On Fri, 4 Oct 2002, waytech wrote:
> hi everyone,
>
> Thanks for all replys about my question.
>
> I know here are lots of perl experts.
>
> it's amazing that someone can short
>
> that long program into one line.
>
> I am a perl newbie, i wonder
>
> whether someone can explain what
>
> these guys wrote(like what
>
> Sudarshan Raghavan wrote
>
> ). I really can not understand.
>
> Since this group called " perl beginners",
>
> i hope i can get some explanations.
>
> Tanks a lot.
>
> kevin
I will explain the one-liners in the order in which they were posted
1) perl -e'while(<>){print unless $seen{$_}++}' <infile >outfile
-e command line switch: Type perldoc perlrun and read about the -e option.
<> (diamond operator): perldoc perlop, search for 'The null filehandle'
print unless $seen{$_}++ is the same as
unless ($seen{$_}++) {
print;
}
2) perl -n -e 'print unless $seen{$_}++' infile >outfile
Removed the while (<>) {} wrapper from the first one. The -n command line
switch will do this for us.
perldoc perlrun # -n switch
3) perl -ne'$s{$_}++||print' infile >outfile
Same as 2, $s{$_}++||print works like this, the print statement is
executed only if $s{$_}++ evaluates to false (lazy evaluation).
It is the same as
while (<>) {
unless ($s{$_}++) {
print;
}
}
4) perl -pe'$+{$_}++&&y+++cd' infile >outfile
# perldoc perlrun for the -p switch
%+ is the hash used here instead of %s or %seen used in the above examples.
If $+{$_}++ evaluates to true then execute y+++cd (lazy evaluation again)
y+++cd is nothing but y///cd or tr///cd, using '+' as a delimiter instead
of '/'
The above one-liner can be written as
while (<>) {
if ($+{$_}++) {
tr///cd;
}
print;
}
For y/// or tr///: perldoc perlop # Search for Quote-like Operators
5) perl -pe'$s{$_}++&&s{.+}$$s' infile >outfile
Borken down to
while (<>) {
if ($s{$_}++) {
s{.+}$$s; # same s/.+//s;
}
}
/s modifier makes '.' match on a newline too (by default it does not)
perldoc perlre
When matching delimiters like '{' and '}' are used to enclose the left
hand side of s/// or tr///, you can use another pair or character to
enclose the right hand side. In this case the character '$' is used to
enclose the RHS.
6) perl -pe'$s{$_}++&&s{$_}++' infile >outfile
Same as above, using '+ to enclose the RHS instead of '$'. Note this will
not work if any regex metacharacters are a part of $_ (the line read in).
You can overcome this by using \Q and \E. perldoc perlre
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