Question

I am trying to make a look up table. Here is the pretext:

Suppose, following is the defines list of certain macros.

#define ENTITY1 0x10001001
#define ENTITY2 0x10001002
.
.
.

The ENTITY_ is the User readable string value of the otherwise unsigned long integer type value and there can be any number of macros (say greater than 200, or even 500).

Now, there is a list which keeps track of which entity exists in which file number. Something like this:

 0x10001001 1
 0x10001002 2
 0x10001003 3
 .
 .
 .

The use of the long unsigned integers for each ENTITY is necessary because of proprietary conventions.

The first list is already present, and the second list needs to be generated through a program by using the macro strings in #defines of the first list as the user enters the record.
Since the number of such entries is very large, hard coding each value is a burdensome task. Also, if the first list is updated, the second list will not update appropriately if additional switch cases are not coded.

When the user makes an entry, he tells that the entry is to be made in ENTITY3 through a string variable, the system should look up if a macro exists by the name ENTITY3. If yes, then open the file with number 3 and do the necessary processing, otherwise, display warning that such an entry does not exist.

So, how do I compare the string variable entered by the user with a macro name without using SWITCH CASE?

I am using C programming. GNU C Library.

---

Edit: Here is the scenario.

The different entities named ENTITYn (n can be any number) can exist in different files which have a certain integer number 1,2,3…
But, the proprietary environment has built up these entities such that they are recognized using certain unsigned long integers like 0x01001001 etc. For each entity, the macros have been defined in some header files corresponding to those entities by the name ENTITY1 ENTITY2…
Now when a certain manager wants to change something, or enter certain data to a particular entity, he would address is by the name ENTITYn, and the program would look up in a lookup table for a corresponding entry. If a match is found, it would use the unsigned long integer code for that entity for subsequent processing internal to the proprietary system, access another lookup table which looks for which file number has this entry and opens that file location for processing.

I need to populate this second table with the unsigned long ints of the Entities and their corresponding locations (let all of them be in a single file 1 for now). I want to circumvent the condition, that the one making that LUT has to know the corresponding entity unsigned long integer codes. The program uses the input string i.e. ENTITY1 and directly maps it.

But now I am beginning to think that hardcoding a LUT would be a better option. 🙂

Answer 1

Macros are preprocessor features, they’re not visible to the C compiler. So you cannot directly reference the “values” of macros from code.

It seem you need two look-up tables, if I get this correctly:

1. One table mapping a string such as ENTITY1 to to a unique unsigned integer, such as 0x10001001.
2. One table mapping an unsigned integer such as 0x10001001 to a “file number” which looks like a (small) unsigned integer such as 1.

Both of these tables can be generated by processing the source code you seem to have. I would recommend gathering the ENTITYn strings into something like this:

struct entity_info
{
  const char *name;
  unsigned int key;
};

Then have your pre-processing code build a sorted array of these:

const struct entity_info entities[] = {
{ "ENTITY1", 0x10001001 },
{ "ENTITY2", 0x10001002 },
/* and so on */
};

Now you can implement an efficient function like this:

unsigned int get_entity_key(const char *entity_name);

It could perhaps use binary-search, internally.

Then you need to do the second step, obviously. I’m not sure of the exact details of these values (how and when they can change); if the “file number” for a given entity is constant, it could of course be added directly into the entity_info structure.