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Re: [PBML] Reading partial array

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  • Matthew K
    The author of Switch posted this. http://www.perlmonks.org/?node_id=486756 For those of you who don t want to click, he lists it under, modules you shouldn t
    Message 1 of 10 , Jul 30, 2012
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      The author of Switch posted this.

      http://www.perlmonks.org/?node_id=486756


      For those of you who don't want to click, he lists it under, "modules you shouldn't use in production because their purpose is to explore and prototype future core language features".


      --
      Matthew Kunzman
      Software Engineer 
      Boise, ID USA


      Email: matt_hew@... 
      LinkedIn: http://www.linkedin.com/pub/matthew-kunzman/b/5ba/94a
      -------------------------------------------------------------------------


      >________________________________
      > From: "merlyn@..." <merlyn@...>
      >To: jana_van <jana_van@...>
      >Cc: perl-beginner@yahoogroups.com
      >Sent: Monday, July 30, 2012 9:44 AM
      >Subject: Re: [PBML] Reading partial array
      >
      >

      >>>>>> "jana" == jana van <jana_van@...> writes:
      >
      >jana> use Switch;
      >
      >Please don't.
      >
      >jana> My requirement is that @temp should print 25 elments of @arr for
      >jana> each iteration and the original array @arr should be
      >jana> intact. Please help here.
      >
      >The general pattern is this:
      >
      >my @copy = @arr;
      >while (@copy) {
      >my @temp = splice(@copy, 0, 25);
      >print "@temp\n"; # or whatever
      >}
      >
      >--
      >Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095
      ><merlyn@...> <URL:http://www.stonehenge.com/merlyn/>
      >Smalltalk/Perl/Unix consulting, Technical writing, Comedy, etc. etc.
      >See http://methodsandmessages.posterous.com/ for Smalltalk discussion
      >
      >
      >
      >

      [Non-text portions of this message have been removed]
    • Shlomi Fish
      Hi Rob, On Mon, 30 Jul 2012 12:36:02 -0400 ... Reading from http://perl-begin.org/tutorials/bad-elements/#switch_pm :
      Message 2 of 10 , Jul 30, 2012
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        Hi Rob,

        On Mon, 30 Jul 2012 12:36:02 -0400
        Rob Richardson <CedricCicada@...> wrote:

        > Randal,
        >
        > Why not use Switch?
        > Since this is a beginner's forum, an explanation would be helpful.
        >

        Reading from http://perl-begin.org/tutorials/bad-elements/#switch_pm :

        <<<
        One should not use Switch.pm to implement a switch statement because it's a source filter, tends to break a lot of code, and causes unexpected problems. Instead one should either use given/when, which are only available in perl-5.10 and above, or dispatch tables, or alterantively plain if/elsif/else structures.
        >>>

        Regards,

        Shlomi Fish

        > RobR
        >
        >
        >
        > On Mon, Jul 30, 2012 at 11:44 AM, <merlyn@...> wrote:
        > >>>>>> "jana" == jana van <jana_van@...> writes:
        > >
        > > jana> use Switch;
        > >
        > > Please don't.



        --
        -----------------------------------------------------------------
        Shlomi Fish http://www.shlomifish.org/
        Stop Using MSIE - http://www.shlomifish.org/no-ie/

        Better be a tail for the lions, than the head of the jackals.
        — Pirkei Avot, 4 15

        Please reply to list if it's a mailing list post - http://shlom.in/reply .
      • BhavinP
        ... my @arr = (1..100); my $start = 0; my $end = 0; my $incr = 25; sub get25{ $end = $start + $incr; my @retarr = @arr[$start..$end]; $start = $end; return
        Message 3 of 10 , Sep 20, 2012
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          --- In perl-beginner@yahoogroups.com, "jana_van" <jana_van@...> wrote:
          >
          > #!/usr/local/roadm/bin/perl
          > # This is compiled with threading support
          > use strict;
          > use warnings;
          > use threads;
          > use threads::shared;
          > use Switch;
          >
          > my @arr=();
          >
          > my $k=0;
          >
          > for($k=0;$k<100;$k++)
          > {
          > push(@arr, $k);
          > }
          >
          > print "\n The array is ", "@arr", "\n";
          > my @temp=();
          >
          > for($k=0; $k<4;$k++)
          > {
          > @temp = @arr[25*$k .. 25];
          > print "\n The fragmented array is ", "@temp", "\n";
          > @temp=();
          > }
          >
          > In the second for loop, for the first iteration @temp reading 25 elements but for the second iteration onwards the values read by @temp are not as expected. whats the fault here ?
          >
          > My requirement is that @temp should print 25 elments of @arr for each iteration and the original array @arr should be intact. Please help here.
          >
          > Thanks
          >



          my @arr = (1..100);
          my $start = 0;
          my $end = 0;
          my $incr = 25;

          sub get25{
          $end = $start + $incr;
          my @retarr = @arr[$start..$end];
          $start = $end;
          return \@retarr;

          }
        • Joe Pepersack
          There are a couple problems with the solution provided by BhavinP, not the least of which is that it doesn t duplicate Jana s requirements. The subroutine is
          Message 4 of 10 , Sep 23, 2012
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            There are a couple problems with the solution provided by BhavinP, not
            the least of which is that it doesn't duplicate Jana's requirements.
            The subroutine is never invoked, and there is a completely useless
            assignment after the slice.

            In Jana's example, there's no need to use threads or Switch, as the code
            doesn't use those features. Don't use a module or feature that's never
            invoked.

            The first part of your code is easy. Rather than using a for loop to
            assign 0 to 99 to an array, we can use the range (..) operator:

            my @arr = (0..99);

            It's more compact and faster.

            The reason your code isn't working is that you're mis-using the range
            operator in your array slice.
            You have:
            @temp = @arr[25*$k .. 25];

            You should have
            @temp = @arr[ ($k*25) .. (($k*25)+24) ];

            When doing an array slice, you are telling it the values of the first
            and second array indexes, not the first index and the number of values
            you want. We also have to remember that array indexes are zero-based,
            so we need to adjust accordingly to avoid off-by-one / fencepost
            errors. Doing the math inline makes the code hard to read, so for clarity:

            my $incr = 25;
            for my $k ( 0 .. 3 ) {
            my $start = $incr * $k;
            my $end = $start + ($incr - 1); # parens for clarity
            my @temp = @arr[ $start .. $end ];
            print "Array slice $start to $end is ( @temp )\n";
            }

            This will give us:
            Array slice 0 to 24 is ( ... )
            Array slice 25 to 49 is ( ... )
            Array slice 50 to 74 is ( ... )
            Array slice 75 to 99 is ( ... )

            Printing the values of $start and $end allows us to easily debug the
            code. If we had, for instance, said:
            $end = $start + $incr;
            it would be immediately obvious what the problem was.

            For style and additional clarity it's probably a good idea factor this
            logic out into a function, but that's another lesson. You also
            generally want to try to avoid using C-style for loops. Making your code
            clear and readable should always be a priority.

            On 9/20/12 10:41 AM, BhavinP wrote:
            >
            >
            >
            > --- In perl-beginner@yahoogroups.com
            > <mailto:perl-beginner%40yahoogroups.com>, "jana_van" <jana_van@...> wrote:
            > >
            > > #!/usr/local/roadm/bin/perl
            > > # This is compiled with threading support
            > > use strict;
            > > use warnings;
            > > use threads;
            > > use threads::shared;
            > > use Switch;
            > >
            > > my @arr=();
            > >
            > > my $k=0;
            > >
            > > for($k=0;$k<100;$k++)
            > > {
            > > push(@arr, $k);
            > > }
            > >
            > > print "\n The array is ", "@arr", "\n";
            > > my @temp=();
            > >
            > > for($k=0; $k<4;$k++)
            > > {
            > > @temp = @arr[25*$k .. 25];
            > > print "\n The fragmented array is ", "@temp", "\n";
            > > @temp=();
            > > }
            > >
            > > In the second for loop, for the first iteration @temp reading 25
            > elements but for the second iteration onwards the values read by @temp
            > are not as expected. whats the fault here ?
            > >
            > > My requirement is that @temp should print 25 elments of @arr for
            > each iteration and the original array @arr should be intact. Please
            > help here.
            > >
            > > Thanks
            > >
            >
            > my @arr = (1..100);
            > my $start = 0;
            > my $end = 0;
            > my $incr = 25;
            >
            > sub get25{
            > $end = $start + $incr;
            > my @retarr = @arr[$start..$end];
            > $start = $end;
            > return \@retarr;
            >
            > }
            >
            >




            [Non-text portions of this message have been removed]
          • BhavinP
            well, my code was just to point in the right direction. a module that will give you chunks of 25 elements from the array each time its invoked. the last
            Message 5 of 10 , Sep 24, 2012
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              well, my code was just to point in the right direction. a module that will give you chunks of 25 elements from the array each time its invoked. the last statement after we got the chunk is to move the array pointer 25 elements forward.

              ofcourse , we need to wrap it in a loop like map { print Dumper get25(); } (1..3) in one line would do it...


              --- In perl-beginner@yahoogroups.com, Joe Pepersack <joe@...> wrote:
              >
              > There are a couple problems with the solution provided by BhavinP, not
              > the least of which is that it doesn't duplicate Jana's requirements.
              > The subroutine is never invoked, and there is a completely useless
              > assignment after the slice.
              >
              > In Jana's example, there's no need to use threads or Switch, as the code
              > doesn't use those features. Don't use a module or feature that's never
              > invoked.
              >
              > The first part of your code is easy. Rather than using a for loop to
              > assign 0 to 99 to an array, we can use the range (..) operator:
              >
              > my @arr = (0..99);
              >
              > It's more compact and faster.
              >
              > The reason your code isn't working is that you're mis-using the range
              > operator in your array slice.
              > You have:
              > @temp = @arr[25*$k .. 25];
              >
              > You should have
              > @temp = @arr[ ($k*25) .. (($k*25)+24) ];
              >
              > When doing an array slice, you are telling it the values of the first
              > and second array indexes, not the first index and the number of values
              > you want. We also have to remember that array indexes are zero-based,
              > so we need to adjust accordingly to avoid off-by-one / fencepost
              > errors. Doing the math inline makes the code hard to read, so for clarity:
              >
              > my $incr = 25;
              > for my $k ( 0 .. 3 ) {
              > my $start = $incr * $k;
              > my $end = $start + ($incr - 1); # parens for clarity
              > my @temp = @arr[ $start .. $end ];
              > print "Array slice $start to $end is ( @temp )\n";
              > }
              >
              > This will give us:
              > Array slice 0 to 24 is ( ... )
              > Array slice 25 to 49 is ( ... )
              > Array slice 50 to 74 is ( ... )
              > Array slice 75 to 99 is ( ... )
              >
              > Printing the values of $start and $end allows us to easily debug the
              > code. If we had, for instance, said:
              > $end = $start + $incr;
              > it would be immediately obvious what the problem was.
              >
              > For style and additional clarity it's probably a good idea factor this
              > logic out into a function, but that's another lesson. You also
              > generally want to try to avoid using C-style for loops. Making your code
              > clear and readable should always be a priority.
              >
              > On 9/20/12 10:41 AM, BhavinP wrote:
              > >
              > >
              > >
              > > --- In perl-beginner@yahoogroups.com
              > > <mailto:perl-beginner%40yahoogroups.com>, "jana_van" <jana_van@> wrote:
              > > >
              > > > #!/usr/local/roadm/bin/perl
              > > > # This is compiled with threading support
              > > > use strict;
              > > > use warnings;
              > > > use threads;
              > > > use threads::shared;
              > > > use Switch;
              > > >
              > > > my @arr=();
              > > >
              > > > my $k=0;
              > > >
              > > > for($k=0;$k<100;$k++)
              > > > {
              > > > push(@arr, $k);
              > > > }
              > > >
              > > > print "\n The array is ", "@arr", "\n";
              > > > my @temp=();
              > > >
              > > > for($k=0; $k<4;$k++)
              > > > {
              > > > @temp = @arr[25*$k .. 25];
              > > > print "\n The fragmented array is ", "@temp", "\n";
              > > > @temp=();
              > > > }
              > > >
              > > > In the second for loop, for the first iteration @temp reading 25
              > > elements but for the second iteration onwards the values read by @temp
              > > are not as expected. whats the fault here ?
              > > >
              > > > My requirement is that @temp should print 25 elments of @arr for
              > > each iteration and the original array @arr should be intact. Please
              > > help here.
              > > >
              > > > Thanks
              > > >
              > >
              > > my @arr = (1..100);
              > > my $start = 0;
              > > my $end = 0;
              > > my $incr = 25;
              > >
              > > sub get25{
              > > $end = $start + $incr;
              > > my @retarr = @arr[$start..$end];
              > > $start = $end;
              > > return \@retarr;
              > >
              > > }
              > >
              > >
              >
              >
              >
              >
              > [Non-text portions of this message have been removed]
              >
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