checking a char **array for values

Question

I got a char **function which returns obviously an Array of chars.... the problem is that I don't know how many strings are filled in the array .... char *data[size_of_data]; has been set at the first, but the function may not need all the array fields....

presume I got int size_of_data = 100 , but it just got 15 Strings .... if I want to break after the 15th String, which condition do I need in C ... I got the data already in a char ** field and tried sth like....

 while(strcmp(data[i],'\0'))
{
    msg_send (session, para[0],data[i]);
    printf("------> %s \n",data[i]);
}

tell me I thought NULL or'\0' but I am wrong... I m still struggling with c ... coming from Java — Fendrix
– Fendrix, Commented Aug 23, 2012 at 19:45

Ari · Accepted Answer · 2012-08-23 19:48:38Z

1

One thing you can do is to also provide the size of the array as an out parameter. Your function signature would look like this

char ** function(size_t * num_strings); //num_strings is an out parameter

Then you do something like so

size_t nstrings = 0;
char **strings = function(&nstrings);
size_t i = 0;
while (i < nstrings) {
    //do stuff
}

There's an advantage to knowing the size, instead of just iterating until you hit a null pointer. Say you prepared an array of 100 char*. What if you use them all? There's no null pointer, and you'll have overrun your buffer. Of course, you could add an extra pointer at the end as a delimiter, but personally I like having the size.

answered Aug 23, 2012 at 19:48

Ari

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2 Comments

bitmask Over a year ago

Out parameters always feel like a hack to me. Even if some standard interfaces use it.

Ari Over a year ago

For stylistic reasons, you could also write the function signature as void function(char **arr, size_t *num) . I think this looks a bit nicer, and to my mind the intent is more clear.

qelper · Accepted Answer · 2012-08-23 19:34:30Z

1

In your char **function you can put a NULL pointer at the end of your array. So in this case put a NULL after 15th string. Then in your while loop check data[i] == NULL, if its that's the end.

answered Aug 23, 2012 at 19:34

qelper

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1 Comment

Fendrix Over a year ago

thnks but is there no other way... what I wanted to know is what happens in c at the char *data[15] if I wouldn't put a Null pointer... sth like '\0' or NULL should be automatically set otherwise to which element does the pointer point on the heap at data[15] if not NULL ?

lordjeff82 · Accepted Answer · 2012-08-23 19:36:00Z

1

The correct way to do this is to have the function place a null pointer (0) into the data array at the first location that is not used.

This can simply be tested by checking to see if the current pointer is 0:

for (i = 0; data[i]; i++) {
    puts(data[i]);
}

answered Aug 23, 2012 at 19:36

lordjeff82

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Comments

bitmask · Accepted Answer · 2012-08-23 20:06:41Z

Basically there are two common things you can do, either the convention of indicating the end-of-array with a nullpointer or return a struct that contains an additional size_t which holds the number of elements. There is also a variation of the second version using the iterator idiom.

end is null:

char** fun() {
  /* ... */
  char** array = /*...*/;
  unsigned end = 15;
  array[end] = NULL;
  return array;
}

The caller will then stop reading from the result array when it encounters a NULL.

separate size

struct {
  size_t size;
  char** array;
} fun() {
   /* ... */
   char** array = /*...*/;
   unsigned end = 15;
   return {end,array};
}

iterator idiom (lent from C++)

struct {
  char** begin;
  char** end;
} fun() {
   /* ... */
   char** array = /*...*/;
   unsigned end = 15;
   return {array,array+end};
}

This has the advantage that is can be used nicely in loops:

for (struct {char** begin; char** end;} it = fun(); it.begin != it.end; ++it.begin) {
  printf("%s\n",it.begin);
}

And you can also determine the size easily by subtracting end - begin.

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