Problem with char and arguments passed by value

[heatedcrab]

Hello,
I'm just started to learn the C++, and tried to make a simple program. In fact, I made a nice problem:

#include <iostream>
using namespace std;

void radius () {
	const double pi=3.141592654; 
	double r;
	cout << "Radius: "; cin >> r;
	cout << "A kor kerulete: " << 2*r*pi << endl;
	cout << "A kor terulete: " << r*r*pi << endl;
	cout << "A gomb felszine: " << 4*r*r*pi << endl; 
	cout << "A gomb terfogata: " << (4*r*r*r*pi)/3 << endl;
}

void ynexit (char& a){
	cout << "Tipe y to restart, n to exit -> ";
	cin >> a;
}

int main () {
	char a;
myprog1:	
	radius ();
myprog2:
	ynexit (a);
	switch (a) {
		case 'y': goto myprog1; break;
		case 'n': return 0;
		default: {cout << "\nTry again.\n"; goto myprog2;} }
}

So the funniest problem is when the program gets to the switch statemants and if the char a contains more than 1 character like yy it go mad and somewhere comes in an infinite loop.
Anyway the problem to resolve is: how can I limit char to contain only 1 character. If the answer is something like char a[1], than how can I bring it out from that void function? Caz when I tried it like above the VC++ Compiler hates me (lot of compile error).
Thanks for the answer, and sorry for my bad English and for goto function.

EDIT: void radius(). (There are some mathematical expression in sequence of characters witch I can't translate to English)

[jsmith]

The implementation of radius() is important, because I strongly suspect you
are reading either an integer or a floating point number via cin there.

cin >> a;

reads only a single character from the input stream. If the input stream is

yy<ENTER>

then the first 'y' is read and the second one is left in the stream. When radius()
attempts to read a number, it sees the second 'y' in the input stream and balks
because 'y' is not a valid number.

There are plenty of examples/posts around here of how to properly read data
from input. I suggest searching for one. Typically the best answer is to use
std::getline() and parse the input yourself rather than using operator>>() on
cin.

There are also many lengthy discussions around here regarding use of goto, which
is a keyword, not a function. I suggest reading them as well before asking about
the pros and cons of its use. (I do not want to start yet another "discussion" on
the topic)

[PanGalactic]

Why on Earth are you using goto in your program?

[heatedcrab]

@jsmith
Ah, i see. You where right about the radius() content. I think I better go back to the documentation... :) Thx

@PanGalactic
I used goto because i learned pascal before and goto "keyword" was something new to me.