String array function problem

I've been banging my head against the wall trying to figure out why this doesn't work. Isn't strs a pointer to a pointer of char? Any help would be appreciated. Thanks.

Code:

#include <stdio.h>
#include <string.h>

#define LIM 10
#define MAX 81

void print_arrs(const char **strs, int num);

int main(void)
{
        char strs[LIM][MAX];

        puts("Enter a string, blank to quit");
        while (cnt < LIM && (gets(strs[cnt]) && strs[cnt] != '\0')) {
                cnt++;
                puts("Enter a string, blank to quit");
        }

        print_arrs(strs, cnt);

        return 0;
}

void print_arrs(const char **strs, int num)
{
        int i;

        for (i = 0; i < num; i++)
                puts(strs[i]);
        return;
}

I get the following warning:
tmp.c:33: warning: passing argument 1 of 'print_arrs' from incompatible pointer type

Originally Posted by albundy

Isn't strs a pointer to a pointer of char?

When an array is passed as an argument, it is converted to a pointer to its first element. Therefore, an array of arrays is converted to a pointer to an array, not a pointer to a pointer, hence the "incompatible pointer type".

See also: Question 6.18


Quzah.

While you're browsing the CLC FAQ, check this entry in particular
Question 12.23

Thanks for the replies. I'm still caught up on this. In print_arrs, strs[i] is a pointer to char (the i'th string), correct? It seems I'm obviously wrong because swap_onetwo is complaining about incompatible types in assignment.

Code:

#include <stdio.h>
#include <string.h>

#define LIM 10
#define MAX 81

void print_arrs(char (*strs)[MAX], int num);
void swap_onetwo(char (*strs)[MAX], int num);

int main(void)
{
        char strs[LIM][MAX];
        int cnt = 0;

        puts("Enter a string, blank to quit");
        while ((gets(strs[cnt]) && strs[cnt][0] != '\0')) {
                if (++cnt == 10)
                        break;
                puts("Enter a string, blank to quit");
        }

        print_arrs(strs, cnt);

        return 0;
}

void swap_onetwo(char (*strs)[MAX], int num)
{
        char *tmp;

        tmp = strs[0];
        strs[0] = strs[1];
        strs[1] = tmp;

        return;
}

void print_arrs(char (*strs)[MAX], int num)
{
        int i;

        for (i = 0; i < num; i++)
                puts(strs[i]);
        return;
}

tmp.c: In function 'swap_onetwo':
tmp.c:46: error: incompatible types in assignment
tmp.c:47: error: incompatible types in assignment


And yes, I fully understand the ramifications of using gets()

Originally Posted by albundy

In print_arrs, strs[i] is a pointer to char (the i'th string), correct?

Code:

void print_arrs(char (*strs)[MAX], int num);

strs is a pointer to an array of characters.
*strs is an array of characters.
strs[ x ] is an array of characters.
strs[ x ][ y ] is a single character.
*strs[ 0 ] is a single character.

That looks about right:

Code:

#include<stdio.h>
int main( void )
{
    char array[ 3 ][ 5 ] = { { 0 }, }; /* an array of arrays */
    char (*ap)[ 5 ] = array; /* a pointer to an array of five characters */
    char *p = NULL; /* a pointer to a character */
    char c = 0; /* a character */
    
    p = *ap; /* assign a pointer to a character to a pointer to a character */
    c = *ap[ 0 ]; /* assign a character to a character */
    c = ap[ 1 ][ 1 ]; /* likewise... */
    
    return 0;
}

Quzah.

I have something that works but I'm still not completely sure why my previous code did not. I think it is because you cannot change the pointer unless you pass a pointer to the pointer.

Code:

#include <stdio.h>
#include <string.h>

#define LIM 10
#define MAX 81

void print_arrs(char *strs[MAX], int num);
void swap_onetwo(char *strs[MAX], int num);

int main(void)
{
        char arr[LIM][MAX];
        char *strs[LIM];

        int cnt = 0;

        puts("Enter a string, blank to quit");
        while ((gets(arr[cnt]) && arr[cnt][0] != '\0')) {
                strs[cnt] = arr[cnt];
                if (++cnt == 10)
                        break;
                puts("Enter a string, blank to quit");
        }

        print_arrs(strs, cnt);
        swap_onetwo(strs, cnt);
        print_arrs(strs, cnt);

        return 0;
}

void swap_onetwo(char *strs[MAX], int num)
{
        char *tmp = NULL;

        tmp = strs[0];
        strs[0] = strs[1];
        strs[1] = tmp;

        return;
}

void print_arrs(char *strs[MAX], int num)
{
        int i;

        for (i = 0; i < num; i++)
                puts(strs[i]);
        return;
}

I've created an array of pointers and set them to point to the beginning of each string. Is this the right way to do it? Thanks.

If you mean your post #5, that didn't work because you were trying to assign array names (that is, strs[0] was an array and strs[1] was an array).