MESSAGE
| DATE | 2004-09-09 |
| FROM | Ruben Safir Secretary NYLXS
|
| SUBJECT | Re: [hangout] Re: two dimensional arrays passed to functions
|
> The right way to do it:
>
> char * insert(char *table, int cols, char **values);
This just won't take in example codes or in the main program I'm
writing.
It compiles but segfaults. This is what I statrted with which has put
me on
2 days of a twisted path.
In essence, everything I'm reading is saying that is you create
a[3][255]
then when you pass a as an argument, the data type sent is an array
pointer
Additionally, strangley enough, what it needs is not the number of
elements defined
in the first index, but the size of the second index, otherwise it is
clueless in the
function to do poiter aritmitic.
http://www-users.cs.umn.edu/~tan/www-docs/C_lang.html
6.18: My compiler complained when I passed a two-dimensional array to
a function expecting a pointer to a pointer.
A: The rule (see question 6.3) by which arrays decay into pointers
is not applied recursively. An array of arrays (i.e. a two-
dimensional array in C) decays into a pointer to an array, not a
pointer to a pointer. Pointers to arrays can be confusing, and
must be treated carefully; see also question 6.13. (The
confusion is heightened by the existence of incorrect compilers,
including some old versions of pcc and pcc-derived lints, which
improperly accept assignments of multi-dimensional arrays to
multi-level pointers.)
If you are passing a two-dimensional array to a function:
int array[NROWS][NCOLUMNS];
f(array);
the function's declaration must match:
f(int a[][NCOLUMNS])
{ ... }
or
f(int (*ap)[NCOLUMNS]) /* ap is a pointer to an array
*/
{ ... }
In the first declaration, the compiler performs the usual
implicit parameter rewriting of "array of array" to "pointer to
array" (see questions 6.3 and 6.4); in the second form the
pointer declaration is explicit. Since the called function does
not allocate space for the array, it does not need to know the
overall size, so the number of rows, NROWS, can be omitted. The
"shape" of the array is still important, so the column dimension
NCOLUMNS (and, for three- or more dimensional arrays, the
intervening ones) must be retained.
If a function is already declared as accepting a pointer to a
pointer, it is probably meaningless to pass a two-dimensional
array directly to it.
See also questions 6.12 and 6.15.
References: K&R1 Sec. 5.10 p. 110; K&R2 Sec. 5.9 p. 113; H&S
Sec. 5.4.3 p. 126.
as an example of much of what I've read.
Even worse, it seems C's implementaitons for 2 dimensional arrarys is
nothing for of
inconsistent. Things I've read that I should be able to do, simple
don't work.
Look at this sample program, for example:
#include
#include
#include
#include
void print_str(char **b);
int main(int argv, char *argc[]){
char a[3][256];
strcpy(a[0], "Ruben");
strcpy(a[1], "Safir");
strcpy(a[2], "222222");
printf("\n%s", a[0]);
printf("\n%s", a[1]);
printf("\n%s", a[2]);
print_str(a);
exit(0);
}
void print_str( char **b ){
printf("\n%s", *b);
b++;
printf("\n%s", *b);
b++;
printf("\n%s\n", *b);
return;
}
ruben-at-flatbush:~> gcc -Wall test_func.c test_func.c: In function `main':
test_func.c:21: warning: passing arg 1 of `print_str' from incompatible
pointer type
It segfualts and according to the literature, it is the wrong pointer
type because it
is only a char *[255] and not a char**
I've tried a dozen variations on this to get it to work, defineing in
main the d** and pointing it
at a and at &a, etc. I've tried sending a[0] and changing the function
definition to print_str( char *)
and to call the function as print_str(a[0]) as recommended in the KN
King book, which is an excellent
C book, and this fails.
Even
char ** d;
d = c;
or d = &c[0][0], etc have failed..............
> Advice: don't use arrays as arguments. It's stupid. Use pointers.
>
I'm wishing I can take this advise!
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