Strange compile error?

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

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Re: Strange compile error? by marto (Cardinal) on Apr 29, 2024 at 16:38 UTC
`my $N= #Org;` [download] If you want to get the length of the array do: `my $N = scalar( @Org );` [download] You may have been thinking of: `my $N = $#Org;` [download] Which gives you the index of the last element, probably not what you're looking for.	[reply] [d/l] [select]
Re^2: Strange compile error? by LanX (Saint) on Apr 29, 2024 at 17:27 UTC
> `my $N = scalar( @Org );` That's explicit and correct, tho for completeness `my $N = @Org;` [download] has the same implicit effect, since it's a scalar assignment. (The left hand side is a scalar) Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery}	[reply] [d/l] [select]
Re: Strange compile error? by LanX (Saint) on Apr 29, 2024 at 17:20 UTC
Others already told you how to do right... ... But you asked: > What do I do wrong? Point is, that # "normally" starts a comment, and you wrote `my $N= #Org; say "N = $N";` [download] So what the compiler effectively sees is a multiline statement, semantically equal to `my $N= say "N = $N";` The declaration `my $N` at the beginning only takes effect after this statement, i.e. behind the semicolon. Hence, the second $N is undeclared. Hope the error messages are clearer now. :) Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery}	[reply] [d/l] [select]
Re: Strange compile error? by hippo (Archbishop) on Apr 29, 2024 at 17:27 UTC
`my $N= #Org;` ... seen this kind of error many times in my programs If you find you make this sort of error frequently then it would probably be worth enabling syntax highlighting in your editor. It should then be really apparent that you have started a comment where you did not mean to. 🦛	[reply] [d/l]
Re: Strange compile error? by kcott (Archbishop) on Apr 29, 2024 at 18:11 UTC
G'day gelbukh, Welcome to the Monastery. "Sorry for a stupid question! But I want to learn: ..." Not a "stupid question"! ++ for "I want to learn". Here's a description of what's going on. A '`#`' introduces a comment. See "perlsyn: Comments". So, when compiling, Perl sees your lines 9 to 12 as: `my $N= say "N = $N";` [download] As Perl doesn't care about the intervening whitespace, that's basically: `my $N= say "N = $N";` [download] Assignment is a right-to-left operation. See "perlop - Perl operators and precedence". That's a fairly long page; the parts most pertinent to your question are: The table near the end of the "Operator Precedence and Associativity" section. The "Assignment Operators" section. So, Perl attempts to compile '`say "N = $N"`' first. It hasn't evaluated the left side of the assignment ('`my $N`') yet, so it sees `$N` as a "Global symbol" and hence the error message. There is a subtle point here that isn't immediately, or intuitively, obvious. A statement such as '`my $x = ...;`' has two distinct parts: '`my $x`' declares `$x` as a lexical variable at compile time. The whole statement '`my $x = ...;`' is compiled at compile time; however, the actual assignment is a runtime operation. I see a couple of posts showing how to get length and last index of an array. Another way to get the length is: `my $N= 0+@Org;` [download] As a further tangential point, a "use VERSION" statement, where VERSION is `5.12` or greater, you get '`use strict;`' automatically. I like to put the use VERSION statement first: if the user does not have a sufficiently high enough version, you might as well stop here rather than doing other (effectively) pointless processing. Accordingly, instead of `use strict; use warnings; use 5.020;` [download] I'd write `use 5.020; use warnings;` [download] Just something to consider for future reference. — Ken	[reply] [d/l] [select]
Re^2: Strange compile error? by afoken (Chancellor) on Apr 30, 2024 at 13:46 UTC
A '`#`' introduces a comment. Oh no! You triggered that annoying little bean counter! So here we go ... A '`#`' does not ALWAYS introduce a comment: `$#array` is the last index of `@array` `#` can be used in place of `/` in `s///`, `m//`, `q//`, `qq//`, `qx//`, `qw//`, `y///`, `tr///`. `$#` was a magic variable, deprecated long ago, removed in Perl 5.10.0 (see perlvar) `#` inside a comment is ignored like any other character until end of line And of course, a `#` in a string constant is just that. It does not start a comment inside a string constant. Alexander -- Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)	[reply] [d/l] [select]
Re^3: Strange compile error? by LanX (Saint) on Apr 30, 2024 at 22:43 UTC
BUT ... Inside POD, you forgot POD. WAIT... After `__DATA__` and `__END__` STOP ... could be inside a here-doc marker too MOMENT ... A source filter could transform them (see half of f*ck!ng `ACME::` ...) HOLD ... what if C-preprocessor macros are activated? AND ... (fill in more nitpicking, yawn 🥱) Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery} UPDATES Inserted 3 more criteria in hindsight... 🙃	[reply] [d/l] [select]
Re^2: Strange compile error? by LanX (Saint) on Apr 29, 2024 at 19:47 UTC
> Assignment is a right-to-left operation That's right, but sorry you are overcomplicating things. Precedence isn't relevant here: `DB<1> use strict; my $N = $N; Global symbol "$N" requires explicit package` [download] Declarations are only effective after the statement, in this case behind the semicolon. This was done to allow accessing equally named variables from a surrounding scope. See perlsub#Private-Variables-via-my() The declared variable is not introduced (is not visible) until after the current statement. Thus, my $x = $x; can be used to initialize the new `$x` with the value of the old `$x`, and the expression my $x = 123 and $x == 123 is false unless the old `$x` happened to have the value `123`. Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery}	[reply] [d/l] [select]
Re^3: Strange compile error? by kcott (Archbishop) on Apr 29, 2024 at 23:08 UTC
I was talking about associativity (as in "right-to-left" operation). The table to which I referred lumps associativity and precedence together: there's not a lot I can do about that. — Ken	[reply]
Re^4: Strange compile error? by LanX (Saint) on Apr 30, 2024 at 00:19 UTC
Re: Strange compile error? by gelbukh (Initiate) on Apr 29, 2024 at 20:03 UTC
OOPS, I don't know how to remove my question or edit it. Yes it was a trivial error. What I wanted to say was: my $N= $#Org, but I forgot the $, so the rest of the line was a comment, and the my statement was unfinished. Now I see. I don't know how to remove this question	[reply]
Re^2: Strange compile error? by LanX (Saint) on Apr 29, 2024 at 20:26 UTC
> OOPS, I don't know how to remove my question No need to remove a question. Many people will read the thread and will learn from the discussion. Even in the far future. That's how a board works :) So don't worry. Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery}	[reply]
Re^2: Strange compile error? by hippo (Archbishop) on Apr 29, 2024 at 22:03 UTC
See the FAQ How do I change/delete my post?, paying particular attention to the section headed "Why shouldn't I delete a node or its content?". 🦛	[reply]
Re: Strange compile error? by 1nickt (Canon) on Apr 29, 2024 at 16:41 UTC
Hi, ~~For the length of an array you need `$#`, i.e. `$#Org`.~~ Ignore this, it's a blunder. The way forward always starts with a minimal test.	[reply] [d/l] [select]

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