Using regex in Map function

narashima has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: Using regex in Map function by merlyn (Sage) on May 02, 2007 at 22:21 UTC
You'd probably be even more surprised by the result of this: `my @in = qw(aaa bbb ccc); my @out = map { s/.$/x/ } @in; print "@in\n=>\n@out\n";` [download] which prints: `aax bbx ccx => 1 1 1` [download] Yeah, the original is modified. Basically: never modify $_ in a map, and remember that the "output" of s/// is the success count/code. To get what you want, you need to localize $_ and reuse it as the last expression evaluated in the block: `my @in = qw(aaa bbb ccc); my @out = map { local $_ = $_; s/.$/x/; $_ } @in; print "@in\n=>\n@out\n";` [download] which indeed shows: `aaa bbb ccc => aax bbx ccx` [download] -- Randal L. Schwartz, Perl hacker	[reply] [d/l] [select]
Re^2: Using regex in Map function by GrandFather (Saint) on May 02, 2007 at 22:47 UTC
or better perhaps: `my @out = map {substr ($_, 0, -1) . 'x'} @in;` [download] Prints: `aaa bbb ccc => aax bbx ccx` [download] DWIM is Perl's answer to Gödel	[reply] [d/l] [select]
Re^2: Using regex in Map function by blazar (Canon) on May 03, 2007 at 09:16 UTC
To get what you want, you need to localize `$_` and reuse it as the last expression evaluated in the block: `my @in = qw(aaa bbb ccc); my @out = map { local $_ = $_; s/.$/x/; $_ } @in; print "@in\n=>\n@out\n";` [download] Or use a proper lexical variable: `my @out = map { (my $s = $_) =~ s/.$/x/; $s } @in;` [download] AIUI in blead/5.10 the following should also work just in the same manner: `my $_; # ... my @out = map { s/.$/x/; $_ } @in;` [download]	[reply] [d/l] [select]
Re^3: Using regex in Map function by narashima (Beadle) on May 03, 2007 at 18:42 UTC
Hi Blazer & Randal, Could you guys please explain why we need to localize $_ inside the map? I am not sure I understand why we need to do this.I seem to get the same result even if I do not localize $_. Is there something I am missing? Thanks!	[reply]
Re^4: Using regex in Map function by blazar (Canon) on May 03, 2007 at 22:43 UTC
Re^2: Using regex in Map function by spandox (Novice) on Feb 05, 2013 at 21:14 UTC
************ Bad Program `my @in = qw(a b c d e f g h); print "Start: ",@in,"\n","RUNNING MAP\n\n"; my @out = map { s/.$/x/} @in; print "in : ",@in,"\n"; print "out: ",@out,"\n"` [download] ********* Output ** `Start: abcdefgh RUNNING MAP in : xxxxxxxx out: 11111111` [download] ***************** Comment #1: Use "r" feature on regex ********** fixed Program `my @in = qw(a b c d e f g h); print "Start: ",@in,"\n","RUNNING MAP\n\n"; my @out = map { s/.$/x/r} @in; print "in : ",@in,"\n"; print "out: ",@out,"\n"` [download] ********* Output ** `Start: abcdefgh RUNNING MAP in : abcdefgh out: xxxxxxxx` [download] ************ Comment #2 But what if you REALLY wanted the return value of the regex - and not modify the original output? (why? I don't know) Localize the input... ********** fixed Program `my @in = qw(a b c d e f g h); print "Start: ",@in,"\n","RUNNING MAP\n\n"; my @out = map { s/.$/x/} my @temp = @in; print "in : ",@in,"\n"; print "out: ",@out,"\n"` [download] ********* Output ** `Start: abcdefgh RUNNING MAP** in : abcdefgh out: 11111111` [download]	[reply] [d/l] [select]
Re^3: Using regex in Map function by merlyn (Sage) on Feb 08, 2013 at 18:40 UTC
You're replying to my note from 2007 with a feature that has only been available for a few releases. Nice. -- Randal L. Schwartz, Perl hacker The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.	[reply]
Re^4: Using regex in Map function by Anonymous Monk on Jan 29, 2016 at 21:12 UTC
Re: Using regex in Map function by jdporter (Paladin) on May 02, 2007 at 21:09 UTC
You need to remember that the result of map is the list of the final value of each pass through its block. The value of `s///` is a boolean indicating whether a replacement was made. You want the final value of the map block to be `$_`. Like so: `@new = map { s/$_/$_,hi/; $_ } <TEST>;` [download] However, your substitution is less than ideal. In particular, you'll have problems if `$_` contains any regex-special characters. If you're simply appending, you could do this: `@new = map { s/$/,hi/; $_ } <TEST>;` [download] or, even ~~better~~worse, this: `@new = map { $_ . ',hi' } <TEST>;` [download] A word spoken in Mind will reach its own level, in the objective world, by its own weight	[reply] [d/l] [select]
Re^2: Using regex in Map function by grinder (Bishop) on May 02, 2007 at 22:00 UTC
The value of s/// is a boolean indicating whether a replacement was made. Err, no, not really. It's actually the number of replacements made. Thus: `# given $x = 'abacad'; # then $x =~ s/a/z/; # returns 1 $x =~ s/a/z/g; # returns 3 $x =~ s/x/z/; # returns empty string, not undef` [download] ... although it can be treated quite nicely in a boolean manner :) • another intruder with the mooring in the heart of the Perl	[reply] [d/l]
Re^2: Using regex in Map function by jettero (Monsignor) on May 02, 2007 at 21:14 UTC
In addition to what jdporter is saying, I'd like to add that this is probably faster and clearer: `@new = <TEST>; s/something/something/ for @new;` [download] I haven't Benchmarked it or anything, but I'm pretty sure it's faster than `map {s/regex/stuff/}` — either way, it's probably more readable. -Paul	[reply] [d/l] [select]
Re^2: Using regex in Map function by pKai (Priest) on May 02, 2007 at 21:21 UTC
or, even better, this That may be subject to judgement. While your 1st and 3rd variant append behind the line-ending newline, the 2nd is probably closer to what the OP wanted, since `$` is matching before that newline.	[reply] [d/l]
Re^2: Using regex in Map function by strat (Canon) on May 03, 2007 at 09:57 UTC
If you need something like the copy - do something more often, you could also create a copy-wrapper for map, let's call it cwmap: `sub cwmap( &@ ) { map { local $_ = $_; # create copy $_[0]->(); # execute code-block $_; # copied element } @_[ 1..$#_ ]; # list } # cwmap # and then use it like map: @new = cwmap { s/$_/$_,hi/ } <TEST>;` [download] Best regards, perl -e "s>>F>e=>y)\martinF)stronat)=>print,print v8.8.8.32.11.32"	[reply] [d/l]
Re: Using regex in Map function by BrowserUk (Patriarch) on May 02, 2007 at 23:13 UTC
I'd do it this way: `my @new; ( $new[ @new ] = $_ ) =~ s[$][,hi] while <DATA>;` [download] Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply] [d/l]
Re: Using regex in Map function by naikonta (Curate) on May 03, 2007 at 04:44 UTC
I'm not really sure about your exact requirement, but should it's really that simple regex, simple concatenation will do as well. `my @orig = qw(a,b c,d e,f); my $extra = 'hi'; my @new = map { $_ .= ",$extra" } @orig; print "orig: [@orig] => new: [@new]\n"; # orig: [a,b,hi c,d,hi e,f,hi] => new: [a,b,hi c,d,hi e,f,hi]` [download] This way, you append the line and return it as well from `map()`. Open source softwares? Share and enjoy. Make profit from them if you can. Yet, share and enjoy!	[reply] [d/l] [select]