Floating point division by zero

//---------------------------------------------------------------------------
 
#include <vcl.h>
#pragma hdrstop
#include "math.h"
#include "Unit1.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
        : TForm(Owner)
{
 
}
double a, b, h, y_min, y_max;
int n;
typedef double (*TFun)(double);
TFun f;
double fun0(double);
double fun1(double);
double fun2(double);
 
 
//---------------------------------------------------------------------------
 
void __fastcall TForm1::FormCreate(TObject *Sender)
{
 
 
 
Edit1 ->Text="-3"; // à
Edit2 ->Text="3"; // b
Edit3 ->Text="50"; // n
RadioGroup1 ->ItemIndex = 0;
 
        
}
//---------------------------------------------------------------------------
 
void __fastcall TForm1::Button3Click(TObject *Sender)
{
 
double x, r;
a=StrToFloat(Edit1 ->Text);
b=StrToFloat(Edit2 ->Text);
n=StrToInt(Edit3 ->Text);
h = (b - a)/n;
switch(RadioGroup1 ->ItemIndex){
case 0: f = fun0; break;
case 1: f = fun1; break;
case 2: f = fun2; break;
}
y_min = y_max = f(a);
for (x = a+h; x<=b; x+=h){
r = f(x);
if (y_min<r) y_min = r;
if(y_max<r) y_max = r;
}
 
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
Chart1 ->Series[0]->Clear(); // Î÷èùåííÿ ãðàô³êó
for(double x=a; x<=b; x+=h)
Chart1 ->Series[0]->AddXY(x, f(x));
 
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Button2Click(TObject *Sender)
{
Chart1 ->CopyToClipboardMetafile(True);
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Button4Click(TObject *Sender)
{
double xmax, ymax, xt, yt, y0, x0;
double hx, hy, x;
Image1 ->Canvas ->Pen ->Color=clBlack; // Óñòàíîâêà êîëüîðó ïåðà
// Ïîøóê êîîðäèíàò ïðàâîãî íèæíüîãî êóòà ïîëîòíà ²màgå
xmax = Image1 ->Width;
ymax = Image1 ->Height;
// Çàôàðáîâóâàííÿ ïîëîòíà ²màgå ïîòî÷íîþ á³ëîþ êèñòþ
Image1 ->Canvas ->Rectangle(0,0, xmax, ymax);
// Ïîøóê ñåðåäèíè ïîëîòíà
y0=ymax/2;  x0=xmax/2;
// Âèêðåñëþâàííÿ êîîðäèíàòíèõ ë³í³é
Image1 ->Canvas ->MoveTo(0, y0);
Image1 ->Canvas ->LineTo(xmax, y0);
Image1 ->Canvas ->MoveTo(x0, 0);
Image1 ->Canvas ->LineTo(x0, ymax);
Image1 ->Canvas ->Pen ->Color=clRed; // Óñòàíîâêà êîëüîðó ïåðà
Image1 ->Canvas ->Pen ->Width=2; // Óñòàíîâêà øèðèíè ïåðà
// Ïîøóê êðîê³â ïî õ ³ ó ç ìàñøòàáóâàííÿì
hx=(b - a)/xmax;    hy=(y_max - y_min)/ymax;
Image1->Canvas ->MoveTo(ceil(x0+a/hx), ceil(y0 - f(a)/hy));
for(x=a; x<=b; x+=h)
Image1 ->Canvas ->LineTo(ceil(x0+x/hx), ceil(y0 - f(x)/hy));
}
 
double fun0(double r){
return sin(r);
}
double fun1(double r){
return r*r;
}
double fun2(double r){
return r*r*r;
}
 
//---------------------------------------------------------------------------

hy = (y_max - y_min) / ymax;
Image1->Canvas ->MoveTo(ceil(x0+a/hx), ceil(y0 - f(a)/hy));