Ошибка в числах с плавающей запятой (калькулятор)

//---------------------------------------------------------------------------
 
#include <clx.h>
#pragma hdrstop
 
#include "Unit1.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.xfm"
TForm1 *Form1;
float a,b; int f; AnsiString st;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
        : TForm(Owner)
{
}
//---------------------------------------------------------------------------
 
void __fastcall TForm1::Button1Click(TObject *Sender)
{
if (f!=0)
Memo1->Text=Memo1->Text+((TButton*)Sender)->Caption;
else{
Memo1->Text=((TButton*)Sender)->Caption;
f=1;}
b=StrToFloat(Memo1->Text);
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Button9Click(TObject *Sender)
{
if (f!=0){
a=StrToFloat(Memo1->Text);
st=((TButton*)Sender)->Caption;
Memo1->Text=((TButton*)Sender)->Caption;
f=0;}
else
Application->MessageBoxA("Спочатку введіть число");
}
//---------------------------------------------------------------------------
 
void __fastcall TForm1::Button16Click(TObject *Sender)
{
 if (st=='+')
{a=a+b; f=0;}
if (st=='-')
{a-=b; f=0;}
if (st=='*')
{a*=b; f=0;}
if (st=='/')
{a/=b; f=0;}
Memo1->Text=FloatToStr(a);
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Button17Click(TObject *Sender)
{
if(f!=0)
Memo1->Text=Memo1->Text+",";
else if (Memo1->Text.Pos(",")==0){
Memo1->Text="0,";
f=1;}
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Button7Click(TObject *Sender)
{
 f=0;
Memo1->Text="0";
}
//---------------------------------------------------------------------------