Beginner's question concerning pointers and arrays

This is a discussion on Beginner's question concerning pointers and arrays within the C Programming forums, part of the General Programming Boards category; I am having trouble understanding why I can't get my array of strings to print using a pointer. Here is ...

  1. #1
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    Beginner's question concerning pointers and arrays

    I am having trouble understanding why I can't get my array of strings to print using a pointer.

    Here is my code so far:

    Code:
    #include <stdio.h>
    
    int main() {
    
      int a[] = {1, 10, 100, 1000};
      int *b = a;
      char *c[] = {"hello", "how", "are", "you?"};
    
      printf("%d\n", *b);
      printf("%d\n", *++b);
      printf("%d\n", *++b);
    
      printf("%s\n", *c);
      printf("%s\n", *++c);
    
      return 0;
    }
    When I run try to compile this code, I receive this error:

    Code:
    pointer-test.c: In function ‘main’:
    pointer-test.c:14: error: lvalue required as increment operand
    Why does the same pointer syntax not work for an int array and a string array?

    Thanks for any help.

  2. #2
    DESTINY BEN10's Avatar
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    c is an array of char pointers, which means c is actually an array, and that's why you can't use ++ operator with it. Whereas b is a poniter(not an array), that's why ++b is legal. Take this as an example.
    Code:
    int a[]={1,2,3,4};
    printf("%p",(void *)++a); // again the same error coz a is an array
    HOPE YOU UNDERSTAND.......

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  3. #3
    spurious conceit MK27's Avatar
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    You can do this:
    Code:
    printf("%s\n", ++(*c));
    but that ++ == sizeof(char), which is not what you want.

    Consider subscript notation:
    Code:
    printf("%s\n", c[1]);
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    Thanks for the help. I appreciate both of your replies.

    I have two further questions about the following code:

    Code:
    #include <stdio.h>
    
    int main(int argc, char *argv[]) {
    
      int a[] = {1, 10, 100, 1000};
      int *b = a;
      char *c[] = {"hello", "how", "are", "you?"};
      char **d = c;
    
      printf("%d\n", *b);
      printf("%d\n", *++b);
      printf("%d\n", *++b);
      printf("%d\n", *++b);
    
      printf("%s\n", *d);
      printf("%s\n", *++d);
      printf("%s\n", *++d);
      printf("%s\n", *++d);
    
      while (--argc > 0)
        printf("%s\n", *++argv);
    
      return 0;
    }
    1) Why is it that I can manipulate argv, which is an array of char pointers, using ++ but couldn't do the same to c (in the original post)?

    2) Is my way of using char **d sloppy at all? After reading BEN10's answer, I had the idea of using a pointer to a pointer to a char, but I don't know if that is a convoluted way of doing things.

    My goal with this little program is to understand pointers better and not use indexing, since I already understand how indexing works.

    Thanks again for any help.

  5. #5
    and the hat of wrongness Salem's Avatar
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    > 1) Why is it that I can manipulate argv, which is an array of char pointers, using ++ but couldn't do the same to c (in the original post)?
    Because it's a pointer.
    char *argv[] could equally be written as char **argv

    Using [] in the context of a function parameter doesn't add any "true array" properties to it.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  6. #6
    DESTINY BEN10's Avatar
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    Quote Originally Posted by rcomj View Post
    Thanks for the help. I appreciate both of your replies.

    I have two further questions about the following code:

    Code:
    #include <stdio.h>
    
    int main(int argc, char *argv[]) {
    
      int a[] = {1, 10, 100, 1000};
      int *b = a;
      char *c[] = {"hello", "how", "are", "you?"};
      char **d = c;
    
      printf("%d\n", *b);
      printf("%d\n", *++b);
      printf("%d\n", *++b);
      printf("%d\n", *++b);
    
      printf("%s\n", *d);
      printf("%s\n", *++d);
      printf("%s\n", *++d);
      printf("%s\n", *++d);
    
      while (--argc > 0)
        printf("%s\n", *++argv);
    
      return 0;
    }
    1) Why is it that I can manipulate argv, which is an array of char pointers, using ++ but couldn't do the same to c (in the original post)?

    2) Is my way of using char **d sloppy at all? After reading BEN10's answer, I had the idea of using a pointer to a pointer to a char, but I don't know if that is a convoluted way of doing things.

    My goal with this little program is to understand pointers better and not use indexing, since I already understand how indexing works.

    Thanks again for any help.
    1. When you use any array as an argument to function it decomposes into pointer and thus you can do ++ with argv. Here's an example.
    Code:
    #include<stdio.h>
    void f(int []);
    int main(void)
    {
    	int a[]={1,2,3};
    	printf("%p",(void *)a);
    	f(a);
    	getch();
    }
    void f(int b[])
    {
    	printf("\n%p",(void *)++b);//++ is legal here
    }
    2. It's fine I guess.
    Last edited by BEN10; 08-11-2009 at 09:14 AM.
    HOPE YOU UNDERSTAND.......

    By associating with wise people you will become wise yourself
    It's fine to celebrate success but it is more important to heed the lessons of failure
    We've got to put a lot of money into changing behavior


    PC specifications- 512MB RAM, Windows XP sp3, 2.79 GHz pentium D.
    IDE- Microsoft Visual Studio 2008 Express Edition

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