Thread: Infinte loop with switch, Why??

Hi,
I have made a simple menu program and use switch to handle the choices. The problem is, if i enter a number, it sends it into an infinite loop instead of the default command to say invalid input.
Does anyone know why this is?
Thanks

Code:

#include <stdio.h>

void menu();

void add()
{
	float a, b, ans;
	a = 0;
	b = 0;
	ans = 0;

	printf("********************************\n");
	printf("          ADDITION\n\n");
	printf("Enter first addend: ");
	scanf("%f", &a);
	printf("Enter second addend: ");
	scanf("%f", &b);
	
	ans = a+b;

	printf("\n %f plus %f equals %f \n", a, b, ans);

	menu();
}

void subtract()
{
	float a, b, ans;
	a = 0;
	b = 0;
	ans = 0;

	printf("********************************\n");
	printf("          SUBTRACTION\n\n");
	printf("Enter Minuend: ");
	scanf("%f", &a);
	printf("Enter Subtrahend: ");
	scanf("%f", &b);
	
	ans = a-b;

	printf("\n %f minus %f equals %f \n", a, b, ans);

	menu();
}

void multiply()
{
	float a, b, ans;
	a = 0;
	b = 0;
	ans = 0;

	printf("********************************\n");
	printf("        MULTIPLICATION\n\n");
	printf("Enter Multiplicand: ");
	scanf("%f", &a);
	printf("Enter Multiplier: ");
	scanf("%f", &b);
	
	ans = a*b;

	printf("\n %f plus %f times %f \n", a, b, ans);

	menu();
}

void divide()
{
	float a, b, ans;
	a = 0;
	b = 0;
	ans = 0;

	printf("********************************\n");
	printf("          DIVISION\n\n");
	printf("Enter Dividend: ");
	scanf("%f", &a);
	printf("Enter Divisor: ");
	scanf("%f", &b);
	
	ans = a/b;

	printf("\n %f divided by %f equals %f \n", a, b, ans);

	menu();
}

void retry()
{
	menu();
}

void menu()
{
	int ans = 0;

	printf("\n\nWhat would you like to do?\n");
	printf("1. Add: \n");
	printf("2. Subtract: \n");
	printf("3. Multiply: \n");
	printf("4. Divide: \n");
	scanf("%d", &ans);

	switch(ans)
	{
	case 1:
		add();
		break;
	case 2:
		subtract();
		break;
	case 3:
		multiply();
		break;
	case 4:
		divide();
		break;
	default:
		printf("Invalid input!!\n");
		retry();
		break;
	}
}


int main()
{
	menu();
	return 0;
}

Originally Posted by quzah

That's not a solution. That's a problem. fflush is NOT for input streams! Ever. Any attempt to do so is undefined which means anything or nothing can happen. It was never ever intended for input streams.

Sorry, quzah, I am new to this board (I have just subscribed today) and in a rush of giving my contribute I have not yet read the fuc*ed FAQ.
I just thought that Kate was using Microsoft Visual C, where fflush(stdin) is perfectly legal (it is effectively seen as an extension to the C standard). Look at http://msdn.microsoft.com/library/en...crt_fflush.asp for a sample code.
In fact, before answering, I tried my suggestion (with MSVC compiler) and it worked...

Originally Posted by quzah

Then you go on to suggest goto? Why not tell them to use gets while you're at it?

I based my suggestion on the result. The result is the way a program will work; the result is the compiled machine code which will allow the program to run.
Now, please, look at the machine code that the compiler (alas... again the Microsoft one...) will generate for a goto and for a while...

Code:

           while (1) a = 3;
                    mov eax,1
                    test eax,eax
                    je ...  // OUT OF LOOP
                    mov dword ptr [a],3
                    jmp ...  // START OF LOOP

and this is for the goto...

Code:

           retry: a = 3; goto retry;
                     mov dword ptr [a],3
                     jmp retry

The second one seems even more compact than the first one, isn't it?

Best regards, bilbo

P.S. Ah, fortunately "goto" is not in the FAQ! Anyway you must give me credit that I did not discredit the purists!

P.P.S Well, I hope we will become friends, if you leave me the time to make some other contribution, instead of flaming as fast as you can!

Thanks guys,

Great help

Originally Posted by quzah

As for fflush, I, as well as many others here, feel it's best to only present people with standard methods for handling problems.

Ok, quzah, got it! Next time I'll specify "this is valid only for MSVC, or GCC, or something else", or - better - I'll avoid to suggest non-standard solutions, as you pointed out.

Originally Posted by quzah

The reason we usually steer people away from goto is that it makes for horrible debugging and readability. It's also one of the reasons I don't like globals in general.

Sorry, quzah, I - not always but sometimes - think goto is more readable because it avoids a lot of indenting in the code lines; or, using it, the creation of helper variables to go out of nidified loops can be avoided.
And I - not always but sometimes - like globals because they do not need to be passed on the stack when calling some related function. I know, it is harmful, collateral effects may arise, etc etc, but I think the skill of the people, among many other things, can be evaluated by the way they are able of playing with fire... Anyway, I respect your point of view.

Best regards, bilbo