the RHS of the statement instead of assigning a variable as an intermediate point. I had it in place for debug reasons.

char Convert(int day){
  
   // Text held within an array
   const char *dayArray[] = {"Text1", "Text2", "Text3", "Text4", "Text5"};

   char dayString = dayArray[(char)day];       

   return dayString; 

...
    // Debug data
    char firstday = Convert(initialDayInt);
    char finalday = Convert(dayCount);

...
    printf("Initial Day: %c\n", firstday);
    printf("Final Day: %c\n", finalday); 
}

The output from the above (no compilation errors or warnings):

printf("Initial Day: %c\n", firstday);
    printf("Final Day: %c\n", finalday);

Hmm, smells like a homework assignment! Luckily I'm on christmas vacation!

Firstly, don't use char * for strings in C++. Use std::string. If you really, really, really must use char *, then use std::experimental::string_view instead (http://en.cppreference.com/w/cpp/header/experimental/string_view).

The code you posted above may have been mispasted. You are returning a single char, not a pointer to an array of char.

Also never index arrays using char which is an enormous security hole and highly prone to corrupting memory. Only ever use size_t to index arrays.

Finally never do unchecked array lookup. Always check if the index being looked up exceeds the array length, otherwise it's another security hole and and memory corruption possibility.

funkey_monkey wrote: »

I'm also wondering if it would be more efficient to use a switch statement instead of the above. Would this be so?

No. In modern C++ there is very little correlation between the code you type and what code is generated. This is why it takes at least 10,000 hours to master C++ to learn the typical transformation patterns used by the big three compilers, and the approx. 10-15% pay bump experienced C++ engineers see over Java ones.

Modern C++ compilers optimise using topological transforms, so imagine a bendy sheet which must be tamped down between partially moveable sharp points and it's a good analogy.

funkey_monkey wrote: »

Alternatively, should I be looking to create dayString to be of enumerate type daysOfWeek and then cast the int day onto dayString? If so, how is this done?

My debug is showing that I'm not getting the dayString variable populated correctly.


Thanks.

It seems awfully like you need to choose one of the many solutions from https://stackoverflow.com/questions/207976/how-to-easily-map-c-enums-to-strings. https://stackoverflow.com/questions/201593/is-there-a-simple-way-to-convert-c-enum-to-string#201792 is also worth reading for a preprocessor method of auto generating strings from enums.

I remember someone proposing a Boost library (http://www.boost.org/) for this stuff, but I don't think it got submitted for peer review.

Niall

funkey_monkey

No - not homework. Unemployed and retraining.

Also, I'm doing the above in C, not C++. Does this affect your reply?

funkey_monkey

14ned wrote: »

Hmm, smells like a homework assignment! Luckily I'm on christmas vacation!

Got made redundant beforce Christmas, so I'm trying to gain some additional skills to broaden my CV.

The code you posted above may have been mispasted. You are returning a single char, not a pointer to an array of char.

Yes, I noticed this when doing some further debugging. I can read the array value directly (e.g. dayArray[2]) but not indirectly using the variable.

14ned

funkey_monkey wrote: »

Got made redundant beforce Christmas, so I'm trying to gain some additional skills to broaden my CV.

Good on you for reacting this way. Whenever I'm between contracts I always take a course, I've got most of the way through an OU Maths degree doing that now which will be my third undergrad degree. Still, it's never easy getting laid off, whatever the circumstances.

funkey_monkey wrote: »

Yes, I noticed this when doing some further debugging. I can read the array value directly (e.g. dayArray[2]) but not indirectly using the variable.

As you're writing all this in C not C++, you're kinda stuck with zero terminated char arrays as the string representation unless you bring in a third party library which in any production code you always would, because C strings are utterly evil and riddled with traps. The biggest lesson I have learned from twenty years plus of writing C is don't write in C

Anyway, warnings aside, your string array looks like this:

static const char *weekdays[]={"Monday", "Tuesday", ... };

The type of weekdays is const char *[7] which will decay on demand to const char **, so a pointer to a pointer to a const char. When you index weekdays like this:

const char *tuesday=weekdays[1];

The type of tuesday is now a pointer to a const char. Indexing that again:

const char u=tuesday[1];

Printing these would be as follows:

printf("%c\n", u); // prints u
printf("%s\n", tuesday); // prints Tuesday
for(size_t n=0; n<sizeof(weekdays)/sizeof(*weekdays); n++)
  printf("%u: %s\n", (unsigned) n, weekdays[n]);
// prints 0: Monday, 1: Tuesday, 2: Wednesday etc.

I suspect your difficulty would be greatly helped if you enable all warnings, so if on GCC or clang use /Wall. If on MSVC make warnings Level 4 and enable Code Analysis.

Also if on MSVC, absolutely use VS2015 with Update 1 applied and forget about all earlier MSVC's. VS2015 is a quantum leap forward for Microsoft's compiler, it finally even has most of C99 support only fifteen years late!

Finally, if all the above looks greek to you, I'd suggest taking an online marked course in the C or C++ type system. I've been told they exist and some are quite good, but I know of none myself.

Niall

funkey_monkey

Thanks - I've got it working now in the funtion. I now have:

const char Convert(int day){
  
   // Text held within an array
   const char *dayArray[] = {"Text1", "Text2", "Text3", "Text4", "Text5"};

   char *dayString = dayArray[day];       

   return *dayString; 

...
    // Debug data
    const char *firstday = Convert(initialDayInt);
    const char *finalday = Convert(dayCount);

...
    printf("Initial Day: %s\n", *firstday);
    printf("Final Day: %s\n", *finalday); 
}

However, I can't get the first day and final day to print out. I'm still not 100% comfortable with my understanding of these pointers.

I can, however get the first character of the text to output correctly though:

// Debug data
    const char firstday = Convert(initialDayInt);
    const char finalday = Convert(dayCount);

...
    printf("Initial Day: %c\n", firstday);
    printf("Final Day: %c\n", finalday); 
}

Help? I just can't visualise what is (and what should be) happenng when I'm trying to use strings.

14ned

funkey_monkey wrote: »

Thanks - I've got it working now in the funtion. I now have:

const char Convert(int day){
  
   // Text held within an array
   const char *dayArray[] = {"Text1", "Text2", "Text3", "Text4", "Text5"};

   char *dayString = dayArray[day];       

   return *dayString; 

...
    // Debug data
    const char *firstday = Convert(initialDayInt);
    const char *finalday = Convert(dayCount);

...
    printf("Initial Day: %s\n", *firstday);
    printf("Final Day: %s\n", *finalday); 
}

However, I can't get the first day and final day to print out. I'm still not 100% comfortable with my understanding of these pointers.

I can, however get the first character of the text to output correctly though:

// Debug data
    const char firstday = Convert(initialDayInt);
    const char finalday = Convert(dayCount);

...
    printf("Initial Day: %c\n", firstday);
    printf("Final Day: %c\n", finalday); 
}

Help? I just can't visualise what is (and what should be) happenng when I'm trying to use strings.

Convert() should definitely be returning a const char *. Use %s to print a const char * as a string.

Niall

funkey_monkey

14ned wrote: »

Convert() should definitely be returning a const char *. Use %s to print a const char * as a string.

Niall

Hi Niall, thanks for your reply - is that not what I'm doing here:

...     
// Debug data     
const char *firstday = Convert(initialDayInt);     
const char *finalday = Convert(dayCount);  
...     
printf("Initial Day: %s\n", *firstday);     
printf("Final Day: %s\n", *finalday);  }

I've also tried it with the printf statements modified as:

printf("Initial Day: %s\n", firstday);     
printf("Final Day: %s\n", finalday);  }

I get the following errors:

main.c:151:28: warning: initialization makes pointer from integer without a cast
     const char *firstday = ConvertDayToString(initialDayInt);
                            ^
main.c:152:28: warning: initialization makes pointer from integer without a cast
     const char *finalday = ConvertDayToString(dayCount);
                            ^

BTW - I'm using NetBeans 8.1 as my IDE.

Also, why did you prefix the dayArray declaration with 'static'?

14ned

funkey_monkey wrote: »

Hi Niall, thanks for your reply - is that not what I'm doing here:

Nope:

const char Convert(int day){

should be:

const char *Convert(int day){

funkey_monkey wrote: »

I get the following errors:

main.c:151:28: warning: initialization makes pointer from integer without a cast
     const char *firstday = ConvertDayToString(initialDayInt);
                            ^
main.c:152:28: warning: initialization makes pointer from integer without a cast
     const char *finalday = ConvertDayToString(dayCount);
                            ^

BTW - I'm using NetBeans 8.1 as my IDE.

Yep you are doing an implicit conversion from a char to an int, and using that to initialise a pointer. Very bad.

I would keep changing the code until you see no more warnings anyway, that's for sure.

funkey_monkey wrote: »

Also, why did you prefix the dayArray declaration with 'static'?

If you don't, it gets auto duration which means the compiler will make a copy of the array every time you execute Convert(). If it has static duration, it doesn't copy the array.

Niall

funkey_monkey

Can you explain to me why the * is needed prior to the name of the Convert function.

Is it normal to have * beside every IO parameter in the function declaration?

The structure I was following has char * prefix on them but I was trying to pass in an attribute of the month array which is a constant char *.
This was giving me a compiler warning which I avoided by prefixing the function declarations with const.

Thanks.

14ned

funkey_monkey wrote: »

Can you explain to me why the * is needed prior to the name of the Convert function.

You are returning a pointer to a char, not a char itself.

Remember, strings in C are simply arrays of char. C does not allow variable length arrays to be returned from functions, so the next best thing to do is to return a pointer to the first item in the array.

funkey_monkey wrote: »

Is it normal to have * beside every IO parameter in the function declaration?

Only if the parameter being passed is a pointer to something rather than a something.

You can also have a pointer to a pointer to something e.g. int **foo;

A pointer to a pointer to a pointer to something also works e.g. float ***foo;

The star isn't a prefix, it is part of the type, same as any const-ness.

Niall

funkey_monkey

Thanks, so it is normal when creating a function to pass the parameters using pointers?

14ned

funkey_monkey wrote: »

Thanks, so it is normal when creating a function to pass the parameters using pointers?

It depends. If you passed the entire array as a parameter, the compiler would copy the entire array every time you enter the function. Similarly if you return an entire array, the compiler will again copy the whole thing on exit.

If you pass a pointer to something, it simply copies the pointer instead.

C is barely above Assembler i.e. it is very stupid. It does exactly what you tell it to do, provides no safety, no hand holding and no helpful behaviours like other languages. Unless you have a very good reason to program in C or C++, choose anything else like Java or Python or C# or Swift or Rust all of which are better choices almost all of the time.

Experienced C++ programmers earn 10-25% more than experienced devs in other languages because it takes at least five, and more usually ten, years of experience to master the complexity that is modern C++ - even those on the ISO C++ standard committee say none of them really understands the language even though they as a collective designed the thing. Even C will take you two or three years of daily use before you are competent, and that is assuming you attend week long training conferences three times a year each costing a thousand euro at least.

I'd urge you down any other path than C or C++ unless you are willing to invest years of your life training yourself up. Most of the big software bugs which wreck lives were written by incompetent C programmers, so most employers now expect a very high standard and are unwilling to train up junior programmers in C.

Niall

funkey_monkey

Thanks for that. I'm applying for a few jobs and trying to avoid anything with C. But still I want to have an initial grip of it. I need to sort myself out with pointers. I undertand them in principal, but in practice, it is causing problems.

Got some good feedback about a job in Java - a language I used a number of years ago. Been using proprietary languages for a number of years now.

Related to pointers, are you able to help with the PM I sent you?


Thanks for your replies.