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Re^3: Multiple foreach loops in single statement

by Athanasius (Monsignor)
on Oct 24, 2012 at 04:11 UTC ( #1000551=note: print w/ replies, xml ) Need Help??


in reply to Re^2: Multiple foreach loops in single statement
in thread Multiple foreach loops in single statement

...we can provide a list too in split function.

Sorry, no. split operates on a single string (a scalar), but not on a list:

13:57 >perl -wE " $s = q[abc]; $t = q[mno]; say for split(//, $s, $t); + " Argument "mno" isn't numeric in split at -e line 1. a b c 14:06 >perl -wE " $s = q[abc]; $t = q[mno]; say for split(//, ($s, $t) +); " Useless use of a variable in void context at -e line 1. m n o 14:06 >perl -wE " @u = (q[abc], q[mno]); say for split(//, @u); " 2 14:06 >
The docs however mention EXPR ... which I assumed to be a scalar.

Yes, your original assumption was correct. But split returns a list, which is why the output of split can be the subject of a foreach or a map.

Hope that helps,

Athanasius <°(((><contra mundum


Comment on Re^3: Multiple foreach loops in single statement
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Re^4: Multiple foreach loops in single statement
by gurpreetsingh13 (Scribe) on Oct 24, 2012 at 13:22 UTC
    No. See it clearly.
    perl -e '@lines=`cat file.pl`;print $#lines,"\n", $lines[2];'

    Output:

    18 use Unicode::Collate::Locale;

    this returns a list infact - a list of strings after pumping in entire file splitted on newline.

    And we are providing this list in place of an EXPR in split which makes it a point that we can provide a list too instead of a string.

      gurpreetsingh13,

      I understand your reasoning, and for most languages you would be correct. But Perl is different: in Perl, the behaviour of functions and operators depends on context. For example, here is what the documentation says about backticks:

      The collected standard output of the command is returned ... In scalar context, it comes back as a single (potentially multi-line) string, or undef if the command failed. In list context, returns a list of lines ..., or an empty list if the command failed.

      So, in your script:

      perl -e 'print chr(ord()+1) foreach(split //,`cat file.pl`);'

      the expression `cat file.pl` is in scalar context (because split expects a scalar expression here), so the output of the cat command is fed to split as a single, multi-line string. But in your other script:

      perl -e '@lines=`cat file.pl`;print $#lines,"\n", $lines[2];'

      the assignment to an array (@lines) puts `cat file.pl` into list context, so the output of the cat command is here not a single string as before, but rather a list of strings (one for each line).

      Context is a central concept in Perl. You can read up on it in the references given by Anonymous Monk, above, or in Chapter 2 of Programming Perl by Christiansen, foy, and Wall (in the section beginning on page 76 of the 4th Edition).

      Hope that helps,

      Athanasius <°(((><contra mundum

      split's second argument is evaluated in scalar context, so it's more like
      my $output = `cat file.pl`;
      than
      my @output = `cat file.pl`;
Re^4: Multiple foreach loops in single statement
by gurpreetsingh13 (Scribe) on Oct 24, 2012 at 14:51 UTC
    I considered myself to have given a good start on perl(as it was in some other languages)....but the wisdom of monks here and my weakness in basics shows there is still a long way to go and a lot to know....

    Thanks for your help. :)

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