fscanf() issue...

[I think the issue is more connected with fseek than fscanf..]

Hi!
Could anybody please explain me why this simple program to print the content of a file in a reverse order doens't work properly whit characters like 'è' or 'à' ecc...?

Code:

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#include <stdio.h>

int main ()
{
        FILE *fPtr=fopen ("reverse1.txt","rb");

        int ch,numBytes=0;
        while ((ch=fgetc(fPtr))!=EOF)
                numBytes++;
        int i;
        char c;
        printf("\n****************\n");
        for (i=0;i<numBytes;i++)
        {
                fseek (fPtr,-(i+1),SEEK_END);
                fscanf(fPtr,"%c",&c);//fread (&c,sizeof(char),1,fPtr);
                printf("%c",c);
        }
        printf("\n****************\n");
        fclose (fPtr);
        return 0;
}

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both fread and fscanf give the same problem: special chars are outputted like this: ��
Aren't they 1byte long like normal chars?

thanks!

There are cases where a character consists of multiple characters - why not print the characters you actually read as numbers (decimal or hex) and compare with the actual character - you would then probably see multiple digits for one character [do this in the forward direction] - in which case your reverse function like that would not work at all.

I'm sure it doesn't matter at all if fgetc(), fscanf() or fread() in this case - they all read a character on it's own. In a mult-byte-character set, the sequence is interpreted as a long character only when it comes to the displaying, not when reading from or writing to files.

--
Mats

Only in extended ASCII (ie unsigned), whereas you're reading them in as signed (and printing as signed).

[edit]
I was talking about something else... but it's still related.
[/edit]

Quote:

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Originally Posted by zacs7

Only in extended ASCII (ie unsigned), whereas you're reading them in as signed (and printing as signed).

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Shouldn't make a difference - the signed or unsigned value is the same binary value as a character - and that is what printf (or putc) will do.

--
Mats

Quote:

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Originally Posted by matsp

- in which case your reverse function like that would not work at all.

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then, what could be an alternative solution? it sounds to be a bit complicated, isn't it?

By the way, I use this post to ask another question related to dealing with files.
Last day I solved an exercise which required to leave only 1 blank line in the case more than 1 consecutives empty lines where found.
I solved it immediately in this way:

Code:

---

........
int ch;
while ((ch=fgetc(fPtr))!=EOF)
        {
                if (ch!='\n') //IT's A CHAR-->copy it
                {
                        fprintf(f2Ptr,"%c",ch);
                        prevIsChar=1;
                        prevIsEmptyLine=0;
                }
                else
                {
                        if (prevIsChar)
                        {
                                fprintf(f2Ptr,"%c",ch);
                                prevIsChar=0;
                                prevIsEmptyLine=0;
                        }
                        else
                        {
                                if (!prevIsEmptyLine)
                                {
                                        fprintf(f2Ptr,"%c",ch);
                                        prevIsChar=0;
                                        prevIsEmptyLine=1;
                                }
                        }
                }
        }
...

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in linux this works fine.. Now, after reading again the text of the exercise, it tells that in UNIX the <CR> corresponds to 10, thing which I didn't consider..
To let it work in Unix, should I seek for sequences of

Code:

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<CR><LF> --> 10 '\n'

---

?

again, thanks for your kind help! :)

It appears that your source file is not encoded in a single-byte character set (like one of the ISO-8859 LATIN formats), and you are expecting it to be. It might be encoded in UTF-8.

When dealing with unicode encoded files, you need to process characters, not bytes, if you want to deal with multi-byte characters.

Quote:

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Originally Posted by Dino

It appears that your source file is not encoded in a single-byte character set (like one of the ISO-8859 LATIN formats), and you are expecting it to be. It might be encoded in UTF-8.

When dealing with unicode encoded files, you need to process characters, not bytes, if you want to deal with multi-byte characters.

---

Yes, and char, despite it's name, is not characters, but bytes [they are only characters when a byte and a character is the same thing, and UTF-8 for example is one of those where this is not true].

Doing "backwards" on UTF-8 files is far from trivial - not sure how you'd go about it really, because the way that UTF works is "in order", so you would have to scan backwards to find the unque markers for multi-character start [I think there are more than one - but not a huge number], then go forward according to the length of the character.

Given this complexity, it may make more sense to treat the file as a list of backwards lines - so read back from the very end until you find a newline, then reverse all characters [not bytes] in the resulting string. At least then it's relatively easy to deal with multi-byte characters, because you can move back and forth within the string itself.

--
Mats

Quote:

---

Originally Posted by Dino

It appears that your source file is not encoded in a single-byte character set (like one of the ISO-8859 LATIN formats), and you are expecting it to be. It might be encoded in UTF-8.

When dealing with unicode encoded files, you need to process characters, not bytes, if you want to deal with multi-byte characters.

---

so which function should I use instead than fseek(which works with bytes) to solve this problem?
thanks!

Quote:

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Originally Posted by smoking81

so which function should I use instead than fseek(which works with bytes) to solve this problem?
thanks!

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It is not fseek as much as the "encoding of each character" that is the problem. You would have to know where the boundaries of a character are, and that is far from trivial.

If it's not strictly a requirement for your assignment that it works with multi-byte characters in UTF, I'd suggest you just mention as a restriction in your implementation that it doesn't support UTF-8 character sets properly.

--
Mats

You can try this if you want. Without knowing the exact encoding of your source file, this is a guess.

If you read this description in the following link, you'll learn that in UTF-8 (my assumption of your source file format), any ISO 8859 character code that is less than X'80' can be represented in a single byte in UTF-8. Latin characters with diacritics need 2 bytes. See the description:

http://en.wikipedia.org/wiki/UTF-8#Description

In the description, you see that each UTF-8 "character" is self defining by inspecting the high order bit(s). If the bit is off, then the character only takes up one byte. If the high order bits AND with X'C0', then it is a two-byte character.

See the examples for how the high end of the LATIN character set are represented in 2-byte UTF-8 character codes.

http://en.wikipedia.org/wiki/UTF-8#Examples

Todd

Ah, ok, so you could actually identify that a character is UTF-8 by just looking at that byte itself.

--
Mats

Yes. I just coded up a small test to see it. To reproduce, just create a UTF-8 encoded file with your favorite editor and insert some diacritics.

Todd

Here a routine that works to reverse a string in a UTF-8 encoded file. (It doesn't take into account CRs or LFs.

Todd

Code:

---

#include <stdio.h>
#include <string.h>

int main (int argc, const char * argv[]) {
        char line[81], newline[81] = {0 }  ;
        char * linep, *newlinep ;
        FILE * file ;

        if ( (file = fopen("/Users/toddburch/Desktop/myutf8.txt","r+")) == NULL) {
                printf("Cannot open FILE\n" ) ;
                return -1 ;
        }

        fgets( line, sizeof(line), file ) ;

        // Reverse the string
        linep = line ;
        newlinep = newline + strlen(line)-3 ;  // point to end of string
        *newlinep-- = 0 ;                      // null term char
        linep += 3 ;  // skip the utf-8 encoding flag bytes
        while (*linep) {  // Do until we hit the null term byte
       
                if ( (*linep & 0xC0) == 0xC0) { // if 2-byte char, move both chars in order
                        *(newlinep-1) = *linep++ ;
                        *newlinep-- = *linep++ ;
                        newlinep-- ;
                }
                else {
                        *newlinep-- = *linep++ ;
                }
        }
        fputs(newline, file) ;  // write reversed string
        fclose(file) ;
    return 0;
}

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The output of which looks like this:

Code:

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Here is my résumé.

.émusér ym si ereH

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