Problems with graphing program

Question

#include <iostream>
using namespace std;

#define CHANGE 0.5
#define YMAX (yourterminallength/(2*(1/CHANGE)))
#define YMIN (-yourterminallength/(2*(1/CHANGE)))
#define XMAX (yourterminalwidth/(2*(1/CHANGE)))
#define XMIN (-yourterminalwidth/(2*(1/CHANGE)))

int main()
{
float m,b,y,x,c;

cout << "Intervals:" <<  CHANGE << "    \tYMAX: " << YMAX << "\tYMIN: " << YMIN <<      "\tXMAX: " << XMAX << "\tXMIN: " << XMIN;
cout << "\nEnter any key to continue...";
cout << "nEnter a linear equation in slope-intercept form: y = Mx + B";
cout << "\nM=";
cin >> m;
cout << "B=";
cin >> b;

c=YMAX;
while(c>YMIN)
{
    x=XMIN;
    while(x<XMAX)
    {
        if(c==0)
        {
            if(x==0)
                cout << char(197);
            else
                cout << char(196);
        }
        else if(x==0)
            cout << char(179);
        else if((m*x+b)==c)
            cout << char(254);
        else
            cout << char(176);
        x+=CHANGE;
    }
    c-=CHANGE;
}
return 0;
}   

The program is meant to graph simple linear equations. It has worked fine on my Windows computer and I simply made a copy of it to run on my Ubuntu OS, but the outcome looks funny. I know I am not giving much information, but I have no idea what is wrong with this.

Answer 1

1. Firstly, you can use environment variables to get the COLUMNS/LINES of your terminal.

   Otherwise, make sure you add newlines because terminal emulators don't usually do line wrapping anymore these days(?).

2. Now, the real problem appears to be the use of float comparisons.

   A line like

   if (c==0)
   

   or

   else if((m*x+b)==c)
   

   are all but completely undefined because of inexact float representations.

3. Finally, your sample appears (?) to assume ASCII or a codepage with block drawing characters. These won't print nicely on a UTF8 enabled terminal. Simply replace the characters by more suitable ones.

4. y is unused in the code. c seems to double it's meaning?