Re^3: Multiple foreach loops in single statement

[Athanasius]

...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,

[gurpreetsingh13]

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. :)

[gurpreetsingh13]

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.

[Anonymous Monk]

No , split doesn't work with lists, it forces scalar context on EXPR, so qx returns a STRING, not a list

$ perl -wle " print for split //, @ARGV " asdf
1

$ perl -wle " print for split //, @ARGV " a s d f
4

$ perl -wle " print for split //, $ARGV[0] " asdf
a
s
d
f

See Tutorials: Context in Perl: Context tutorial, "List" Is a Four-Letter Word, Easy Reference for Contexts

[Athanasius]

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 4^th Edition).

Hope that helps,

[ikegami]

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`;