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



Dear Monks,

What is the best way to to combine variables to create one virtual variable? For example with the following piece of code I don't want to have to repeat this statement 4 times because the amout of Perl code generated becomes difficult to manage. If possible I want the code to make it obvious that we are dealing with a fusion of two variable names. So, for example,  $sth_Count_Ref_vs_Comp.$letter_tag Then I just need to change the letter tag within a loop. 
      
      
      my $sth_Count_Ref_vs_Comp = $dbh_B->prepare($sql_count_A_Ref_vs_
+Comp) or die "Cannot prepare: " . $dbh_B->errstr();
      $sth_Count_Ref_vs_Comp->execute() or die "$sth_Count_Ref_vs_Comp
+->errstr\n";    
      
      my $sth_Count_Ref_vs_Comp_B = $dbh_B->prepare($sql_count_A_Ref_v
+s_Comp_B) or die "Cannot prepare: " . $dbh_B->errstr();
      $sth_Count_Ref_vs_Comp_B->execute() or die "$sth_Count_Ref_vs_Co
+mp_B->errstr\n";    
      
      my $sth_Count_Ref_vs_Comp_C = $dbh_B->prepare($sql_count_A_Ref_v
+s_Comp_C) or die "Cannot prepare: " . $dbh_B->errstr();
      $sth_Count_Ref_vs_Comp_C->execute() or die "$sth_Count_Ref_vs_Co
+mp_C->errstr\n";    
      
      my $sth_Count_Ref_vs_Comp_D = $dbh_B->prepare($sql_count_A_Ref_v
+s_Comp_D) or die "Cannot prepare: " . $dbh_B->errstr();
      $sth_Count_Ref_vs_Comp_D->execute() or die "$sth_Count_Ref_vs_Co
+mp_D->errstr\n";

[download]

Replies are listed 'Best First'.
Re: Combining variables to create a single virtual variable
by davido (Cardinal) on Sep 21, 2012 at 08:18 UTC

    Identifier names like $sql_count_A_Ref_vs_Comp_B might be indicative of a deeper design problem. But a solution to the immediate problem presented could be:

    foreach my $sql ( 
    $sql_count_A_Ref_vs_Comp, 
    $sql_count_A_Ref_vs_Comp_B, 
    $sql_count_A_ref_vs_Comp_C, 
    $sql_count_A_Ref_vs_Comp_D 
) {
    my $sth = $dbh_B->prepare( $sql ) or die "Cannot prepare: " . $dbh
+_B->errstr();
    $sth->execute() or die $sth->errstr;
}

    [download]

    This lets $sql stand in as an alias for each of those sql_count_A_blah_blah_blah variables. Then you just loop over them, repeating the same process for each.


    Dave

[reply]
[d/l]
[select]
      Would it then be possible to incorporate this as well? It might be better if I kept the code all strung out and just tidied up the formating :< 
      
      my @row_A_count_A_Ref_vs_Comp;
      my @fields_A_count_A_Ref_vs_Comp;
      my @record_A_count_A_Ref_vs_Comp;
      

      print BILATERAL_COMPARISONS_A   "$sql_count_A_Ref_vs_Comp";


      while(my @row_A_count_A_Ref_vs_Comp = $sth_Count_Ref_vs_Comp->fe
+tchrow_array()) { 
          
               
          
          print BILATERAL_COMPARISONS_A   "\t\t\t\t";
                    
          print BILATERAL_COMPARISONS_A  "$entry_no_new_MINUS_ONE\t"; 
+    
          
          local $\ = "\n"; 
          local $, = "\t"; 
          
          print BILATERAL_COMPARISONS_A @row_A_count_A_Ref_vs_Comp;   
+  
          
          my @record_A_count_A_Ref_vs_Comp = @row_A_count_A_Ref_vs_Com
+p;          
          push(my @fields_A_count_A_Ref_vs_Comp, @record_A_count_A_Ref
+_vs_Comp);  
          
      }
      $sth_Count_Ref_vs_Comp->finish();

      [download]
[reply]
[d/l]
Re: Combining variables to create a single virtual variable
by Athanasius (Archbishop) on Sep 21, 2012 at 08:23 UTC

    I don’t know what you mean by a “virtual variable”, but why not just use hashes:

    my %sql_count_A_Ref_vs_Comp;
my %sth_Count_Ref_vs_Comp;

for (qw( A B C D ))
{
    $sth_Count_Ref_vs_Comp{$_} = $dbh_B->prepare($sql_count_A_Ref_vs_C
+omp{$_}) or die "Cannot prepare: " . $dbh_B->errstr();
    $sth_Count_Ref_vs_Comp{$_}->execute() or die "$sth_Count_Ref_vs_Co
+mp{$_}->errstr\n";
}

    [download]

    Adjust as needed to accommodate $letter_tag as the hash key.

    Hope that helps,

    Athanasius <°(((>< contra mundum

[reply]
[d/l]
Re: Combining variables to create a single virtual variable
by locked_user sundialsvc4 (Abbot) on Sep 21, 2012 at 12:59 UTC

    Another perhaps even-more-obvious way to present it would be to make one or more correspondings lists of letter-tag names, then iterate through that list making interpolations to build up the SQL string.   (Variable-names only, of course ... from a known fixed list, of course ...)   If there is a natural symmetry that you want to emphasize to the next reader of the source code, which of course is a very desirable thing to do, then I think that this would be a “clean and obvious” way to do that.

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