Confusing of "Access Violation " - c++

Question

I don't Understand with this case but this is really really important for me, Please Help me...
void __fastcall TForm1::Button4Click(TObject *Sender)
{
String masuk, keluar, kosong;
int i, x, j, n = 0;
masuk = Edit2->Text;
keluar = masuk;
kosong = " ";
n = 0;
x = 0;
mulai:
i = 1;
j = 0;
j = j + n;
i = i + j;
if (masuk[i] == 'a')
{
keluar[i] = 't';
}
else if (masuk[i] == 't')
{
keluar[i] = 'a';
}
else if (masuk[i] == 'c')
{
keluar[i] = 'g';
}
else if (masuk[i] == 'g')
{
keluar[i] = 'c';
}
else
{
Application->MessageBoxA("Masukan Anda Salah", "Peringatan", MB_OK | MB_ICONWARNING);
keluar = kosong;
goto end;
}
n = n + 1;
if (i < 10)
goto mulai;
else
goto end;
end:
Memo1->Text = keluar;
}
if I make masukan more than 10 (i<10 (10 as default value)), it is ok but if it less than 10, it will make message exception Class EAccessViolation..

Answer 1

Taking a shot in the dark, but I think what you're actually trying to do might be this. I'm assuming that you're taking a single string of 10 characters which represents one half of a genome and you're generating another string of the pair values.
void __fastcall TForm1::Button4Click(TObject *Sender)
{
String masuk, keluar;
masuk = Edit2->Text;
keluar = masuk;
char kosong = ' ';
for (int i=0; i < 10; i++)
{
switch(masuk[i]) {
case 'a':
keluar[i] = 't';
break;
case 't':
keluar[i] = 'a';
break;
case 'c':
keluar[i] = 'g';
break;
case 'g':
keluar[i] = 'c';
break;
default:
Application->MessageBoxA("Masukan Anda Salah", "Peringatan", MB_OK | MB_ICONWARNING);
keluar[i] = kosong;
break;
}
Memo1->Text = keluar;
}