Unusual initializer for char array in C

Question

I have the initializer for char array:

static const char dtmf_positions[] = "123A-------" "456B-------" "789C-------" "*0#D-------" "----E------" "-----F-----" "------G----" "-------H---" "--------J--" "---------K-" "----------L";

Can somebody explain, how to get some symbol by index, eg symbol '4'. Thank you.

Ok, then I have additional question. Is there any convenient way to access array elements by row and column index in above kind of array, like we do with two dimensional array?

Answer 1

From C99 specs, 5.1.1.2 Translation phases

6. Adjacent string literal tokens are concatenated.

You can find similar text in other C specs also.

So

"abc" "def" would become "abcdef" during translation phase.

So your definition is similar to:

static const char dtmf_positions[] = "123A-------456B-------789C-------*0#D-----------E-----------F-----------G-----------H-----------J-----------K-----------L";

I hope now you can find index for any symbol :)

---

EDIT: Your additional question:

 
                               /* Col 012345678910 */
static const char dtmf_positions[] = "123A-------"  /* Row 0 */
                                     "456B-------"  /* Row 1 */
                                     "789C-------"  /* Row 2 */
                                     "*0#D-------"  /* Row 3 */
                                     "----E------"  /* Row 4 */
                                     "-----F-----"  /* Row 5 */
                                     "------G----"  /* Row 6 */
                                     "-------H---"  /* Row 7 */
                                     "--------J--"  /* Row 8 */
                                     "---------K-"  /* Row 9 */
                                     "----------L"; /* Row 10 */
#define NCOLS (sizeof("123A-------") - 1)
#define FETCH_CHAR(ROW,COL) dtmf_positions[ROW * NCOLS + COL]

After this you can access any character with FETCH_CHAR(R,C)

Answer 2

This is a single C literal. The language allows you to split string literals by adding quotes on both sides of a "gap", and inserting whitespace in between (demo):

char *s = "AB" "CD";
printf("%s\n", s);

is the same as

char *s = "ABCD";
printf("%s\n", s);

This is usually done for convenience when formatting your code: line breaks are allowed, too, so you can make parts of the literal appear in vertical columns:

static const char dtmf_positions[] = "123A-------"
                                     "456B-------"
                                     "789C-------"
                                     "*0#D-------"
                                     "----E------"
                                     "-----F-----"
                                     "------G----"
                                     "-------H---"
                                     "--------J--"
                                     "---------K-"
                                     "----------L";

So if you would like to find the index of character '4', start counting at zero, and skip over the "gaps" in your literal to get the index (you should get 11).