sscanf and parsing invalid floats?

This is a discussion on sscanf and parsing invalid floats? within the C Programming forums, part of the General Programming Boards category; I'm using sscanf to scroll through a file. The file has 4 values divided by spaces: 92832 John Doe 23.33 ...

  1. #1
    Learner Axel's Avatar
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    sscanf and parsing invalid floats?

    I'm using sscanf to scroll through a file. The file has 4 values divided by spaces:

    92832 John Doe 23.33

    the problem i'm facing is if the file has an invalid float

    92832 John Doe 23/33 and i try assigning it to a variable that value is rounded down to 23.00 how do i detect if it's an invalid float? I know that sscanf returns a number of items it has successfully read i.e. with the above example it would return 4. But even with the invalid float it still returns 4 so i'm kinda stuck??

  2. #2
    The C eater *munch*
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    what have you tried doing? we might be able to see your problem from the code?

  3. #3
    Learner Axel's Avatar
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    my code is a bit long... ill post down the relevant bits in a sec.

  4. #4
    Learner Axel's Avatar
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    ok this should describe my problem:

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    struct salesRecord
    {
       float budget;
    };
    
    typedef struct salesRecord saleNode;
    
    int main ()
    {
      char sentence []="23/33";
      
      saleNode *test = (saleNode *) malloc(sizeof(saleNode));
    
      sscanf (sentence,"%f", &test->budget);
      
      printf("%f", test->budget);
      
      return 0;
    }
    ./test.c

    23.000000

    rounds it to 23 ?? :|

  5. #5
    The C eater *munch*
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    you want to make 23/33 to 23 or 23/33 to 23.33?

  6. #6
    and the hat of wrongness Salem's Avatar
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    > saleNode *test = (saleNode *) malloc(sizeof(saleNode));
    Don't cast malloc in C - see the FAQ

    > char sentence []="23/33";
    > sscanf (sentence,"%f", &test->budget);
    But 23 is a valid float as per sscanfs rules. It stops conversion at the first character which fails to match a floating point number.

    If you want to see what that character is, then perhaps
    Code:
    int n;
    sscanf (sentence,"%f%n", &test->budget,&n);
    if ( sentence[n] ... )
    The %n conversion tells you how many characters have been used so far, and hence which would be the next to be converted. If that character is "invalid" by whatever rules you choose, then you know what to do about it.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  7. #7
    Learner Axel's Avatar
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    ok the %n conversion returns "20" basically i just an if that says if it is less than or equal i exit out. Is there anyway to do this without the use of such a number? as it just makes the code more harder to read with random numbers and no meanings (yes i can comment it but it should be naturally readable anyway)

  8. #8
    Learner Axel's Avatar
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    nevermind. I used fscan instead and checked the return value.

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