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

#!/usr/bin/perl -w
use strict;
my $x = 3;
for(1..3){
    my $y = $x;
    $y++;
    $x -= $y;
}

[download]

The above little programme, if I put an acton on line 6 ($y++) the action occurs at line 6, line 7 then at line 5 where it is invalid ($y not defined)

To illustrate here is my session in the debugger...

  DB<3> a 6 print ("\$y $y\n")
  DB<4> n
  DB<4> 
  DB<4> 
$y 3
  DB<4> 
$y 4
  DB<4> 
Use of uninitialized value $y in concatenation (.) or string at (eval 
+9)[/usr/share/perl/5.18/perl5db.pl:732] line 1.
 at (eval 9)[/usr/share/perl/5.18/perl5db.pl:732] line 1.
    eval 'no strict; ($@, $!, $^E, $,, $/, $\\, $^W) = @DB::saved;pack
+age main; print ("\\$y $y\\n");
' called at /usr/share/perl/5.18/perl5db.pl line 732
    DB::eval called at /usr/share/perl/5.18/perl5db.pl line 2661
    DB::DB called at test.pl line 5
$y 
  DB<4>

[download]

Clearly I misunderstand the manual. How do I do an action so it happens on only one line?

Replies are listed 'Best First'.
Re: Debugger actions: On which lines?
by james28909 (Deacon) on Aug 18, 2015 at 05:23 UTC
    Not trying to go off topic, but isnt that code slightly redundant? after the first loop the output will always be the same. But back to the question. I do not get any errors using strict or warnings, or in the debugger.
    Read more... (491 Bytes)

    EDIT: Updated code
[reply]
[d/l]

      Aha! Adding back the shebang line gives the error. Taking it out and it performs as one would expect!

      To be crystal try:

      #!/usr/bin/perl -w
use strict;
use warnings;
my $x = 3;
my $y;
for(1..3){
    $y = $x;print "line 6 ---- x = $x -- y = $y\n";
    $y++; print "line 7 ---- x = $x -- y = $y\n";
    $x -= $y; print "line 8 ---- x = $x -- y = $y\n";
}

      [download]

      For me that has the error and...

      use strict;
use warnings;
my $x = 3;
my $y;
for(1..3){
    $y = $x;print "line 6 ---- x = $x -- y = $y\n";
    $y++; print "line 7 ---- x = $x -- y = $y\n";
    $x -= $y; print "line 8 ---- x = $x -- y = $y\n";
}

      [download]

      ....does not

[reply]
[d/l]
[select]

        It's not the shebang, it's the -w switch. Take out -w and only use use warnings instead and the error goes away because warnings is lexically scoped but -w is global. Using -w turned warnings on globally which is why your error came from the debugger code as it told you.

        I don't know why the action is executed on each line of the loop; I agree that docs indicate that it should be executed only on that line each time through the loop.

        Update: Realized the example code had both -w and use warnings.

        The way forward always starts with a minimal test.
[reply]
[d/l]
[select]
        It works with or without the shebang for me, go figure... I have windows 10 64bit and active-state perl 5.16.3 32bit. I think you should declare $y before the loop though like my $y = 0; that would probably get rid of your errors.
[reply]
[d/l]

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