Cartesian plane infinite line? - c++

Question

I'm trying to make a program that makes a cartesian plane; you put in input the basic information of the line and he draws the line.
Too bad that it doesn't work well. Basically this is the loop that puts an X char in every point of the line:
for(int x=0;x<MAP_HEIGHT;x++)
{
piano.griglia[x][a*x+b]='X';
}
But it does not work well! here is the result:
http://i.imgur.com/u01Bdk7.jpg
notice how there are 3 lines where it should be only one. Here's the code of the plane, it may help:
class Cartesiano
{
public:
char griglia[MAP_LENGHT+1][MAP_HEIGHT+1];
Cartesiano( void )
{
for(int y=0;y<=MAP_HEIGHT;y++)
{
for(int x=0;x<=MAP_LENGHT;x++)
{
if (y==0)griglia[x][y]='_';
else
{
if(x==0)griglia[x][y]='|';
else griglia[x][y]=' ';
}
}
}
}
void Draw( void )
{
for(int y=MAP_HEIGHT;y>=0;y--)
{
for(int x=0;x<=MAP_LENGHT;x++)cout<<griglia[x][y];
cout<<"\n";
}
}
}piano;
Any ideas?

Answer 1

When your y values exceed 49 (MAP_HEIGHT - 1) you are overflowing your array. If in your draw loop you calculate y=a*x+b and print Z instead of X you'll see the first line is X's and the overflowed errors are all Z's
for(int x=0;x<MAP_LENGTH;x++)
{
int y = a*x+b;
if(y<MAP_HEIGHT) {
piano.griglia[x][y]='X';
} else {
piano.griglia[x][y]='Z';
}
}
This is due to the way memory for the arrays is stored in C++, it just happens writing outside one array here causes you to write into the next one. In general doing this will crash your program.
Also note that here the x loop should be over MAP_LENGTH.
I would also strongly advise getting a proper graphics library