Thread: My program won't stop looping

I am new to programming and I have a problem. The problem is that i have to produce a menu driven program which will allow a user to make a choice from a menu. I would really appreciate it if some one could tell me why if I input a letter instead of a number my program goes into an infinte loop. I have included the offending bit of code as a text file please help me cheers Davie

David,

The offender is in here:

Code:

printf("\n\n\n\t\tPlease Enter Your Menu Choice:  ");

/* read in valid menu option */
scanf("%i",&choice);

while((choice==0)||(choice>5))
{
  printf("\n\n\t\tPlease Enter Menu Between 1 and 5 :  ");
  scanf("%i",&choice);
}/* end of while */

At the first scanf(), we type in some letter ('x', for example). scanf() is unable to assign the 'x' from stdin to 'choice' because of the conversion character instructing scanf() to put an integer into 'choice'. So, scanf() returns zero and nothing is moved (the 'x' is still sitting inside stdin).

When we reach the second scanf(), the same thing happens: scanf() looks at stdin, finds the 'x', can't match it against the conversion character %i, and returns zero. Since the value of 'choice' is never updated, the loop runs forever (as long as choice had a value of zero or greater than five when you started out).

Now that you know why it doesn't work, I'll let you ponder the solution for a while. Come on back if you're still stuck.

Solution...

Use fgets with the stdin stream. It is so much better than scanf. You are just going to have to do a sscanf on the string to get the int out. Something like this:

Code:

char line[256];
int choice;

fgets(line, sizeof(line), stdin);
sscanf(line, "%d", &choice);

That is the best input method. scanf also has some other less desirable traits. You're better off sticking with fgets and sscanf.

When we reach the second scanf(), the same thing happens: scanf() looks at stdin, finds the 'x', can't match it against the conversion character %i, and returns zero. Since the value of 'choice' is never updated, the loop runs forever (as long as choice had a value of zero or greater than five when you started out).

Deckard is quite right,
also the range that you are asking the user to input is not being tested for.

if typed in 1,2,3,4,5 it'll loop forever,

> if typed in 1,2,3,4,5 it'll loop forever,

Actually, it wouldn't loop at all, since 1-5 are not 0 or >5.

A couple other things.

1) Get out of the habit of using void main now, while you're still beginning. main returns an int.

2) There are better methods to clear the screen than using system. They're detailed in the FAQ.

3) David's right about global variables - they may seem like they help a lot, but it's good practice to use them as spaingly as possible.

I pretty much agree with Govt on all of these topics. It is darn near close to a sin to use 'void main', as Govt said. Also, don't use the system command. It could be considered a short cut. Look up the function(s) to follow out this process. And as you make larger and more practical programs, then code is going to get copias. Therefore, globals will be a sloppy style of code and bad practice.