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Only the first statement prints the reversed result correctly. I'm
confused why the other three don't print as I expected. How to print
the reversed result without using the intermediate variable $rstring?

$ cat reverse.pl
#!/usr/bin/perl

use warnings;

$string = "abc";
$rstring=reverse($string);
print "$rstring\n";
print reverse($string), "\n";
print(reverse($string), "\n");
print((reverse($string)), "\n");

$ ./reverse.pl
cba
abc
abc
abc

On Dec 31, 2:45*pm, Peng Yu <pengyu...@gmail.com> wrote:
> Only the first statement prints the reversed result correctly. I'm
> confused why the other three don't print as I expected. How to print
> the reversed result without using the intermediate variable $rstring?
>
> $ cat reverse.pl
> #!/usr/bin/perl
>
> use warnings;
>
> $string = "abc";
> $rstring=reverse($string);
> print "$rstring\n";
> print reverse($string), "\n";
> print(reverse($string), "\n");
> print((reverse($string)), "\n");
>
> $ ./reverse.pl
> cba
> abc
> abc
> abc

Hello Peng,

You need to consider that the print operator expects a list. The text
below is copied from 'perldoc -f print'.

Because print takes a LIST, anything in the LIST is evaluated in list
context...

In your code,

print reverse($string), "\n";

print is expecting a list, and so reverse tries to return a list in
reverse order. However, the only item in the list is '$string', so it
gives that to print.

print scalar reverse($string), "\n";


C:\perlp>perldoc -f reverse
reverse LIST
In list context, returns a list value consisting of the
elements
of LIST in the opposite order. In scalar context,
concatenates
the elements of LIST and returns a string value with all
characters in the opposite order.

Chris

Peng Yu <> writes:

> Only the first statement prints the reversed result correctly. I'm
> confused why the other three don't print as I expected. How to print
> the reversed result without using the intermediate variable $rstring?
>
> $ cat reverse.pl
> #!/usr/bin/perl
>
> use warnings;
>
> $string = "abc";
> $rstring=reverse($string);
> print "$rstring\n";
> print reverse($string), "\n";
> print(reverse($string), "\n");
> print((reverse($string)), "\n");
>
> $ ./reverse.pl
> cba
> abc
> abc
> abc

john@ecce:~$ perl -e 'print reverse("abc"), "\n"'
abc
john@ecce:~$ perl -e 'print scalar(reverse("abc")), "\n"'
cba
perl -e 'print reverse("abc") . "\n"'
cba

perldoc -f reverse
reverse LIST
In list context, returns a list value consisting of the
elements of LIST in the opposite order. In scalar context,
concatenates the elements of LIST and returns a string value
with all characters in the opposite order.

--
John Bokma

Read my blog: http://johnbokma.com/
Hire me (Perl/Python): http://castleamber.com/

Peng Yu <> wrote:
>Only the first statement prints the reversed result correctly. I'm
>confused why the other three don't print as I expected. How to print
>the reversed result without using the intermediate variable $rstring?
>
>$ cat reverse.pl
>#!/usr/bin/perl
>
>use warnings;
>
>$string = "abc";
>$rstring=reverse($string);
>print "$rstring\n";
>print reverse($string), "\n";
>print(reverse($string), "\n");
>print((reverse($string)), "\n");

Use reverse() in scalar instead of in list context:
print scalar(reverse($string)), "\n";

In your examples you are just swapping the position of the string and
the "\n".

jue

On Jan 1, 2:43*pm, John Bokma <j...@castleamber.com> wrote:
> Peng Yu <pengyu...@gmail.com> writes:
> > Only the first statement prints the reversed result correctly. I'm
> > confused why the other three don't print as I expected. How to print
> > the reversed result without using the intermediate variable $rstring?
>
> > $ cat reverse.pl
> > #!/usr/bin/perl
>
> > use warnings;
>
> > $string = "abc";
> > $rstring=reverse($string);
> > print "$rstring\n";
> > print reverse($string), "\n";
> > print(reverse($string), "\n");
> > print((reverse($string)), "\n");
>
> > $ ./reverse.pl
> > cba
> > abc
> > abc
> > abc
>
> john@ecce:~$ perl -e 'print reverse("abc"), "\n"'
> abc
> john@ecce:~$ perl -e 'print scalar(reverse("abc")), "\n"'
> cba
> perl -e 'print reverse("abc") . "\n"'
> cba
>
> perldoc -f reverse
> * * * *reverse LIST
> * * * * * * * *In list context, returns a list value consisting of the
> * * * * * * * *elements of LIST in the opposite order. *In scalar context,
> * * * * * * * *concatenates the elements of LIST and returns a string value
> * * * * * * * *with all characters in the opposite order.

I'm wondering how to define a user function that is context dependent.
Could you give me a simple example?

Peng Yu <> wrote:
>I'm wondering how to define a user function that is context dependent.

perldoc -f wantarray

jue

Tad McClellan <> wrote:
>Jürgen Exner <> wrote:
>> Peng Yu <> wrote:
>>>Only the first statement prints the reversed result correctly. I'm
>>>confused why the other three don't print as I expected. How to print
>>>the reversed result without using the intermediate variable $rstring?
>>>
>>>$ cat reverse.pl
>>>#!/usr/bin/perl
>>>
>>>use warnings;
>>>
>>>$string = "abc";
>>>$rstring=reverse($string);
>>>print "$rstring\n";
>>>print reverse($string), "\n";
>>>print(reverse($string), "\n");
>>>print((reverse($string)), "\n");
>>
>> Use reverse() in scalar instead of in list context:
>> print scalar(reverse($string)), "\n";
>
>
>That is correct.
>
>
>> In your examples you are just swapping the position of the string and
>> the "\n".
>
>That is not correct, as the newline is not an argument to reverse()...

Ooops, you are right of course.

jue

Tad McClellan wrote:
> Peng Yu <> wrote:

>> I'm wondering how to define a user function that is context dependent.
>> Could you give me a simple example?
>
> -----------------------
> #!/usr/bin/perl
> use warnings;
> use strict;
>
> my $rstring = context();
> print context(), "\n";
>
> sub context {
> if ( wantarray )
> { warn "list context\n" }
> else
> { warn "scalar context\n" }
> }


This one also knows about void context:


sub trim {
#
# Remove whitespace from tail and head
#
# IN: 1 .. N values to trim
# (in-place when called in *void* context)
# OUT: 1 .. N trimmed values

no warnings 'uninitialized';

if ( wantarray ) { # list context
my @value = @_; # copy
s{\s+\z}{}, s{\A\s+}{} for @value;
return @value;
}
elsif ( defined wantarray ) { # scalar context
my $value = $_[0]; # copy
s{\s+\z}{}, s{\A\s+}{} for $value;
return $value;
}

# void context, so change in-place
s{\s+\z}{}, s{\A\s+}{} for @_;
return;
}

--
Ruud