Probelm with Arrays, Pointers, or Loop/ Function Calls, Im not sure which

[wilnos]

Warning: Noob post.

Messing around with a few concepts learned in this website's tutorial and I cant wrap my mind around why I cannot rename one element of an array to which I am pointing. The code structure is a do-while loop in main that sets two rand shorts, then calls two functions. The first fills a pointer pointing to a char array, the second prints it out how I want it to.
Here's where I think the problem is, the first function:

void FillMap () {
	
	for (n=0; n<(x * c); n++) {
			a[n] = '.';
		}
	for (n= (x-1); n!=x; n++) {
		a[n] = '@';
	}

}

But, the issue created here could be in my main ():

int main () {
	
	

	do {
	
	srand (time (NULL));
	x = rand () % 10 + 4;
	c = rand () % 8 + 6;

	a = new char [x*c];

		FillMap ();
	
		PrintMap ();
	
	std::cout << std::endl;
	std::cin >> end;

	} while (end == false);

	return 0;
}

The specific issue I'm having is that I want PrintMap () to print out the array that's being pointed to, but with an '@' that can be moved around. Right now, I cant even get the '@' symbol to appear in the array, and I don't know why. Right now, I just want it to print out a '@' at the array's rand element.

For those of you who enjoyed NetHack, yes this C++ beginner is testing concepts on this a model of this game.

I would love advice on how to do this an easier way, but for the purpose of mastering basic concepts, I am much more interested in why my primitive solution isn't working, so responses to either end would be appreciated.

[Mythios]

Not to sure how exactly you want your fill map to work but as you want to display to a map - you'll need to use a 2d array instead of your char[]. You would more need to use something like "char* map[ySize][xSize];"

And then fill it like

for(int x = 0; x < 24; ++x)
{
	for(int y = 0; y < 78; ++y)
	{
		map[x][y] = " ";
	}
}

map[playerClass.x][playerClass.y] = "@";

for(int x = 0; x < 24; ++x)
{
	for(int y = 0; y < 78; ++y)
	{
		std::cout << map[x][y];
	}
	std::cout << std::endl;
}

Hope that gives you some idea :) If not let me know and I'll make you a basic one you can build from

[wilnos]

I did that at first, but not with classes (if I read line 9 correctly?), and an error that I couldn't fix popped up. Instead, I used a single dimensional array, because the tutorial suggests multi-dimensional arrays are for fanciful thinking, and are barely different than a single dimensional array the size of two integers multiplied. So I took my two random shorts and multiplied them. This prints a map successfully, exactly how I want it, with a random height and random width, filled inside with '.' and bordered by '|', '_', and '-', it just doesnt seem to be able to transfer the information for a change to one element- the '@'.

One thing I'm confused about in your example is why you have constant integers 24 and 78 in your for loops. Are they arbitrary, or do they mean something I can't see?

Regardless, here's the entirety of what I have going now, if it helps:

#include <iostream>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

short x, n, f, c, t;
char * a;
bool end (false);
void FillMap () {
	
	for (n=0; n<(x * c); n++) {
			a[n] = '.';
		}
	for (n= (x-1); n!=x; n++) { /* this is the line whose info doesnt seem to transfer*/
		a[n] = '@';
	}

}
void PrintMap () {

	
	for (n=0; n<(x+2); n++) {
		std::cout << "_";
	}
	for (f=0; f<c; f++){
	std::cout << std::endl << "|";

	for (n=0; n<x ; n++) {
		std::cout << a[n]; 
	}
	std::cout << "|" ;
	}
	std::cout << std::endl;
	for (n=0; n< (x+2); n++) {
		std::cout  << "-";
	}
}
int main () {
	
	

	do {
	
	srand (time (NULL));
	x = rand () % 10 + 4;
	c = rand () % 8 + 6;

	a = new char [x*c];

		FillMap ();
	
		PrintMap ();
	
	std::cout << std::endl;
	std::cin >> end;

	} while (end == false);

	return 0;
}
	

If I'm not understanding your concept, then I would appreciate a brief example. Just don't forget that I'm a self taught noob of less than a week, with only minor experience in visual BASIC as a kid :)

[psault]

how many @'s are you expecting to fill in with that line?

n = x - 1; n != x; n++ ?

You are making n one less than x, then adding 1 to it... your loops executes only 1 time.

~psault

[guestgulkan]

The program does what it is programmed to do:
When I run it, for a value of x=5 and c=10
this is what it displays


_______
|....@|
|....@|
|....@|
|....@|
|....@|
|....@|
|....@|
|....@|
|....@|
|....@|
-------

I do agree with psault - a loop that is only ever going to loop once - isn't really much of a loop.

[Faldrax]

I think you have the basic approach wrong for what you want to do.
If I understand corrctly you want to draw a map with an @, then have the @ move?
EG

_______
|....@|
|.....|
|.....|

_______
|.....|
|....@|
|.....|

_______
|.....|
|...@.|
|.....|

I would create the map in the array (you might as well make it 2D, it makes the code more obvious), then add the @.
To Move the @ put a . in the array where it was, then put an @ at the new position.
This way you are only updating the minimum information required to do the move.
As an additonal point, you have the srand call inside the loop - it should be called just once before the loop starts.
The reason for this is that time(NULL) is at 1 second precision, so all calls to srand(time(NULL)) within the same second will initialise to the same point.
See

int main()
{
   int a,b;
   //srand(time(NULL));   //Run once like this then uncomment this line
   for (i=0; i < 1000; i++)
   {
      srand(time(NULL));   // Run once like this, then comment this line
      a = rand () % 10 + 4;
      b = rand () % 8 + 6;
      count << a << "\t" << b << endl;
   }
   return 0;
}

to demonstate the difference.

[Mythios]

Does it make any difference if you make your = '@'; turn into ="@";?

[wilnos]

@ psault:
I tried an IF statement, which had no effect, so I ran a one time loop instead, which had the exact effect that guestgaulkan printed up for us.

@ Faldrax:
That is exactly the goal.
Im going to try and work my way through your idea and see if it changes anything, but I feel like its something I've already tried. Who knows, a second time around might fix things. I think what you're saying is have the pointer a point to the value '@' and then move it around in the array based on char input? Now, will i need extra code to erase the elements of the array that a pointed to before the last input? (as you can tell, im still wrapping y head around arrays and pointers before i try moving onto classes, the next lesson in the tutorial)
The srand and rand shorts were inside the loop for previous tests on printing several different sizes of maps. I just forgot to change it back.

[Mythios]

Are you planning on trying to get the '@' moving around a little bit A.I like?

[Faldrax]

If you use a 2D array then all you need is to hold two pairs of info

int Old_x, Old_y;
int New_x, New_y;

the to move you just do

Old_x = New_x;
Old_y = New_y;
FindNewPosition(New_x,New_y);   //Function to calculate new values
Map[Old_x][Old_y] = '.';        //Set the old postion to a dot
Map[New_x][New_y] = '@';        //Set the new position to a @
DrawMap(Map);                   //Draw the new map