On 24 Nov 2006 at 23:55, D. Bolliger wrote:
> Beginner am Freitag, 24. November 2006 14:48:
> > Hi,
> >
> > I have a number of jpegs I wanted to rename. I wrote a short script
> > to do it but the new file name is not always generated correctly. The
> > script should find the last letter in the filename (before the
> > extension) and substitute it for '_a'.
>
> Hi Beginner
>
> I assume that you mean "substitue with _a".
>
> > If you look at the results below you'll see that 'a' and 'b' fail but
> > 'c' worked. I don't understand why.
> >
> > DSC00092a.jpg -> DSC00092a.jpg a
> > DSC00093b.jpg -> DSC00093b.jpg b
> > DSC00094c.jpg -> DSC00094_a.jpg c
> > ...snip
> >
> > Here the script, there isn't much to it. Can anyone explain why the
> > substitute fails?
>
> I should not mention that in the public ;-) but, just to demonstrate one way
> to search for a reason for a malfunction:
>
> Because I did not see an error at first glance, I...
>
> > #!/bin/perl
> > # Active State 5.8.6.811
> >
> > use strict;
> > use warnings;
> > use File::Basename;
> >
> > my $dir = 'D:/Temp/jpegs/thumbs/';
> > my @files = glob("${dir}*.jpg");
>
> ...replaced these two lines with simply
>
> my @files=qw(DSC00092a.jpg DSC00094c.jpg); # etc
>
> and everything worked fine.
> Then, I created these files in the current directory, and again everything
> worked fine.
>
> Then, I made a subdirectory, moved the file over, ...
>
>
> > foreach my $f (@files) {
> > (my $l) = ($f =~ /([a-z]|[a-z][a-z])\.jpg/);
> > (my $new = $f) =~ s/$l/_a/;
>
> ...placed here a
>
> warn "new=$new";
>
> and got (excerpt):
>
> new=/home/d_ani/ramsch/thumbs/DSC00092a.jpg at ./script.pl line 17. # !!
> new=/home/dani/ramsch/thum_as/DSC00093b.jpg at ./script.pl line 17. # !!
> new=/home/dani/rams_ah/thumbs/DSC00094c.jpg at ./script.pl line 17. # !!
> new=/home/dan_a/ramsch/thumbs/DSC00100i.jpg at ./script.pl line 17. # !!
> new=/home/dani/ramsch/thumbs/DSC00101_a.jpg at ./script.pl line 17.
>
> > my $basef = basename($f);
> > my $basenew = basename($new);
> > print "$basef -> $basenew $l\n";
> > }
>
> And now it's extraordinary obvious that the error is
>
> (my $new = $f) =~ s/$l/_a/;
>
> which simply searches for the first char contained in $l and replaces it
> with '_a'. This makes the malfunction dependent from the contents in $dir.
>
> Instead, this line should be more specific, f.ex:
>
> (my $new = $f) =~ s/$l\.jpg$/_a\.jpg/;
>
> (Note that I anchor with $ since "DSC00092a.jpg" is a valid path name :-) )
>
> Of course it would have been sufficient to only present this last
> substitution
> to lead you to a "aha!", but I think it's important to have a personal
> strategy to search for errors in the dark :-)
>
>
> btw, the foreach code can at least be shortened to:
>
> foreach my $basef (map basename ($_), @files) {
> (my $l) = ($basef =~ /([a-z]{1,2})\.jpg$/);
> # above line is still problematic: What if the match failes?
>
> (my $basenew = $basef) =~ s/$l\.jpg$/_a\.jpg/;
> print "$basef -> $basenew $l\n";
> }
>
> and certainly optimized further in several ways (f.ex if you don't need the
> last print statement, $l could possibly be eliminated), but I'm so tired and
> brain dead at the time :-)
Thanx Dani and John,
I should have realised that the that I was making the substitiution
on the full path and not the basename.
I appreciate you showing me how to shorten the code. Can I ask if I
am reading it right.
foreach my $basef (map basename ($_), @files) {
(my $l) = ($basef =~ /([a-z]{1,2})\.jpg$/);
Does this basename everything in @files and make it $basef?
In John's example I am not sure what is happening with this RegEx:
( my $new = $f ) =~ s/([a-z]{1,2})(?=\.jpg\z)/_a/;
There are 2 sets of parentheses but one lvalue, $new. So is that any
character a-z, 1 or 2 times and the ? mean 1 or more times? What is
the \z switch here? I can find it is perlre.
Thanx again.
Dp.
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