Check for error in scanf()

This is a discussion on Check for error in scanf() within the C Programming forums, part of the General Programming Boards category; if i have a scanf for example: scanf("%d",&x); this is set to scan in a int data type to varabila ...

  1. #1
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    Check for error in scanf()

    if i have a scanf for example:

    scanf("%d",&x);

    this is set to scan in a int data type to varabila x.

    say the user inputs 2.5, i know the 2.5 will be trunkated to 2 but how useing a if statememt coukd i check if an int has been inputed.

    hope the question makes sense...

    thanks in advance....
    Dangerous Dave

  2. #2
    Code Goddess Prelude's Avatar
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    I don't see why you want to do this. If your program is supposed to take an integer and the user enters a float value, that value is truncated into an integer and the input is correct.
    If you want a float and the user enters an integer, once again the value is made to be the type you wanted and the input is correct.

    To answer your question, I don't think you can do that, but if I think of something I'll be sure to let you know.

    -Prelude
    My best code is written with the delete key.

  3. #3
    Registered User C_Coder's Avatar
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    You cant, the scanf will stop reading numbers when it hits the '.' so your input will always be integer, you can't test for a float you dont read.
    My advice is don't use scanf at all, especially with number inputs as it will leave the newlines in the input buffer, use fgets instead.
    Code:
    char temp[10], *temp_ptr;
    
    temp_ptr = temp;
    
    printf("\nEnter number ");
    fgets(temp, 10, stdin);
    
    while(*temp_ptr != '\n')
    {
         /* move along string checking for a '.' which would mean the user entered a float */
    }
    Not great, but you see what I mean.
    All spelling mistakes, syntatical errors and stupid comments are intentional.

  4. #4
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    would i be correct in saying that the scanf function returns a value of 0 if the correct input has been entered and a value of 1 if incorrect input has been entered??

    and if so how would i check for this returned value?
    Dangerous Dave

  5. #5
    Registered User C_Coder's Avatar
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    scanf returns the number of succesful conversions it has made, so if you want to read 2 letters scanf will return 2 if succesful.
    Code:
    printf("Enter 2 letters ");
    if(scanf("%c%c", &a, &b) != 2)
         printf("Error");
    I still say don't use scanf though
    All spelling mistakes, syntatical errors and stupid comments are intentional.

  6. #6
    Code Goddess Prelude's Avatar
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    I agree, scanf is easy but error prone both in how you code it and in how it functions. In my opinion that makes it doubly bad. fgets is a little more involved, but your code will be better because of it.

    Though I'd be lying if I said I didn't use scanf sometimes when I was lazy or going for compact code.

    p.s. Thanks C_Coder, until now I had no idea what scanf returned, just that it did in fact return a value.

    -Prelude
    My best code is written with the delete key.

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