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Re^4: Multiple foreach loops in single statement

by gurpreetsingh13 (Scribe)
on Oct 24, 2012 at 13:22 UTC ( #1000635=note: print w/replies, xml ) Need Help??

in reply to Re^3: Multiple foreach loops in single statement
in thread Multiple foreach loops in single statement

No. See it clearly.
perl -e '@lines=`cat`;print $#lines,"\n", $lines[2];'


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.

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Re^5: Multiple foreach loops in single statement
by Anonymous Monk on Oct 24, 2012 at 13:30 UTC
Re^5: Multiple foreach loops in single statement
by Athanasius (Chancellor) on Oct 24, 2012 at 13:56 UTC


    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`);'

    the expression `cat` 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`;print $#lines,"\n", $lines[2];'

    the assignment to an array (@lines) puts `cat` 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

Re^5: Multiple foreach loops in single statement
by ikegami (Pope) on Mar 02, 2013 at 22:54 UTC
    split's second argument is evaluated in scalar context, so it's more like
    my $output = `cat`;
    my @output = `cat`;

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[shmem]: at least, some Ook.
[erix]: another simple Pg thing is to running a database off a tsv (alas, readonly)
[erix]: I'm waiting for that zipfile-reading to show up in DBD::SQLite :)
[Corion]: hippo, erix: That sounds promising :)
[Corion]: erix: I misinterpreted that. It only allows to read a zipfile like a table, not reading a database from a zipfile :-(
shmem is grabbed by nostalgia for Bolivia
[erix]: aah, ok.
[erix]: oh well, in each life some rain must fall

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