Question about inserting a character into a 2dimensional sequence.

RHLinuxGUY · 11-27-2005, 02:37 PM

I've been trying to figure out how I would be able to replace a character in my 2 dimensional array that I have with another character. Heres the code...

#include <cstdlib>
#include <cstdio>
#include <iostream>
using namespace std;

class Map
{
public:
char* MapA[10][10];
void mkmap()
{
for(int i = 0;i < 10;i++)
{
MapA[i][10] = "OOOOOOOOOO";
cout << MapA[i][10] << endl;
}
return;
}
};

int main(int nNumberofArgs, char* pszArgs[])
{
Map mp;
mp.mkmap();
mp.MapA[0][10] = "A";
cout << endl << endl;
for (int i = 0;i < 10;i++)
{
cout << mp.MapA[i][10] << endl;
}
return 0;
}

... Now on line 25, I believe that is where I have to change in order to replace a character without erasing the entire line. I have tried...

mp.MapA[0][1] = "A"; but to no prevail. How do I change a character in a sequence without resorting to erasing the entire line? Thanx in advance!

--EDIT--
Sry, here is the output...

george@linux:~> ./Array
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO

A
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
george@linux:~>

exvor · 11-27-2005, 04:17 PM

array element [0][10] is outside what you specifyed for your array size.

you inlized an array with 10 elements so the elements are 0 - 9 not 0 - 10

the element [0][10] would be the 11th element not the 10th one.

besides this the other problem you have here is when you intlize the array.

char * creates a read only array. Now it really does more then that but thats the simplist way to think about it.

should be intlized like

char space[10][10];

your not creating an array of pointers but rather an array of strings. in the example above.

theres more going on with this code then that but the core of the problem is with the intlization and the out of bounds stuff. Comple with -Wall with gcc and this would have warned you about such actions.

RHLinuxGUY · 11-27-2005, 05:07 PM

The reason why I changed that char was to make it const (?) as said in this output without a const(?) char. (or a pointer char, correct me please)

george@linux:~> g++ Array.cpp -o Array
Array.cpp: In member function `void Map::mkmap()':
Array.cpp:14: error: assignment of read-only location
Array.cpp:14: error: invalid conversion from `const char*' to `char'
Array.cpp: In function `int main(int, char**)':
Array.cpp:23: error: structure `mp' with uninitialized const members
Array.cpp:25: error: assignment of read-only location
Array.cpp:25: error: invalid conversion from `const char*' to `char'
george@linux:~>

... putting char* Map[][]; fixed that problem and enabled me to run the program.

exvor · 11-27-2005, 05:26 PM

Yea i got the same error when i tried to compile your code with the minor changes. also your code dident generate gcc errors oddly

Im better with C then with C++ so other then that i dont think i can be much help. Ill try to make a similar pice of code in C and see if i can do it there.

xhi · 11-27-2005, 07:49 PM

here is the c++ way to do it (with as few changes to your code as possible).. the string class takes away much of your work.. if you end up needing a char*, to can use the c_str() member function of string......

Code:

class Map
{
    public:
        string MapA[10];
        void mkmap(){
            for(int i = 0;i < 10;i++){
                MapA[i] = "OOOOOOOOOO";
                cout << MapA[i] << endl;
            }
        }
};

int main(int nNumberofArgs, char* pszArgs[])
{
    Map mp;
    mp.mkmap();
    mp.MapA[0][5] = 'A';
    cout << endl << endl;
    for (int i = 0;i < 10;i++){
        cout << mp.MapA[i] << endl;
    }
    return 0;
}

RHLinuxGUY · 11-27-2005, 10:07 PM

Thank you xhi, that was helpful. I'll need to research into chars a little more, tho, I still don't understand why it wasn't working probably with my code before? Can anyone explain for future reference?

xhi · 11-27-2005, 10:52 PM

now I am assumin on you intentions here
that you wanted an array of char 10x10 ... right..
so like this
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx

each x is char right...
well when you did
>> char* MapA[10][10];

you ended up with a 10x10 array of pointers... so when you did
>> MapA[i][10] = "OOOOOOOOOO";
you were actually getting

xxxxxxxxx"OOOOOOOOOO"
xxxxxxxxx"OOOOOOOOOO"
xxxxxxxxx"OOOOOOOOOO"
xxxxxxxxx"OOOOOOOOOO"
xxxxxxxxx"OOOOOOOOOO"
xxxxxxxxx"OOOOOOOOOO"
xxxxxxxxx"OOOOOOOOOO"
xxxxxxxxx"OOOOOOOOOO"
xxxxxxxxx"OOOOOOOOOO"
xxxxxxxxx"OOOOOOOOOO"
xxxxxxxxx"OOOOOOOOOO"

where an x is representing an uninitialized ptr and the "ooooo" are string literals.. now this is a perfectly valid data structure, however the reason it was not working is because you cannot change a string literal... you have to either do some clever copies on it, or run through it char by char until you get to where you want, then add char, then finish out the string... or just use arrays and run through char by char

again changed as little as possible...

Code:

#include <cstdlib>
#include <cstdio>
#include <iostream>
using namespace std;

class Map
{
    public:
    char MapA[10][10];
    void mkmap(){
        for(int i = 0;i < 10;i++){
            for(int j=0; j<10; ++j){
                MapA[i][j] = 'O';
                cout << MapA[i][j];
            }
            cout << endl;
        }
    }
};

int main(int nNumberofArgs, char* pszArgs[])
{
    Map mp;
    mp.mkmap();
    mp.MapA[4][5] = 'A';
    cout << endl << endl;
    for (int i = 0;i < 10;i++){
        for(int j=0; j<10; ++j){
            cout << mp.MapA[i][j];
        }
        cout << endl;
    }
    return 0;
}