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Re: [PBML] returning arrays and hashes from a subroutine

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  • Charles K. Clarkson
    ... So this is what Japhy meant. Shame on you Japhy, I can t believe you d let Mark do it this way. :) ... my $sql = SELECT doc_name FROM $table WHERE
    Message 1 of 12 , May 2, 2002
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      "markveerman" <mark@...> wrote:

      : I figured it out! Although, I don't really understand the, but
      : here's what it was:
      :
      : Below within the sub I originally had: my @fieldnames = $db-
      : >FieldNames(); return @fieldnames; When I removed the word "my",
      : @fieldnames was returned correctly. Also, I didn't realize you could
      : call the function like @array = function() - so that was helpful too.
      :
      : thanks for your help japhy!

      So this is what Japhy meant. Shame on you Japhy, I
      can't believe you'd let Mark do it this way. :)

      : use Win32::ODBC;
      : my $dsn = "mysql";
      : my $table = "table";
      : my $sql = "SELECT doc_name FROM " . $table . " WHERE status = 'Q'";

      my $sql = "SELECT doc_name FROM $table WHERE status = 'Q'";

      : odbc($dsn, $table, $sql);

      my @field_names = odbc($dsn, $sql) ||
      die "There's an uncaught error in odbc()";

      : foreach $fieldnames(@fieldnames){
      : print $fieldnames . "\n";
      : }

      print "$_\n" foreach @field_names;

      : sub odbc {
      : my ($dsn, $table, $sql) = @_;
      : use Win32::ODBC;

      sub odbc {
      my ($dsn, $sql) = @_;

      : my $db = new Win32::ODBC($dsn);
      :
      : if (!$db){
      : die "Error connecting: " . Win32::ODBC::Error() . "\n";
      : }

      my $db = new Win32::ODBC($dsn) ||
      die 'Error connecting: ', Win32::ODBC::Error();

      : if ($db->Sql($sql)) {
      : print "Error submitting SQL statement: " . $db->Error
      : () . "\n" . $sql;
      : }

      if ( $db->Sql($sql) ) {
      print "Error submitting SQL statement: $db->Error\n$sql";
      # Are you sure you don't want to die here?
      }

      : else{
      : @fieldnames = $db->FieldNames();
      : return @fieldnames;

      return $db->FieldNames();
      : }

      return;


      HTH,

      Charles K. Clarkson
      --
      Head Bottle Washer,
      Clarkson Energy Homes, Inc.

      CJ Web Work - Domains for Real Estate Investors.
      E Pluribus Unum -- One from many.
    • Charles K. Clarkson
      ... When I started with perl last year, almost everything I managed to get working was an example of poor coding. A subroutine should be able to be pulled out
      Message 2 of 12 , May 2, 2002
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        "markveerman" <mark@...> wrote:

        : Thanks Charles - Are you telling me that even though it
        : works it's not good code? :-).

        When I started with perl last year, almost everything I
        managed to get working was an example of poor coding.
        A subroutine should be able to be pulled out of the
        program it was written in and placed in another and run
        with minimal adjustment.

        When writing a subroutine, think of it as if a manager
        just handed you an assignment. You have no idea how this
        sub will be used in the program. You know what it should
        accept, what it should return, and what it should do.

        You can cut some corners for short programs, but the
        larger and more complex your script gets the more you
        need to follow this or a more refined approach to
        building subroutines.

        : As if it's not totally obvious - I'm a novice.

        A novice what?

        : > So this is what Japhy meant. Shame on you Japhy, I
        : > can't believe you'd let Mark do it this way. :)
        :
        : In defense of Japhy, today was the first day I've ever
        : written a sub routine, so the issues with scoping and
        : local/global variables is entirely new.

        Is was meant as a joke. I knew Japhy didn't mean want
        you thought he did.


        HTH,

        Charles K. Clarkson
        --
        Head Bottle Washer,
        Clarkson Energy Homes, Inc.

        CJ Web Work - Domains for Real Estate Investors.
        E Pluribus Unum -- One from many.
      • markveerman
        ... I m trying to write a function that that will use the Win32::ODBC module to query a database and return an array containing the fieldnames of the
        Message 3 of 12 , May 2, 2002
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          > You've shown the function, but not what you're doing with it.

          I'm trying to write a function that that will use the Win32::ODBC
          module to query a database and return an array containing the
          fieldnames of the recordset, and also a hash with the data.

          If I use the following within the sub routine:

          <snip>
          my @fieldnames = $db->FieldNames();
          foreach $fieldnames(@fieldnames){
          print $fieldnames . "\n";
          </snip>

          It works fine, so I know that $fieldnames is a working array, but
          when I try to use return @fieldnames; and put the foreach loop
          outside of the function I get nothing.

          Does it have something to do with the type of array or the format of
          the data in the array? I tried your sample and that worked fine. Am
          I missing something obvious?
          thanks,
          /mark
        • Jeff 'japhy' Pinyan
          ... You re returning the array, but what are you returning it TO? ... I think so. My code stores the return value of the function: @output = function(); Does
          Message 4 of 12 , May 2, 2002
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            On May 2, markveerman said:

            >> You've shown the function, but not what you're doing with it.
            >
            >If I use the following within the sub routine:
            >
            ><snip>
            >my @fieldnames = $db->FieldNames();
            >foreach $fieldnames(@fieldnames){
            >print $fieldnames . "\n";
            ></snip>
            >
            >It works fine, so I know that $fieldnames is a working array, but
            >when I try to use return @fieldnames; and put the foreach loop
            >outside of the function I get nothing.

            You're returning the array, but what are you returning it TO?

            >Does it have something to do with the type of array or the format of
            >the data in the array? I tried your sample and that worked fine. Am
            >I missing something obvious?

            I think so. My code stores the return value of the function:

            @output = function();

            Does yours? You still haven't shown the ACTUAL invokation of the
            function.

            --
            Jeff "japhy" Pinyan japhy@... http://www.pobox.com/~japhy/
            RPI Acacia brother #734 http://www.perlmonks.org/ http://www.cpan.org/
            ** Look for "Regular Expressions in Perl" published by Manning, in 2002 **
            <stu> what does y/// stand for? <tenderpuss> why, yansliterate of course.
            [ I'm looking for programming work. If you like my work, let me know. ]
          • markveerman
            I figured it out! Although, I don t really understand the, but here s what it was: Below within the sub I originally had: my @fieldnames = $db- ...
            Message 5 of 12 , May 2, 2002
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              I figured it out! Although, I don't really understand the, but
              here's what it was:

              Below within the sub I originally had: my @fieldnames = $db-
              >FieldNames(); return @fieldnames; When I removed the word "my",
              @fieldnames was returned correctly. Also, I didn't realize you could
              call the function like @array = function() - so that was helpful too.

              thanks for your help japhy!
              /mark

              use Win32::ODBC;
              my $dsn = "mysql";
              my $table = "table";
              my $sql = "SELECT doc_name FROM " . $table . " WHERE status = 'Q'";

              odbc($dsn, $table, $sql);

              foreach $fieldnames(@fieldnames){
              print $fieldnames . "\n";
              }

              sub odbc {
              my ($dsn, $table, $sql) = @_;
              use Win32::ODBC;
              my $db = new Win32::ODBC($dsn);

              if (!$db){
              die "Error connecting: " . Win32::ODBC::Error() . "\n";
              }

              if ($db->Sql($sql)) {
              print "Error submitting SQL statement: " . $db->Error
              () . "\n" . $sql;
              }
              else{
              @fieldnames = $db->FieldNames();
              return @fieldnames;
              }

              }



              --- In perl-beginner@y..., Jeff 'japhy' Pinyan <japhy@p...> wrote:
              > On May 2, markveerman said:
              >
              > >> You've shown the function, but not what you're doing with it.
              > >
              > >If I use the following within the sub routine:
              > >
              > ><snip>
              > >my @fieldnames = $db->FieldNames();
              > >foreach $fieldnames(@fieldnames){
              > >print $fieldnames . "\n";
              > ></snip>
              > >
              > >It works fine, so I know that $fieldnames is a working array, but
              > >when I try to use return @fieldnames; and put the foreach loop
              > >outside of the function I get nothing.
              >
              > You're returning the array, but what are you returning it TO?
              >
              > >Does it have something to do with the type of array or the format
              of
              > >the data in the array? I tried your sample and that worked fine.
              Am
              > >I missing something obvious?
              >
              > I think so. My code stores the return value of the function:
              >
              > @output = function();
              >
              > Does yours? You still haven't shown the ACTUAL invokation of the
              > function.
              >
              > --
              > Jeff "japhy" Pinyan japhy@p...
              http://www.pobox.com/~japhy/
              > RPI Acacia brother #734 http://www.perlmonks.org/
              http://www.cpan.org/
              > ** Look for "Regular Expressions in Perl" published by Manning, in
              2002 **
              > <stu> what does y/// stand for? <tenderpuss> why, yansliterate of
              course.
              > [ I'm looking for programming work. If you like my work, let me
              know. ]
            • Jeff 'japhy' Pinyan
              ... No! You re not returning anything then! You re just setting a global array. The concept of returning something from a function is that you ACTUALLY send
              Message 6 of 12 , May 2, 2002
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                On May 2, markveerman said:

                >Below within the sub I originally had: my @fieldnames = $db-
                >>FieldNames(); return @fieldnames; When I removed the word "my",
                >@fieldnames was returned correctly. Also, I didn't realize you could
                >call the function like @array = function() - so that was helpful too.

                No! You're not returning anything then! You're just setting a global
                array. The concept of returning something from a function is that you
                ACTUALLY send content FROM the function BACK to the program. Also, your
                approach of removing 'my' makes your code possibly less safe, since you
                might end up ruining some OTHER @fieldnames array. Using my() helps keep
                your variables scoped properly.

                Compare:

                sub sum {
                $x = 0;
                for (@_) { $x += $_ }
                }

                sub set_sum {
                my $x = 0;
                for (@_) { $x += $_ }
                return $x;
                }

                sum(1,2,4,8);
                my $n = set_sum(1,3,5,7);

                The first function sets a global variable $x with the value 15; the second
                stores the value 16 in $n. With the first method, you have no control
                over WHERE the data goes -- the function does. With the second method,
                YOU have control where the data.

                > else{
                > @fieldnames = $db->FieldNames();
                > return @fieldnames;
                > }

                There's no point in returning @fieldnames. You're not returning it TO
                anything.

                --
                Jeff "japhy" Pinyan japhy@... http://www.pobox.com/~japhy/
                RPI Acacia brother #734 http://www.perlmonks.org/ http://www.cpan.org/
                ** Look for "Regular Expressions in Perl" published by Manning, in 2002 **
                <stu> what does y/// stand for? <tenderpuss> why, yansliterate of course.
                [ I'm looking for programming work. If you like my work, let me know. ]
              • markveerman
                ... global ... you ... your ... you ... helps keep ... Okay, I think I understand what you re saying, however, If I don t return @fieldnames, then I can t do
                Message 7 of 12 , May 2, 2002
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                  > No! You're not returning anything then! You're just setting a
                  global
                  > array. The concept of returning something from a function is that
                  you
                  > ACTUALLY send content FROM the function BACK to the program. Also,
                  your
                  > approach of removing 'my' makes your code possibly less safe, since
                  you
                  > might end up ruining some OTHER @fieldnames array. Using my()
                  helps keep
                  > your variables scoped properly.

                  Okay, I think I understand what you're saying, however, If I don't
                  return @fieldnames, then I can't do anything with them ... like print
                  them out. I'm going to be using this function to query a db and
                  return the results. I can't do anything with the results within the
                  function, because I want it to remain generic enough so I can use it
                  for all my db queries. So, in order for me to USE the info I get
                  from the queries, I've got to return the data BACK to the program.

                  > There's no point in returning @fieldnames. You're not returning it
                  TO anything.

                  I thought I was returning it to the program so I could then loop
                  through it and print out the field names. Bear with me, I'm really
                  trying to understand :-).
                  /mark

                  >
                  > --
                  > Jeff "japhy" Pinyan japhy@p...
                  http://www.pobox.com/~japhy/
                  > RPI Acacia brother #734 http://www.perlmonks.org/
                  http://www.cpan.org/
                  > ** Look for "Regular Expressions in Perl" published by Manning, in
                  2002 **
                  > <stu> what does y/// stand for? <tenderpuss> why, yansliterate of
                  course.
                  > [ I'm looking for programming work. If you like my work, let me
                  know. ]
                • markveerman
                  Thanks Charles - Are you telling me that even though it works it s not good code? :-). As if it s not totally obvious - I m a novice. /mark ... In defense of
                  Message 8 of 12 , May 2, 2002
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                    Thanks Charles - Are you telling me that even though it works it's
                    not good code? :-). As if it's not totally obvious - I'm a novice.
                    /mark

                    > So this is what Japhy meant. Shame on you Japhy, I
                    > can't believe you'd let Mark do it this way. :)

                    In defense of Japhy, today was the first day I've ever written a sub
                    routine, so the issues with scoping and local/global variables is
                    entirely new.

                    Thanks to you both for the assistance.
                    /mark
                  • Jeff 'japhy' Pinyan
                    ... Gaaah! No, Mark misunderstood my code, Charles. Mark, the return() function does not tell Perl that variables you used in your function can be used
                    Message 9 of 12 , May 2, 2002
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                      On May 2, Charles K. Clarkson said:

                      >"markveerman" <mark@...> wrote:
                      >
                      >: I figured it out! Although, I don't really understand the, but
                      >: here's what it was:
                      >:
                      >: Below within the sub I originally had: my @fieldnames = $db-
                      >: >FieldNames(); return @fieldnames; When I removed the word "my",
                      >: @fieldnames was returned correctly. Also, I didn't realize you could
                      >: call the function like @array = function() - so that was helpful too.
                      >:
                      >: thanks for your help japhy!
                      >
                      > So this is what Japhy meant. Shame on you Japhy, I
                      >can't believe you'd let Mark do it this way. :)

                      Gaaah! No, Mark misunderstood my code, Charles.

                      Mark, the return() function does not tell Perl that variables you used in
                      your function can be used OUTSIDE the function. It does nothing of the
                      sort. All it does is tell Perl that it can put the VALUES of those
                      variables in place you can get them from; specifically, from the return
                      value of the function:

                      sub cube {
                      my $n = shift;
                      return $n ** 3;
                      }

                      $big = cube(1234);

                      The function cube() RETURNS ONE VALUE. If you don't use the return value
                      somehow (like storing it in a variable, or printing it) then it
                      disappears.

                      Here are some examples to see if it becomes clear to you.

                      sub foobar {
                      my @bah = (1, 2, 3, 4);
                      return @bah;
                      }

                      my @bah = (5, 6);
                      foobar();
                      print "@bah\n"; # 5 6

                      ###########################

                      sub foobar {
                      my @bah = (1, 2, 3, 4);
                      }

                      my @bah = (5, 6);
                      foobar();
                      print "@bah\n"; # 5 6

                      ###########################

                      sub foobar {
                      my @bah = (1, 2, 3, 4);
                      return @bah;
                      }

                      my @bah = (5, 6);
                      @bah = foobar();
                      print "@bah\n"; # 1 2 3 4

                      ###########################

                      sub foobar {
                      my @bah = (1, 2, 3, 4);
                      }

                      my @bah = (5, 6);
                      @bah = foobar(); # foobar() returns the last thing evaluated, @bah
                      print "@bah\n"; # 1 2 3 4 (but only because you're lucky!)

                      ###########################

                      sub foobar {
                      @bah = (1, 2, 3, 4); # THIS SETS THE OUTSIDE @bah (bad idea)
                      return @bah;
                      }

                      my @bah = (5, 6);
                      foobar();
                      print "@bah\n"; # 1 2 3 4

                      ###########################

                      sub foobar {
                      my @bah = (1, 2, 3, 4); # DITTO
                      }

                      my @bah = (5, 6);
                      foobar();
                      print "@bah\n"; # 5 6

                      --
                      Jeff "japhy" Pinyan japhy@... http://www.pobox.com/~japhy/
                      RPI Acacia brother #734 http://www.perlmonks.org/ http://www.cpan.org/
                      ** Look for "Regular Expressions in Perl" published by Manning, in 2002 **
                      <stu> what does y/// stand for? <tenderpuss> why, yansliterate of course.
                      [ I'm looking for programming work. If you like my work, let me know. ]
                    • Franki
                      well you just taught me something as well.. I hadn t really given it much thought, but now that I have, its kinda obvious and I am ashamed of myself for not
                      Message 10 of 12 , May 3, 2002
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                        well you just taught me something as well..

                        I hadn't really given it much thought, but now that I have, its kinda
                        obvious and I am ashamed of myself for not realising it..

                        I had been wondering why not all of my "return" code works...

                        Thankyou..


                        regards

                        Frank

                        -----Original Message-----
                        From: Jeff 'japhy' Pinyan [mailto:japhy@...]
                        Sent: Friday, 3 May 2002 10:16 AM
                        To: perl-beginner@yahoogroups.com
                        Subject: Re: [PBML] returning arrays and hashes from a subroutine


                        On May 2, Charles K. Clarkson said:

                        >"markveerman" <mark@...> wrote:
                        >
                        >: I figured it out! Although, I don't really understand the, but
                        >: here's what it was:
                        >:
                        >: Below within the sub I originally had: my @fieldnames = $db-
                        >: >FieldNames(); return @fieldnames; When I removed the word "my",
                        >: @fieldnames was returned correctly. Also, I didn't realize you could
                        >: call the function like @array = function() - so that was helpful too.
                        >:
                        >: thanks for your help japhy!
                        >
                        > So this is what Japhy meant. Shame on you Japhy, I
                        >can't believe you'd let Mark do it this way. :)

                        Gaaah! No, Mark misunderstood my code, Charles.

                        Mark, the return() function does not tell Perl that variables you used in
                        your function can be used OUTSIDE the function. It does nothing of the
                        sort. All it does is tell Perl that it can put the VALUES of those
                        variables in place you can get them from; specifically, from the return
                        value of the function:

                        sub cube {
                        my $n = shift;
                        return $n ** 3;
                        }

                        $big = cube(1234);

                        The function cube() RETURNS ONE VALUE. If you don't use the return value
                        somehow (like storing it in a variable, or printing it) then it
                        disappears.

                        Here are some examples to see if it becomes clear to you.

                        sub foobar {
                        my @bah = (1, 2, 3, 4);
                        return @bah;
                        }

                        my @bah = (5, 6);
                        foobar();
                        print "@bah\n"; # 5 6

                        ###########################

                        sub foobar {
                        my @bah = (1, 2, 3, 4);
                        }

                        my @bah = (5, 6);
                        foobar();
                        print "@bah\n"; # 5 6

                        ###########################

                        sub foobar {
                        my @bah = (1, 2, 3, 4);
                        return @bah;
                        }

                        my @bah = (5, 6);
                        @bah = foobar();
                        print "@bah\n"; # 1 2 3 4

                        ###########################

                        sub foobar {
                        my @bah = (1, 2, 3, 4);
                        }

                        my @bah = (5, 6);
                        @bah = foobar(); # foobar() returns the last thing evaluated, @bah
                        print "@bah\n"; # 1 2 3 4 (but only because you're lucky!)

                        ###########################

                        sub foobar {
                        @bah = (1, 2, 3, 4); # THIS SETS THE OUTSIDE @bah (bad idea)
                        return @bah;
                        }

                        my @bah = (5, 6);
                        foobar();
                        print "@bah\n"; # 1 2 3 4

                        ###########################

                        sub foobar {
                        my @bah = (1, 2, 3, 4); # DITTO
                        }

                        my @bah = (5, 6);
                        foobar();
                        print "@bah\n"; # 5 6

                        --
                        Jeff "japhy" Pinyan japhy@... http://www.pobox.com/~japhy/
                        RPI Acacia brother #734 http://www.perlmonks.org/ http://www.cpan.org/
                        ** Look for "Regular Expressions in Perl" published by Manning, in 2002 **
                        <stu> what does y/// stand for? <tenderpuss> why, yansliterate of course.
                        [ I'm looking for programming work. If you like my work, let me know. ]



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