clearing scanf

This is a discussion on clearing scanf within the C Programming forums, part of the General Programming Boards category; In c++ you can clear out the io stream with the cin.ignore(). What do you use with scanf()? my problem ...

  1. #1
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    Question clearing scanf

    In c++ you can clear out the io stream with the cin.ignore(). What do you use with scanf()? my problem is when a user enters in two numbers (space between numbers) when prompted for one. The second number is picked up by the next scanf()....I want to clear it out of the stream! Any ideas?

    simple example:

    #include <cstdio>
    int main ()
    {
    int num=0;
    while (true)
    {
    printf("enter number (-1 to exit)\n");
    scanf("%d", &num);
    if (num==-1)
    break;
    printf("entered = %d \n", num);
    }
    return 0;
    }

  2. #2
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    how simple!!! Thanks!

  3. #3
    Banned Troll_King's Avatar
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    It's usually just the newline character that is left in the standard input buffer:

    scanf("%n %*",&iNum);

    That will work in most cases as well as the above.

    In C++ isn't it:

    cin.clear();

    ?

  4. #4
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    cin.clear() clears the eof, bad, fail and good status bits

  5. #5
    Registered User moonwalker's Avatar
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    question...

    dumb question... but...
    why did it work ??

    Code:
    #include <stdio.h>
    
    int main ()
    {
    	int num=0;
    	while (true)
    	{
    		printf("enter number (-1 to exit)\n");
    		scanf("%d", &num);
    		
    		while ( getchar() != '\n' );
    
    		if (num==-1)
    		{
    			break;
    		}
    
    		printf("entered = %d \n", num);
    
    	}
    
    	return 0;
    }
    what does this:
    while ( getchar() != '\n' );

    have to do with two numbers seperated by spaces and the second number being ignored ?

  6. #6
    End Of Line Hammer's Avatar
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    Re: question...

    Originally posted by moonwalker
    dumb question... but...
    why did it work ??
    <snip code>
    what does this:
    while ( getchar() != '\n' );

    have to do with two numbers seperated by spaces and the second number being ignored ?
    The loop
    >>>while ( getchar() != '\n' );
    will basically eat any input in the buffer up until the newline character.

    With your code, if you entered the following as input on the command prompt:
    12 17 -1
    The scanf() function would read the first number and assign it to the variable. The while loop would then eat the 17 and -1, so the rest of your code never sees it.
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

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