using a function to move a pointer(I think :| ).

[crinkelite]

Hi folks I think this is my first post so please let extend my greetings and salutations.

and now of course I'd like a little help. :P

I'm trying to get my crappy code to work but I'm having a little trouble.
the program prints a ASCII small multidimensional array in the form of a 7x7 square of '*' characters, my intention is to have a pointer that starts in the centre and marks it's location with a '?' character. the user inputs a direction and the '?' moves around the board. I taught I was close to accomplishing this but the but as it turns out I am unable to get the function to make permanent changes to the pointer location. It al;ways returns to the centre when the program returns to main.

Should I be using a pointer to a pointer???

here's the code anyway. I'd be greatful for your opinions/advice

 #include <cstdlib>
#include <iostream>

using namespace std;

char print(char board[7][7]);
char move(char *p);

int main(int argc, char *argv[])
{
    char board[7][7];                   //declare board and it's dimensions
    for(int a=0;a<7;a++){               //
             for(int b=0;b<7;b++){      //
                     board[a][b]='*';}  //loop to fill board with astrix's
                     }                  //
    
    char *p;                            //Declare pointer p
    p = &board[3][3];                   //place p at board centre 
    *p = '?';                           //mark centre using pointer p
    
    print(board);                       //print board
    
    for(int x=0;x<100;x++){             //loop to keep program running (100 moves). 
            move(p);
            print(board);               //call print board
            }
    
    system("PAUSE");
    return EXIT_SUCCESS;
}




char print(char board[7][7]){
     system("cls");                     //clear screan to avoid clutter
     for(int a=0;a<7;a++){              // for loop to print board
             for(int b=0;b<7;b++){
                     cout << board[a][b];
                     }
                     cout << "\n";       //newline to keep visual dimensions 
             }
}     


char move(char *p)
{
     
     char direct;                        //declare wariable to hold user input
     cin >> direct;                      //get use input
     if(direct=='w'){*p = '*';           //evaluate input against predefined direction        
     p = p - 7;                          //replace marker with board unit
     *p = '?';}                          //place a new marker on destination square
     else if (direct == 's'){*p = '*';
     p = p + 7;
     *p = '?';}
     else if (direct == 'a'){*p = '*';
     p = p - 1;
     *p = '?';}
     else if (direct == 'd'){*p = '*';
     p = p + 1;
     *p = '?';}
     else {cout << "please enter the correct controls \n"; // prompt user to enter corect control
     
     system("pause");                                      //pause to allow messege to be read
     return (*p);
     }
     }
     

I was under the impression that if I passed pointer to a function the change would persist in main.
Thanks for reading.

ps. I know I should have used definitions and stuff but I'm just trying to get this working at the moment.

[Slambofett]

I don't think you can change the value of a pointer inside a function without returning anything, you are thinking of passing a value by reference. What I think you can do is change the pointers location inside the function then change the value outside the function.

for(int x=0;x<100;x++) //loop to keep program running (100 moves). 
           {     
            *p = '*'        // sets value to '*'
           p = move(p);
            *p = '?'    // sets current position of your pointer to value  '?'
            print(board);               //call print board
            }

char move(char p)
{
     
     char direct;                        //declare variable to hold user input
     cin >> direct;                      //get use input
     if(direct=='w')           //evaluate input against predefined direction        
     p = p - 7;                                               
     else if (direct == 's')
     p = p + 7;
     else if (direct == 'a')
     p = p - 1;
     else if (direct == 'd')
     p = p + 1;
     else
     break;

     return (p); // this will return a position not a value
}
 

I didn't get a chance to test this because i am on a computer that doesn't have a C++ compiler.
Someone that knows more then me please come in and help me understand this better as well, I haven't worked with passing pointers as to functions very much.

[firedraco]

@slambofett: That would almost work, except your function needs to take and return char*, not just char.

@OP: You can't change the value of the pointer (i.e. p = p - 2;) You can change the value it is pointing to (i.e. *p = 'q';)

Slambofett's way would probably work the best, although you would need to add checks to make sure that the person isn't going out of bounds or anything.

[jRaskell]

Should I be using a pointer to a pointer???

Yes, you need to use a pointer to a pointer for that. So...

//Incomplete code, but you should be able to update the rest of the code:

char*p = &board[3][3];
move(&p);

char move(char** p){
  **P = '*';
  *p = *p-7;
  **p - '?'
}

I don't think you can change the value of a pointer inside a function without returning anything

That's exactly what a pointer to a pointer (or a reference to a pointer), allows you to do.

[crinkelite]

Hello again. Thank you for the replies!

I have managed to get this working anyway so here's the revised code

#include <cstdlib>                      //don't know if this is needed (Dev-c++ default entry)
#include <iostream>

#define WIDTH 20                        //defenition for dimensions
#define HEIGHT 20                       //

using namespace std;

char print(char board[WIDTH][HEIGHT]);  //print function prototype


int main(int argc, char *argv[])
{
    char board[WIDTH][HEIGHT];          //declare board and it's dimensions
    for(int a=0;a<HEIGHT;a++){          //
             for(int b=0;b<WIDTH;b++){  //
                     board[a][b]='*';}  //loop to fill board with astrix's
                     }                  //
    
    char *p;                            //Declare pointer p
    p = &board[WIDTH/2][HEIGHT/2];      //place p at board centre 
    *p = '?';                           //mark centre using pointer p
    
    print(board);                       //print board
    
    for(int x=0;/*x<100*/;x++){
            print(board);                       //loop to keep program running (infinite). 
            char direct;                        //declare wariable to hold user input
            cin >> direct;                      //get use input
            if(direct=='w'){*p = '*';           //evaluate input against predefined direction        
            p = p - WIDTH;                      //replace marker with board unit
            *p = '?';}                          //place a new marker on destination square
            else if (direct == 's'){*p = '*';
            p = p + WIDTH;
            *p = '?';}
            else if (direct == 'a'){*p = '*';
            p = p - 1;
            *p = '?';}
            else if (direct == 'd'){*p = '*';
            p = p + 1;
            *p = '?';}
            else {cout << "please enter the correct controls \n"; // prompt user to enter corect control
            system("pause");                                      //pause to allow messege to be read
            }
            } 
    
    system("PAUSE");
    return EXIT_SUCCESS;
}




char print(char board[WIDTH][HEIGHT]){
     system("cls");                     //clear screan to avoid clutter
     for(int a=0;a<HEIGHT;a++){              // for loop to print board
             for(int b=0;b<WIDTH;b++){
                     cout << board[a][b];
                     }
                     cout << "\n";       //newline to keep visual dimensions 
             }
}     

@Slambofett: thank you for your suggestion but I don't think I explained the symptoms of my problem in enough detail. the pointer p was marking the square correctly but the pointer itself was returning to the centre at the start of every go.
I was ending up with something like this at the end of 4 trys

******
**?***
*?*?**
**?***
******

you got me thinking though, about where the pointer move was being implemented so I decided to scrap the entire "move" function and I placed it's code in main, inside the infinite loop.
now the changes are persistent and the '?' appears to move around the board like an object.

@firedraco: I understand that I can't change the value of the pointer but I was attempting to change the position of it. I was only changing the value of the pointer's address so I'd have a visual representation of where the pointer is.

@jRaskell: I've seen this in a book somewhere but I have no Idea how to implement it correctly :(