help w/ pointers vs arrays

PTBmilo · 04-09-2003, 06:58 PM

I'm trying to understand how these things work right now and am running into a question that isn't answered in the book.

suppose that I have something like this:

Code:

.........
       char line[MAXLINE],
             *malloc(),
             *sptr,
             *strtbl[NLINES];

        while((nchars = get_line (line,MAXLINE)) != EOF &&
              (sptr = malloc ((unsigned) nchars +1)) != NULL )
        {
                strcpy (sptr, line);
                strtbl[i++] = sptr;
        }
.......

is it also legal to do this?:

Code:

        char *line,
                *sptr,
                **strtbl;
       
       while((nchars = get_line (line,MAXLINE)) != EOF &&
              (sptr = malloc (nchars +1)) != NULL )
        {
                strcpy (sptr, line);
                *(strtbl + i++) = sptr;
        }

I ask because it does work just the same both ways, but it seems like *(strtbl + i++) would end up pointing to someting else?

is this legal?

AltF4 · 04-09-2003, 08:04 PM

char *strtbl[NLINES] ... reserves an array of NLINES char pointers in memory, strtbl is initialized to point to the first element of the array

char **strtbl ... is a pointer to a char pointer, but is not initialized to point anywhere (and no memory is reserved for storing the data array)

to make things more visible consider this:

typedef char *CPTR;
CPTR strtbl[NLINES]; /* reserves array and points to 1st element */

versus

typedef char *CPTR;
CPTR *strtbl; /* points nowhere */

hope it helps :-)

PTBmilo · 04-09-2003, 09:57 PM

that does help. thanks.

is there a way around having the constant? maybe something like:

typedef char *CPTR;
CPTR *strtbl = malloc( nArraySize * sizeof(*int));

would this allocate space for nArraySize number of int* 's ?

AltF4 · 04-10-2003, 05:13 PM

>typedef char *CPTR;
>CPTR *strtbl = malloc( nArraySize * sizeof(*int));

*int = not correct
int * = what you need

> would this allocate space for nArraySize number of int* 's ?

would allocate space for nArraySize INT pointers, but "strtbl" would point to CHAR pointers, which is probably not what you inteded :-)