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Re^2: Multiple foreach loops in single statement

by gurpreetsingh13 (Scribe)
on Oct 24, 2012 at 03:36 UTC ( #1000547=note: print w/ replies, xml ) Need Help??


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

Great. Just noticed that we can provide a list too in split function. The docs however mention EXPR --split /PATTERN/, EXPR-- which I assumed to be a scalar.

Anyways thanks for the help. This was what I wanted to achieve:

perl -e 'print chr(ord()+1) foreach(split //,`cat file.pl`);'
(a simple one liner to encrypt a file via ascii shift).


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Re^3: Multiple foreach loops in single statement
by Athanasius (Monsignor) on Oct 24, 2012 at 04:11 UTC
    ...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

      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`;
      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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