Discard wrong data from stream

This is a discussion on Discard wrong data from stream within the C Programming forums, part of the General Programming Boards category; I'm not understanding how to discard unwanted data in a file stream. For example I want to read in a ...

  1. #1
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    Discard wrong data from stream

    I'm not understanding how to discard unwanted data in a file stream. For example I want to read in a list of doubles and only doubles from a file, if a non double is in the stream, some text for example, I need to flush it out of the stream and get the next item from the stream. The following code is in my textbook.
    Code:
    #define FLUSH while (fgetc(sp) !='\n')
    I just don't understand how to use it.

    Note: sp is the pointer to the file stream.

  2. #2
    Woof, woof! zacs7's Avatar
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    Don't forget about EOF.

    I wouldn't use a macro at all here, see the FAQ - http://faq.cprogramming.com/cgi-bin/...&id=1043284392
    Perhaps a function or just inline code.

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    That code works, I guess that my problem is that while the buffer gets flushed it stops reading from the buffer. I need to keep reading from the buffer until EOF

  4. #4
    and the Hat of Guessing tabstop's Avatar
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    So, what's wrong with what you posted -- it discards input until a new-line is reached.

    Is this interactive? If so, you won't go wrong not worrying about EOF; if it's not, you may want to check (files don't always end with a \n character).

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    I guess that I'm not sure where to call the macro: Here is the relevant code:
    Code:
       printf("Enter File Name:  ");
        scanf("%s",fileName);
        printf("\n\n");
        
        sp = fopen(fileName,"r");
       
        if(!sp){ /* there is no file with given filename*/
            printf("Error could not open file %s for read. \n\n",fileName);
            exit(0);
        }else if( fgetc(sp)==EOF){ /* no data in file,fileName valid */
            printf("There were 0 files in %s.\n",fileName);
            exit(0);
        }
        
        /*************
         the fscanf function reads data in from the stream
         it takes 3 parameters: stream pointer,conversion code, where to place data
         *************/
     while (( getchar()) != '\n' & sp != EOF);
        while(k=(fscanf(sp,"%lf",&data[i++]) == 1)) {
            b += k++ ;
            
        }
     
        fclose(sp);

  6. #6
    and the Hat of Guessing tabstop's Avatar
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    Probably because there's nowhere for you to call in that code, since it assumes that there is no such thing as bad input.

    You can't try to read directly into data[i++], since i will change even if nothing got read in. Read into a temp variable (in a while loop that checks for EOF, not ==1); if good, assign temp to data[i++]; if not, FLUSH.

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    Wow, thx for the help.I give that a shot

  8. #8
    and the hat of wrongness Salem's Avatar
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    Yet another example of how fscanf leads you down the garden path.

    Code:
    char buff[BUFSIZ];
    while ( fgets( buff, sizeof buff, sp ) != NULL ) {
        // do something with buff
        // maybe replace the fscanf call with a sscanf call
    }
    If the file is empty, then the body of the loop doesn't happen. So there is no need for something like if( fgetc(sp)==EOF)
    Which, if there is actually data in the file consumes a character, and just puts you out of sync for the first true read of the file with fscanf.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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