Re: ifstream question

James Kanze <>
Fri, 1 Feb 2008 04:21:46 -0800 (PST)
On Jan 31, 5:24 am, Jerry Coffin <> wrote:

In article <3t6oj.21$az...@newsfe07.lga>, says...

[ ... ]

BUT: O, beautiful synergy: as it happens, the first file
line's three numbers (the rest have five) are not intended
to go into my x,y,z,r,c database, so all I had to do was
read these three and their two char type commas (but _not_
read the newline) _outside_ of the loop, which then gave the
loop exactly what it expected (a char first --the \n-- in
the loop-read) when I went inside the loop to fill the
database with numbers. IOW, it was merely a matter of
which, a number or a character, I read first in the loop.
If the number first, it missed the last number, and if the
char first, it missed the whole file. (I could also have
forced my file_writer_ to stick a comma into position 0 in
the file, but that'd have been a major kludge; ugly and
obviously a very clumsy workaround). Thanks for the more
info, Andy (and Alf), but this seems to have been a case of
a neophyte (me) doing something _so_ obvious and stupid that
the experts here (you included) didn't see what sheer idiocy
I had perpetrated upon myself. (*grin*)

I wouldn't be quite so hard on yourself -- things like this
really can be a pain for almost anybody to get right.

There are other ways that can be a bit easier though. For one
example, when you read numbers from a stream, the stream uses
a locale to actually read the numbers, as well as to classify
the other characters that get read from the stream. When you
read numbers, it treats white- space as delimiters between the

You're telling someone whose having problems groking streams to
write a locale!

Actually, I rather find that sort of use of locales an
abuse---the character class is called "space", and not
"separator". I'd much rather write some sort of manipulator to
handle separators. Much more flexible, and much more accurate.
Done correctly, for example, it wouldn't allow multiple
commas to be treated as a single separator. Something like:

        std::istream& source )
        source >> std::ws ;
        if ( source.peek() == ',' ) {
            source.get() ;

This allows either white space alone or a comma with optional
white space to be treated as a separator, e.g.:

    std::cin >> x >> separ >> y >> separ >> z ;

will accept "1 2 3", "1, 2 3", "1 2 , 3", etc.

James Kanze (GABI Software)
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