Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

hi, i'm at the exercises part of Chap3 of the llama book and i'm stuck with the reverse function. i have this script: 
#!/usr/bin/perl

use warnings;
use strict;

my $test = '';

$test = "apples";
print reverse $test;

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it still prints out the word "apples" but not in the reverse way. i have tried using "@test" too but it doesn't reverse. appreciate your help.

Replies are listed 'Best First'.
Re: reverse is not reversing
by LanX (Saint) on Nov 26, 2018 at 13:13 UTC
    Print is imposing a list context.

    You need scalar context for string reverse

    print scalar reverse $test;

    What's happening is that the one element list is reversed now, which is of course useless.

      DB<5> print reverse qw/a1 b2 c3/               # inverting list
c3b2a1
  DB<6> print scalar reverse qw/a1 b2 c3/        # inverting string
3c2b1a

    [download]

    Cheers Rolf
    (addicted to the Perl Programming Language :)
    Wikisyntax for the Monastery FootballPerl is like chess, only without the dice

    update

    added code

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      oh. it only works with more than one word. i thought it would reverse "apple" into "elppa".
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        No, it will reverse "apple" into "elppa", but only in scalar context. Read more about context in the book Modern Perl.

        Update: Fixed the book title. Thanx LanX.

        ($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

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        > it only works with more than one word

        that's wrong, context matters (LHS) not input (RHS)

          DB<9> print reverse "text"            # list context
text
  DB<10> print scalar reverse "text"    # scalar context
txet
  DB<11> $a= reverse "text"             # scalar

  DB<12> print $a                       # still print LIST, but $a is 
+already reversed
txet

        [download]

        Cheers Rolf
        (addicted to the Perl Programming Language :)
        Wikisyntax for the Monastery FootballPerl is like chess, only without the dice

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Re: reverse is not reversing
by 1nickt (Canon) on Nov 26, 2018 at 13:14 UTC

    Hi, reverse operates on lists: 

        reverse LIST
            In list context, returns a list value consisting of the el
+ements
            of LIST in the opposite order. In scalar context, concaten
+ates the
            elements of LIST and returns a string value with all chara
+cters in
            the opposite order.

                print join(", ", reverse "world", "Hello"); # Hello, w
+orld

                print scalar reverse "dlrow ,", "olleH";    # Hello, w
+orld

            Used without arguments in scalar context, "reverse" revers
+es $_.

                $_ = "dlrow ,olleH";
                print reverse;                         # No output, li
+st context
                print scalar reverse;                  # Hello, world

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    Add a call to scalar: 
    #!/usr/bin/perl

use warnings;
use strict;

my $test = '';

$test = "apples";
print scalar reverse $test;

    [download]

    Hope this helps!


    The way forward always starts with a minimal test.
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