Thread: Reading in char for float with scanf

  1. #1
    Registered User
    Join Date
    Nov 2006

    Reading in char for float with scanf

    I'm using scanf to read floats from input and want to put in error handling to detect when the user is inputting something that isn't a number (that is, a string). When I type in a string, it seems to make a float that is a very tiny number that is close to, but not equal to, 0. The problem is that if I try to account for this by saying floor(inputvariable) can't equal 0, then I can't input a float that's between 0 and 1. What can I do to handle this error?

  2. #2
    Frequently Quite Prolix dwks's Avatar
    Join Date
    Apr 2005
    Check the return value of scanf(); scanf() returns the number of items succesfully read. That is, for scanf("%f"), the return value will be 1 if all was successful or something else (0 or EOF). For scanf("%f%f%f"), the return value could be from 1-3 (or 0 or EOF), indicating how many numbers were read successfully.

    If scanf() detects an error, remove the offending characters from the input stream and resume reading. Something like this:
    int c;  /* must be an int to hold EOF */
    double percent;
    printf("Enter your percent: ");
    while(scanf("%lf", &percent) != 1) {
        printf("Try again: ");
        while((c = getchar()) != EOF && c != '\n');

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  3. #3
    Registered User
    Join Date
    Nov 2006
    Thanks! Got it now

  4. #4
    Just Lurking Dave_Sinkula's Avatar
    Join Date
    Oct 2002
    And my after advice is to simply avoid scanf and use fgets with sscanf.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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