4

I need to translate a value into a human readable string. Normally for things I define I would use values that start at zero and create a simple array of strings with the values as the index.

static const char *foo[] = { "foo", "bar", "etc" };
if (val < 3) printf("%s\n", foo[val]);

For this circumstance I have values that do not start at zero and there are some gaps between them. Is there a good way to do this without having to manually code in a bunch of empty strings for the indexes without a matching value/string pair?

static const char *foo[] = { "", "", "", "foo", "", "bar", "", "", "etc" };
jmfox24
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4 Answers4

7

As of C99, you can use designated initialisers:

static const char *foo[] = { 
   [3] = "foo", 
   [5] = "bar",
   [8] = "etc"
};

This is equivalent to the array definition you posted and will generate an array with 9 entries. There is a similar syntax for the initialisation of structs:

struct Person joe = {
    .name = "Joe", .age = 24, .favcolor = "mauve"
};

Note that this is a C feature only and will not work in C++.

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M Oehm
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  • This is what I have been doing but when referencing an index we didn't define, like 4 in your example, it returns null. My concern is this unsafe behavior; is this ok and I just need to handle the nulls appropriately? – jmfox24 May 21 '15 at 05:25
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    Yes, it is safe. Uninitialised values are initialised to a null or zero value, in your case the `NULL` pointer. The gaps are treated in the same way as trailing uninitialised elements are: In `int a[3] = {2, 5};` the value of `a[2]` is guaranteed to be 0. – M Oehm May 21 '15 at 05:31
  • (Note: My suggestion is only a nicer representation of your solution. It essentially creates the same array, including a lot of empty entries. If you need a sparse array with index-value pairs, you have to implement that explicitly.) – M Oehm May 21 '15 at 05:34
3

If there aren't too many gaps, you can encode each contiguous sequence as a separate array, and then do a little bounds-checking to find the appropriate one to use. Here's a quick-and-dirty example:

#include <stdio.h>
#include <stdlib.h>

static const int array_first_indices[] = {3, 15, 28, 32};
static const char * array0[] = {"foo"};
static const char * array1[] = {"bar", "baz"};
static const char * array2[] = {"bloop", "blorp", "blat"};
static const char * array3[] = {"glop", "slop", "bop"};

#define check_array(whichArray, idx) { \
   unsigned int relIdx = idx - array_first_indices[whichArray]; \
   if (relIdx < (sizeof(array##whichArray)/sizeof(const char *))) \
      return array##whichArray[relIdx]; \
   }

const char * LookupWord(int idx)
{
   check_array(0, idx);
   check_array(1, idx);
   check_array(2, idx);
   check_array(3, idx);
   return NULL;
}

int main(int args, char ** argv)
{
   for (int i=0; i<50; i++) printf("   LookupWord(%i) = %s\n", i, LookupWord(i));
   return 0;
}

For a fully general lookup mechanism, you'd probably need to use a data structure like a hash table or a tree; C implementations of those data structures are available, although if you have the option of using C++ it would be easier to use those data structures in that language, as they are provided by the standard library.

Jeremy Friesner
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1

Create a sorted array that maps IDs to strings and use the bsearch() function to look up the string:

#include <stdio.h>
#include <stdlib.h>

struct id_msg_map {
    int id;
    char const* str;
};


int comp_id_string( const void* key, const void* element)
{
    int key_id     = ((struct id_msg_map*) key)->id;
    int element_id = ((struct id_msg_map*) element)->id;

    if (key_id < element_id) return -1;
    if (key_id > element_id) return  1;
    return 0;
}

static struct id_msg_map msg_map[] = {
    {3, "message 3"} ,
    {12, "message 12"},
    {100, "message 100"},
    {32000, "message 32000"},
};

#define ELEMENTS_OF(x) (sizeof(x) / sizeof((x)[0]))
char const* get_msg(int x)
{
    struct id_msg_map key = {x};
    struct id_msg_map* msg = bsearch(&key, msg_map, ELEMENTS_OF(msg_map), sizeof(msg_map[0]), comp_id_string);

    if (!msg) return "invalid msg id";

    return msg->str;
}

void test_msg(int x)
{
    printf("The message for ID %d: \"%s\"\n", x, get_msg(x));
}

int main(void)
{
    test_msg(0);
    test_msg(3);
    test_msg(100);
    test_msg(-12);
    return 0;
}
Michael Burr
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0

You can use designated initialisers, as described in M. Oehm's post, but that silently introduces the same gaps you were referring to earlier (with implicit 0 values). That option is most suitable when you know 0 will never be an actual selection, when the table doesn't change dynamically (particularly in size) and when the size of the table is small.

If the table is particularly large, but items are never added or removed from it you can use qsort and bsearch on a key/value-pair style structure. For example:

struct foo_pair {
     int key;
     char *value;
};

int foo_pair_compare(void *x, void *y) {
    struct foo_pair *a = x, *b = y;
    return (a->key > b->key) - (a->key < b->key);
}

int main(void) {
    struct foo_pair foo[] = { { .key = 3, .value = "foo" },
                              { .key = 5, .value = "bar" },
                              { .key = 6, .value = "etc" } };

    /* qsort needs to be done at the start of the program,
       and again each time foo changes */
    qsort(foo, sizeof foo / sizeof *foo, sizeof *foo, foo_pair_compare);

    /* bsearch is used to retrieve an item from the sorted array */
    struct foo_pair *selection = bsearch(&(struct foo_pair) { .key = 5 },
                                         foo, sizeof foo / sizeof *foo,
                                         sizeof *foo, foo_pair_compare);
}

When items are routinely added or removed from the collection, it will make more sense to select a hashtable or some kind of ordered map. If you can't be bothered writing and testing your own of these collections, I imagine there are plenty of tried & tested libraries on the internet that you could check out.

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autistic
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