CODEWITHSUNDEEP
carrayscharliterals
hoverjet
hoverjet

Reputation: 31

Unusual initializer for char array in C

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?

Upvotes: 3

Views: 133

Answers (6)

Mohit Jain
Mohit Jain

Reputation: 30489

From C99 specs, 5.1.1.2 Translation phases

  1. 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)

Upvotes: 7

Nic30g
Nic30g

Reputation: 659

simply: 

char c = dtmf_positions[11];

Upvotes: 1

khanak gupta
khanak gupta

Reputation: 63

In case of arrays in C, the index starts from 0. So if you want to access the first element of array you can use dtmf_positions[0]. In case you donot know the index of the elements either you can use a loop to traverse out the array and check for the elements or you can create some kind of maps which would store the indexes of all the elements (in case of character repetition) and than whenever you want to check if the element exists or not and what their positions are you can use this map.

Upvotes: 1

Gopi
Gopi

Reputation: 19864

What is confusing you?

char a[] = "someString";

is same as

char a[] = "some" "String";

So if you know how to access the first array then you are good to do the same for the second one.

int i;
size_t len = strlen(a);
for(i=0;i<len;i++)
printf("%c ",a[i]);

Upvotes: 2

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726479

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).

Upvotes: 2

Spikatrix
Spikatrix

Reputation: 20244

The declaration

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

is the same as

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

which creates a static const char array and initializes it with "123A-------456B-------789C-------*0#D-----------E-----------F-----------G-----------H-----------J-----------K-----------L"

If you want to get a character just use dtmf_positions[n] where n is a number between 0 and 121(121 is the \0 character)

Upvotes: 0

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