String array function problem

This is a discussion on String array function problem within the C Programming forums, part of the General Programming Boards category; I've been banging my head against the wall trying to figure out why this doesn't work. Isn't strs a pointer ...

  1. #1
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    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

  2. #2
    C++ Witch laserlight's Avatar
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    Quote 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".
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  3. #3
    ATH0 quzah's Avatar
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    See also: Question 6.18


    Quzah.
    Hope is the first step on the road to disappointment.

  4. #4
    and the hat of wrongness Salem's Avatar
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    While you're browsing the CLC FAQ, check this entry in particular
    Question 12.23
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  5. #5
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    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()

  6. #6
    ATH0 quzah's Avatar
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    Quote Originally Posted by albundy View Post
    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.
    Hope is the first step on the road to disappointment.

  7. #7
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    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.

  8. #8
    and the Hat of Guessing tabstop's Avatar
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    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).

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