while语句中的无限循环

Question

Here is a snippet from my code

scanf("%d", &s);

while(s!=1 && s!=2) {
    if(scanf("%d", &s) != 1) {
        show_warning(); //just print some info msg
    }
}

The idea is to execute show_warning function only if user enter something different of 1,2 and entered value to be only integer. With the code above it jumps in infinity loop. How to fix that ?

Answer 1

The problem is that the failed input operation doesn't extract any characters from the stream (and you'll keep reading the invalid input over and over), so you have to empty the input manually. For example:

char buf[1000];

// ...

if(scanf("%d", &s) != 1)
{
    puts("Error, try again: ");
    fgets(buf, 1000, stdin);
}

As I suggested in the other question , if you use fgets from the start to always read one line and process it later, you overcome this problem.

(The same philosophy is true in C++: read the whole line first so the input stream can move on, and process the line later to see if its valid.)

Answer 2

Why are you using a while loop? Do you want show_warning(); to be called once, or repeatedly? If you answered once then you only need an if-statement.

Answer 3

Also what happens when no scanf succeeded? The variable s remains unchanged (and perhaps with an undefined value, if you did not initialize it). You should set it, or change the condition of the while !

And you really should learn to compile with warnings enabled and debugging information (ie gcc -Wall -g on Linux) and to use a debugger (eg gdb on Linux)

Answer 4

What makes you think it is looping until infinity?

You only get the warning message when you have not entered a number because it did not read a token.

Changing the && to || though is a certain way to ensure it will run to infinity as your loop will never break then.

You might want an alternative message to prompt the user to enter data when they did enter a number but not 1 or 2?

Answer 5

I'm assuming you're having issues when a non-integer is input. This is because scanf leaves non-matching characters in the buffer, so subsequent calls are seeing the same input and looping.

What you need to do is if the scanf call returns any number other 1 is: read that character in using scanf("%c",&somechar) so you can print it and tell the user that it isn't accepted. The non-accepted input will then have been removed so the next call to scanf will give you the next input rather than the one you saw on the previous iteration.