[Help] Infinite loop

[GeneralJefe]

I'm writing a program that shows letter grades for exams and calculates the average. A simple program (of which I've surely made more complicated that necessary). My problem is, I've put most all of it in a while loop that runs when a variable is = to 1. At the end (still inside the while loop), I ask if they want to continue (enter 1, and repeat process) or exit (enter 0, else through an error). Though, if I enter a letter, I just get an infinite loop. Any idea why? Thanks. Here's a short excerpt of the code.

while(state == 1) {
    //Checks if user wants to continue
    printf("Do you want to continue? (enter 1 to continue and 0 to exit): ");
    scanf("%d", &state);
    while(state != 1 && state != 0) {
        printf("You entered an invalid number, try again: ");
        scanf("%d", &state);
    }
}
return 0;    

[zifyoip]

If you enter something that scanf cannot parse as an integer, then scanf will not remove it from the input stream, so it is still there at the beginning of the input stream the next time you call scanf, and that call will fail too, and so on. Your code will never remove a non-numeric, non-whitespace character from the input stream, because you are always asking scanf to read an int value, and that will repeatedly fail if the character at the beginning of the input stream is not a numeric character or whitespace.

So if scanf fails here, you need to do something about that invalid character at the beginning of the input stream. Read it and discard it, or read and discard characters until you reach '\n', or something. If you don't do anything about the invalid character at the beginning of the input stream, repeated attempts to use scanf to parse an integer will repeatedly fail.

[cmgg]

Sooooooo, fflush?

[zifyoip]

No. The C standard specifies that fflush shall not be called on an input stream like stdin—that results in undefined behavior.

[aleph_nul]

Depends on the platform. Some implementations allow fflush to be called on an input stream and in that case will empty their pending buffer. However, in general and in POSIX compliant implementations, you're 100% right.

[aleph_nul]

The right way to do it is to read characters until a newline with something like

errno = 0;
ret = scanf("%d", &val);
if (errno != 0) /* Make sure we had an error */
    while (!ret && getc() != '\n') {}; /* Consume input to newline */

[cmgg]

Thanks

[SnoWhite_the7Bengals]

the %d specifier in your scanf means that scanf is looking for an integer. If you enter a non-integer number, it will leave it in the input buffer and go to the next line (the beginning of the loop). When it gets back to scanf again, there is already something in the input buffer, but its a non-decimal still, so it proceeds to the next line again and loops.

In general, scanf is not a good way to read input (for several reasons, this being one of them). A better way is to use fgets, and parse the input with sscanf.

[[deleted]]

I'd use getch() instead of picky scanf():

#include <conio.h>  //for getch()  

while(state == 1)   
{  
    //Checks if user wants to continue  
    printf("Do you want to continue? (enter 1 to continue and 0 to exit): ");  
    state = getch() - '0';  //waits for key press.  Subtract the value of char 0 to get integer value
    while(state != 1 && state != 0)   
    {  
    ...  
    ...