Enum and strings

Question

int main()
{     
    enum a {cat, dog, elephant};
    enum a b;

    b=dog;
    if (b==dog) printf("Y"); else printf("N");

    return 0;
}

The program above works. It shows Y. But I do not see it very useful, so I want to make something similar, but with strings:

int main()
{     
    enum a {cat, dog, elephant};
    enum a b;

    char s[100];
    gets(s);

    b=dog;
    if (b==s) printf("Y"); else printf("N");

    return 0;
}

Even if s=="dog", It doesn't work.

It won't compile because you compare a char array to an enum. — Hans Klünder, Dec 30 '14 at 09:19
enum are integer, in order to convert the string to the enum you need to parse it. — mpromonet, Dec 30 '14 at 09:20

score 0 · Answer 1 · answered Dec 30 '14 at 09:20

0

It does not work because you are comparing enum to a pointer.

While char s[100] can indeed hold a string, s simply points to a memory location of the start of your string. In other words, s is a memory chunk, 100 bytes long.

answered Dec 30 '14 at 09:20

Andrejs Cainikovs

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I know what is the problem. But I do no know how to create a program with those functions. enum a {"cat", "dog", "elephant"}; do not work with "". – webpersistence Dec 30 '14 at 09:22
@Abbey: First answer Deduplicator's question: Should the program be C or C++? The solutions will be different. – M Oehm Dec 30 '14 at 09:23
@MOehm , I've removed the [tag:c++] tag from the question as OP mentions "It should be in C" in [one of his comments](http://stackoverflow.com/questions/27702564/enum-and-strings#comment43817575_27702564) – Spikatrix Dec 30 '14 at 09:24

score 0 · Answer 2 · edited May 23 '17 at 10:33

You could play C preprocessor tricks notably with X-macros (see e.g. this).

You could have a macro invoking some macro P on each animal

#define DO_ANIMALS(P) \
  P(cat) \
  P(dog) \
  P(elephant)

then use it to declare the enum

enum animals_en {
#define DECLARE_ANIMAL(An) En,
DO_ANIMALS(DECLARE_ANIMAL)
};

and later use it to convert a string s to the enum animals_en

enum animals_en convert_string_to_animal(const char*s) {
#define CHECK_STRING_ANIMAL(An) if (!strcmp(s, #An)) return An;
DO_ANIMALS(CHECK_STRING_ANIMAL)
   fprintf(stderr, "bad animal %s\n", s); exit(EXIT_FAILURE);
}

^{You may want to look at the preprocessed form obtained with gcc -Wall -C -E yoursource.c > yoursource.i then use some editor or pager on the generated yoursource.i to understand more how that works.}

Read documentation of GNU cpp, notably stringification & concatenation.

You could also improve this code to generate something similar to the answer from Vlad from Moscow.

Such tricks would work both in C and in C++, but are more C-like. (In C++11 you would rather try having templates).

Vlad from Moscow · Accepted Answer · 2014-12-30T09:35:28.977

Try the following approach

#include <stdio.h>
#include <string.h>

int main(void) 
{
    enum Animal { cat, dog, elephant };
    char * animal_name[] = { "cat", "dog", "elephant" };
    enum Animal b;
    size_t n;

    char s[100];
    fgets( s , sizeof( s ), stdin );

    n = strlen( s );
    if ( s[n - 1] == '\n' ) s[n - 1] = '\0';

    b = dog;
    if ( strcmp( animal_name[b], s ) == 0 ) printf( "Y\n" ); 
    else printf( "N\n" );

    return 0;
}

If to enter dog then the output is

Also before comparing strings you could convert the entered string to the lower case that is used in strings of the array of animal names.

Or you could use C# instead of C where enumerators can be converted to strings. :)

Enum and strings

3 Answers3

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