Splitting in while loop

tel2 has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Splitting in while loop by haukex (Archbishop) on Oct 07, 2021 at 09:03 UTC
The usual "don't use a regex when a real parser exists" applies here too. Single quotes are technically a valid character in email addresses, as are commas and spaces when quoted. The following uses Email::Address to correctly parse and split such (admittedly very unusual and not recommended) addresses. `use warnings; use strict; use Email::Address; while (<DATA>) { for my $addr (Email::Address->parse($_)) { print $addr->address, "\n"; } } __DATA__ 'me'@here.com, "West, Casey" <casey@localhost> "those,foo"@there.com others@there.com you@there.com,them@there.com "Hello, World"@example.com` [download] Outputs: `'me'@here.com casey@localhost "those,foo"@there.com others@there.com you@there.com them@there.com "Hello, World"@example.com` [download]	[reply] [d/l] [select]
Re^2: Splitting in while loop by hippo (Bishop) on Oct 07, 2021 at 10:59 UTC
Single quotes are technically a valid character in email addresses For those of you who think this is just pedantry I can assure you that it is not. Many years ago, when defensive programming was less widespread than now, a customer of $WORK did indeed have such an email address. It followed the pattern of `jack.o'malley@bigcorp.com` and caused no end of trouble for various systems which poor, hapless Jack was required to use. It even showed up one or two areas of $WORK's systems where such a potential injection was either mismanaged or misreported. These days Jack should have no trouble with systems from respsonsible coders but there are still plenty of slapdash operators out there who will struggle with this, even now. If you persist in using home-grown regexen to parse email addresses (or HTML or XML or SQL or ...) then you should be aware that sooner or later it will come back to bite you. 🦛	[reply] [d/l]
Re^3: Splitting in while loop by afoken (Chancellor) on Oct 07, 2021 at 18:58 UTC
... email address. It followed the pattern of jack.o'malley@bigcorp.com ... Hmm, let me guess: The first name was Robert. ;-) Alexander -- Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)	[reply]
Re^3: Splitting in while loop by tel2 (Pilgrim) on Oct 08, 2021 at 00:44 UTC
Points taken thanks guys, but in my case I'm just using them as a quick hack to visually show the limits of the address in the output, e.g. there is no leading/trailing spaces, etc. In the rare event that I did end up with this kind of thing being output: 'jack.o'malley@bigcorp.com' it wouldn't matter.	[reply]
Re: Splitting in while loop by tybalt89 (Monsignor) on Oct 07, 2021 at 05:00 UTC
Or: `#!/usr/bin/perl use strict; use warnings; while( <DATA> ) { for ( split /[, \n]+/ ) { print "'$_'\n"; } }` [download]	[reply] [d/l]
Re: Splitting in while loop by tybalt89 (Monsignor) on Oct 07, 2021 at 04:53 UTC
`#!/usr/bin/perl use strict; use warnings; while (split(/[, ]+/, <DATA>)) { chomp; print "'$_'\n"; } __DATA__ me@here.com those@there.com others@there.com you@there.com,them@there.com` [download] Outputs: Use of uninitialized value $_ in scalar chomp at ./pm11137286.pl line +8, <DATA> line 1. Use of uninitialized value $_ in concatenation (.) or string at ./pm11 +137286.pl line 9, <DATA> line 1. '' Use of uninitialized value $_ in scalar chomp at ./pm11137286.pl line +8, <DATA> line 2. Use of uninitialized value $_ in concatenation (.) or string at ./pm11 +137286.pl line 9, <DATA> line 2. '' Use of uninitialized value $_ in scalar chomp at ./pm11137286.pl line +8, <DATA> line 3. Use of uninitialized value $_ in concatenation (.) or string at ./pm11 +137286.pl line 9, <DATA> line 3. '' Use of uninitialized value in split at ./pm11137286.pl line 9, <DATA> +line 3. [download] You probably want: `#!/usr/bin/perl use strict; use warnings; for (map { split /[, ]+/ } <DATA>) { chomp; print "'$_'\n"; } __DATA__ me@here.com those@there.com others@there.com you@there.com,them@there.com` [download]	[reply] [d/l] [select]
Re^2: Splitting in while loop by tel2 (Pilgrim) on Oct 07, 2021 at 05:16 UTC
Nice work, thanks for both your solutions, tybalt89! Do you understand why mine wasn't working?	[reply]
Re^3: Splitting in while loop by kcott (Archbishop) on Oct 07, 2021 at 07:43 UTC
G'day tel2, "... why mine wasn't working?" I believe you're working on the erroneous assumption that `split(/[, ]+/, <DATA>)` will assign its result to `$_`. That assignment to `$_` only occurs in a select number of cases. From "perlsyn: Compound Statements": "If the condition expression of a `while` statement is based on any of a group of iterative expression types then it gets some magic treatment. The affected iterative expression types are `readline`, the `<FILEHANDLE>` input operator, ... If the condition expression is one of these expression types, then the value yielded by the iterative operator will be implicitly assigned to `$_`." For each of your `while` iterations, the condition is true three times, once for each DATA line; however, `$_` is not set for any of those iterations. On the fourth iteration, there are no more DATA lines and the loop ends. Always add `use strict; use warnings;` [download] to the top of your code. In this instance, although it wouldn't give a detailed explanation of the problem as I've provided, it would have hinted at a starting point for investigating the issue. — Ken	[reply] [d/l] [select]
Re^4: Splitting in while loop by tel2 (Pilgrim) on Oct 08, 2021 at 00:54 UTC
Re^5: Splitting in while loop by kcott (Archbishop) on Oct 08, 2021 at 04:38 UTC
Re^5: Splitting in while loop by eyepopslikeamosquito (Archbishop) on Oct 08, 2021 at 01:21 UTC
Re^3: Splitting in while loop by AnomalousMonk (Archbishop) on Oct 07, 2021 at 07:44 UTC
Further to jwkrahn's post: tel2: Note that implicit `split` to `@_` has been deprecated for a long time: `Win8 Strawberry 5.8.9.5 (32) Thu 10/07/2021 3:29:50 C:\@Work\Perl\monks >perl -Mstrict -Mwarnings my $data = <<'EOD'; me@here.com those@there.com others@there.com you@there.com,them@there.com EOD open my $fh, '<', \$data or die "opening: $!"; while (split(/[, ]+/, <$fh>)) { chomp @_; print "'$_'\n" for @_; } ^Z Use of implicit split to @_ is deprecated at - line 14. 'me@here.com' 'those@there.com' 'others@there.com' 'you@there.com' 'them@there.com'` [download] Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^3: Splitting in while loop by AnomalousMonk (Archbishop) on Oct 07, 2021 at 07:21 UTC
`while (split(/[, ]+/, <DATA>)) { ... }` [download] Only a `while`-loop condition expression like `while (<DATA>) { ... }` [download] assigns implicitly to `$_` (see tybalt89's example) (update: but see kcott's post for a more complete discussion of `$_` assignment special-casing). An arbitrary expression like `split(...)` does not. Had you had warnings enabled, Perl would have at least hinted at this problem. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^3: Splitting in while loop by jwkrahn (Abbot) on Oct 07, 2021 at 07:21 UTC
`while (split(/[, ]+/, <DATA>))` [download] Is interpreted by perl as: `while ( @_ = split( /[, ]+/, <DATA> ) )` [download]	[reply] [d/l] [select]
Re^4: Splitting in while loop by tel2 (Pilgrim) on Oct 08, 2021 at 00:34 UTC
Re^5: Splitting in while loop by kcott (Archbishop) on Oct 08, 2021 at 03:23 UTC
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Re: Splitting in while loop by LanX (Saint) on Oct 07, 2021 at 13:16 UTC
Summary RULE: `while` doesn't assign any variables (like `foreach` ) it's a boolean check alike `if(CONDITION) { DO LOOP }` Some operations assign automatically to some vars (side-effect) and return true on success. E.g. regexes setting `$1` etc can be combined with a boolean check. Some like `<FH>` do this only inside `while(<FH>)` context for exceptinal DWIM Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery}	[reply] [d/l] [select]
Re^2: Splitting in while loop by perlfan (Vicar) on Oct 12, 2021 at 16:09 UTC
In non-slurp mode, this works via assignment using the diamond operator (`<>`) - or just the same if you open an file handle, `$FH`: `# test.pl while (my $line = <>) { chomp $line; print qq{$line\n}; # yes I know, pointless use of chomp }` [download] `> perl test.pl < test.pl # test.pl while (my $line = <>) { chomp $line; print qq{$line\n}; # yes I know, pointless use of chomp }` [download] Similarly, you could do something destructive with pop or shift, like, `while (my $item = pop @my_array) { # do stuff with $item }` [download]	[reply] [d/l] [select]
Re^3: Splitting in while loop by haukex (Archbishop) on Oct 12, 2021 at 16:41 UTC
`while (my $line = <>)` ... Similarly, ... `while (my $item = pop @my_array)` `use warnings; use strict; while (my $line = <DATA>) { chomp $line; print "<$line>\n"; } my @array = ("Foo","0","Bar"); while (my $item = pop @array) { print "[$item]\n"; } __DATA__ Hello 0 World` [download] Outputs: `<Hello> <0> <> <World> [Bar]` [download] I'd call that a pretty serious caveat. As per I/O Operators, the first is equivalent to `while ( defined( my $line = <> ) )`, and readline returns `undef` at EOF, while the second one will stop at any false value, and arrays can contain `undef`s too.	[reply] [d/l] [select]


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