jar00n has asked for the wisdom of the Perl Monks concerning the following question:

I'm fairly new to Perl and my searches for an answer to this question drew thousands of irrelevant matches due to the vagueness of my issue. I was toying with repeating a sequence and Perl appears to be doing arithmetic with the 'x' operator: 

#!/usr/bin/perl

use strict;
use warnings;

print "Enter some strings:\n";
my @lines = <>;

print 0..9 x 2, "\n";
foreach (@lines) {
        printf "%20s", $_;
}

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This produces a header line: 

$ ./ex05-02.pl
Enter some strings:
asdfasdfasdf
asdfa
asdfasdfasd
asdf
0123456789101112131415161718192021222324252627282930313233343536373839
+404142434445464748495051525354555657585960616263646566676869707172737
+475767778798081828384858687888990919293949596979899
       asdfasdfasdf
              asdfa
        asdfasdfasd
               asdf

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Not 01234567890123456789 as I had expected it to, ultimately I wanted a 20 character header but 0..9 x 2 is being interpreted unexpectedly for me. I would love for anyone to be able to shed some light on why this happening.

Replies are listed 'Best First'.
Re: Seemingly odd behavior of sequence
by oiskuu (Hermit) on Dec 04, 2013 at 11:47 UTC
    The expression 0..9 x 2 evaluates as 0 .. (9 x 2). Put parentheses around the range op: 
        print +(0..9) x 2, "\n";

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    See perlop for the list of operator precedences. Hmm, or perlop (man page).
    The table appears somewhat mangled on [doc://perlop].
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[d/l]
[select]
      Thanks for the reply, I figured that was happening in a moment of genius in the shower... "What if the 'x' operator has a higher precedence than the sequence expansion?"
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Re: Seemingly odd behavior of sequence
by Kenosis (Priest) on Dec 04, 2013 at 17:27 UTC

    If you're curious how Perl parses an expression--perhaps for debugging purposes or just for the pure joy of it on a rainy afternoon--you can use the module B::Deparse at the command line as follows:

    perl -MO=Deparse,-p -e 'print 0..9 x 2, "\n";'

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    Output:

    print((0 .. (9 x 2)), "\n");
-e syntax OK

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    Note that the output echoes oiskuu's spot-on assessment of your expression's evaluation.

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[d/l]
[select]
      And many thanks to you for showing me this debugger.
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        You're most welcome, jar00n!

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