How a for() assignment works

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

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Re: How a for() assignment works by jwkrahn (Abbot) on Mar 06, 2008 at 21:24 UTC
Perhaps you want: `for my $var ( map lc, @vars ) { print "Value: " . $hash_ref->{$var} . "\n" if exists $hash_ref->{$ +var}; }` [download]	[reply] [d/l]
Re^2: How a for() assignment works by Herkum (Parson) on Mar 06, 2008 at 21:28 UTC
Had not thought of doing something inside the (), though I guess I should have known better.	[reply]
Re: How a for() assignment works by NetWallah (Canon) on Mar 06, 2008 at 21:31 UTC
This could be done thus: `for my $var (map {lc} @vars) { print "Value: " . $hash_ref->{$var} . "\n" if exists $hash_ref->{$ +var}; }` [download] Update: Looks like jwkrahn beat me to it... "As you get older three things happen. The first is your memory goes, and I can't remember the other two... " - Sir Norman Wisdom	[reply] [d/l]
Re: How a for() assignment works by amarquis (Curate) on Mar 06, 2008 at 21:32 UTC
Wow, you caused me to have quite the headslap moment. At first I was thinking, why does the lc get assigned to a new variable? So I played around with it and realized for the first time that $var in this construct is an alias and not a copy. headslap Could you not, at least in this example, use a map? Like, `map { print ("Value: " . $hash{lc($_)}); } @values`Is it uncouth to use map this way? Edit: Destroyed by better and faster answers!	[reply] [d/l]
Re^2: How a for() assignment works by NetWallah (Canon) on Mar 06, 2008 at 21:36 UTC
Yes - I consider it uncouth. "map" (and "grep") return lists. If you are not using the returned values, and merely looping, you should use "for". "As you get older three things happen. The first is your memory goes, and I can't remember the other two... " - Sir Norman Wisdom	[reply]
Re^3: How a for() assignment works by Argel (Prior) on Mar 07, 2008 at 20:27 UTC
~~Your use of "uncouth" seems a bit harsh, especially given that~~ FYI, starting with Perl 5.8.1 "map" no longer returns anything when called in void context. See Perl 5.8.1 Miscellaneous-Enhancements for more. Update: Sorry about that NetWallah. I missed it in the OP due to the poor formatting. Thanks for pointing it out! I will strike out that portion of my message (and bold the updates).	[reply]
Re^4: How a for() assignment works by NetWallah (Canon) on Mar 08, 2008 at 01:09 UTC
Re^5: How a for() assignment works by Argel (Prior) on Mar 08, 2008 at 01:42 UTC
Re: How a for() assignment works by CountZero (Bishop) on Mar 07, 2008 at 07:07 UTC
`for my $var (map lc, grep exists $hash_ref->{lc $_}, @vars) { print "Value: " . $hash_ref->{$var} . "\n" ; }` [download] or even `for my $var (map $hash_ref->{lc $_}, grep exists $hash_ref->{lc $_}, @ +vars) { print "Value: " . $var . "\n" ; }` [download] finally leading to `print "Value: " . $_ . "\n" for map $hash_ref->{lc $_}, grep exists $hash_ref->{lc $_}, @vars;` [download] but if we swap the `map` and `grep` we can eliminate the duplicate `lc` `print "Value: " . $hash_ref->{$_} . "\n" for grep exists $hash_ref->{$_}, map lc, @vars;` [download] Of course now we have three loops running :-( CountZero A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James	[reply] [d/l] [select]
Re^2: How a for() assignment works by jwkrahn (Abbot) on Mar 07, 2008 at 13:24 UTC
Or you could always do it like this: `print map exists $hash_ref->{lc()} ? "Value: $hash_ref->{lc()}\n" : '' +, @vars;` [download]	[reply] [d/l]
Re: How a for() assignment works by igelkott (Priest) on Mar 06, 2008 at 21:38 UTC
`print map {"Value: $hash_ref->{lc $_}\n"} @vars;` Update:Over zealously forgot to consider "if exists". Better solutions offered.	[reply] [d/l]
Re^2: How a for() assignment works by ikegami (Patriarch) on Mar 07, 2008 at 06:01 UTC
You did get rid of the duplication. But it no longer prints what the OP wants it to print. And it issues warnings. And memory usage was needless increased. One step forwards, three steps back.	[reply]
Re^3: How a for() assignment works by igelkott (Priest) on Mar 07, 2008 at 09:30 UTC
Guess I got a little carried away and overlooked the important "if exists" requirement. This is the cause of the warnings. Memory increase doesn't seem to be that big a deal but I didn't check this on anything big. Thanks for the reminder to look both ways before jumping into traffic.	[reply]
Re: How a for() assignment works by repellent (Priest) on Mar 07, 2008 at 17:05 UTC
Here is how I think about for() assignments: For a code like, `for ($passed_in) { # inside BLOCK }` [download] Inside the BLOCK, `$_` is equal to `$$ref` where `$ref = \$passed_in` Except that if you did change `$_`, you will also modify `$passed_in`. They are magically tied together, `$_` and the reference, that is. It is faster this way, since only a REFerence is passed_in. No copy of `$passed_in` is made. However, for the following code, `for my $scoped_copy ($passed_in) { # inside BLOCK }` [download] A copy of `$passed_in` is made to `$scoped_copy`. The same reasoning above applies to other loop constructs, sub arguments, or CODE BLOCKS of map(), grep(), sort(), etc. for the `$_ @_ $a $b` variables. Again, this is just my observation. Would someone with knowledge of Perl guts confirm that I could continue with this train of thought?	[reply] [d/l] [select]
Re^2: How a for() assignment works by jwkrahn (Abbot) on Mar 07, 2008 at 23:25 UTC
As Foreach Loops explains, the variable, whether it is implicitly `$_` or explicitly `my $scoped_copy`, is an alias to the current element of the list.	[reply] [d/l] [select]