A very nice post. Learned something, thanks!
Matt
--- Michael Lamertz <[EMAIL PROTECTED]> wrote:
> On Wed, Feb 06, 2002 at 03:05:29PM +0100, Jorge Goncalvez wrote:
> > I tried this:
> > @Symlink=("$program_path\\install.lnk",
> > "$program_path\\linux.ram.lnk",
> > "$program_path\\alize\\install.lnk",
> > "$program_path\\alize\\startup.txt.lnk");
> >
> > foreach (@Symlink)
> > {
> > if (-e $_)
> > {
> >
> > system "rm $_";
> >
> > }
> > }
> >
> > but it seems not to work?Why?
>
> Looking at the '\\' in your filenames suggests that you're running
> Windows? The 'rm' program is a Unix program and although there are
> versions for Windows too - using Cygwin e.g. - you most probably just
> don't have 'rm'.
>
> Look here:
> ---------- snip ----------
> kanku-dai:~$ perldoc -f unlink
> unlink LIST
> unlink Deletes a list of files. Returns the number of
> files successfully deleted.
>
> ...
> If LIST is omitted, uses $_.
> ---------- snip ----------
>
> The perl builtin 'unlink' does what 'rm' does, but from within perl, so
> you don't need to rely on "external" software.
>
> Handily, it accepts lists, so given your above example you can remove
> your 'foreach' with
>
> unlink @Symlink;
>
> BUT WARNING: The checks are still missing.
>
> That's no real problem since 'unlink' simply ignores the missing files,
> but here's how you could do them anyways:
>
> a.)
> unlink grep { -e $_ } @Symlink;
>
> b.)
> -e $_ && unlink foreach (@Symlink);
>
> Check the documentation for grep (perldoc -f grep) to learn more about
> that. Basically grep gets a piece of code and a list. It runs the code
> for each element in the list and returns as result a new list that
> contains all elements that let the passed code return a 'true' value.
>
> You could do the same like this:
> my @newlist;
> foreach (@Symlink) {
> if (-e $_) {
> push @newlist, $_
> }
> }
> unlink @newlist;
>
>
> b. looks a bit more cryptic but I think still explainable: We use 2
> programming idioms here: shortcut evaluation and modifiers.
>
> First, lets talk about the modifiers:
>
> ---------- snip from 'perldoc perlsyn' ----------
> Simple statements
>
> ...
>
> Any simple statement may optionally be followed by a SIN-
> GLE modifier, just before the terminating semicolon (or
> block ending). The possible modifiers are:
>
> if EXPR
> unless EXPR
> while EXPR
> until EXPR
> foreach EXPR
> ---------- snip ----------
>
> What this short sentence implies is that you can add a clause to any
> simple expression.
>
> if (-e $my_file) {
> unlink $my_file;
> }
>
> is equivalent to
>
> unlink $my_file
> if -e $my_file;
>
> Same goes for the 'foreach' loop:
>
> foreach (@Symlink) {
> unlink
> }
>
> could also be written
>
> unlink
> foreach @Symlink;
>
> Above statement already does the job, but we wanted to include the test
> for existence of the file, which brings us to shortcuts:
>
> Shortcut evaluation is the simple concept of bailing out of the
> condition as soon as it cannot any longer be true.
>
> A '&&' test is only true if both sides of it are true. So if the left
> side of the '&&' fails there's no need to look at the right side, so the
> whole expression fails immediately.
>
> Since functions like 'unlink' return a value, we can use them as part of
> that '&&' statement, which brings us back to our example:
>
> -e $_ && unlink
>
> If the file doesn't exist, perl skips the left half of the '&&' so the
> 'unlink' is never called for non-existant files.
>
> Now we have a working expression that we can round up with our
> 'modifier' foreach
>
> -e $_ && unlink
> foreach (@Symlink)
>
> Voila.
>
> --
> If we fail, we will lose the war.
>
> Michael Lamertz | [EMAIL PROTECTED]
> Nordstr. 49 | http://www.lamertz.net - http://www.perl-ronin.de
> 50733 Cologne | Priv: +49 221 445420
> Germany | Mobile: +49 171 6900 310
>
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