in reply to Help needed to make a conditional statement

First, look at what you're doing here: 

my %lookup;
@lookup{@in} = ();
# For every $elem in @db ... 
foreach my $elem (@db) {
  # see if it exists in the lookup hash ...
  if (exists $lookup{$elem}) { 
    # and if it does then search @db (*again*) looking for
    # items that do not appear in the lookup hash. You're
    # also modifying the array you're iterating over (@db),
    # which can yield unpredictable results, so don't do it ;)
    @db = grep {not exists $lookup{$_}} @db;
  }
}

[download]

In the second case, you could utilize a grep very similar to the one above, and simplify the if/push syntax.

How about this as a solution: 

my (%db_lookup, %in_lookup);
@db_lookup{@db} = ();
@in_lookup{@in} = ();

if ($condition1) { # Dunno what your test conditions are, so...
  @db = grep { not exists $in_lookup{$_} } @db;
} elsif ($condition2) {
  push @db, grep { not exists $db_lookup{$_} } @in;
}

[download]

Hope that helps.

Replies are listed 'Best First'.
Re: Re: Help needed to make a conditional statement
by jonnyfolk (Vicar) on Dec 04, 2002 at 16:42 UTC
    After much fiddling around I have arrived at the following: 
    #!/usr/bin/perl -w
#use strict;
use CGI ':standard';

my @db = (1, 3, 5, 7, 9, 11);
my @in = (1, 2, 5, 8, 9, 10, 13);
my $var = 7; # I have this variable available to indicate the length o
+f the array
my (%db_lookup, %in_lookup);
@db_lookup{@db} = ();
@in_lookup{@in} = ();



$count = 0;
while ($count < $var) {
     if (exists $in_lookup{$count}, @db) {
      @db = grep { not exists $in_lookup{$_} } @db;
      $count += 1;
      } elsif (not exists $in_lookup{$count}, @db) {
      push @db, grep { not exists $db_lookup{$_} } @in;
      $count += 1;
}
}
      
      print "view: @db";

    [download]
    That does what I need when strings match. However when strings don't match the db is unchanged from the original, wheras I need the contents of @in pushed into @db: 
    
my @db = (1, 3, 5, 7, 9, 11);
my @in = (21, 22, 25, 28, 29, 210, 213);

    [download]
    Where am I going wrong?
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[d/l]
[select]

      A couple of problems here. First, $count will need to be specified in a more maintainable manner, such as using scalar on your arrays and finding the greater value. Second, you're looking at the value of $count in your evaluations, not the value of the index in your arrays that $count points to. Finally, by adding @db to your conditionals i.e.,if (exists $in_lookup{$count}, @db) you are ensuring that the test will return 'true' always unless the array is empty. So, the way you've got it now it executes the first conditional branch 7 times and then exits.

      I'm still struggling to see what exactly you're attempting -- if element A in array @in exists in array @db, remove A from @db. Else, if element A does not exist in array @db, append it to @db. Is this right?

[reply]
[d/l]
[select]
        Sorry - I see I wasn't very clear, although you're absolutely right in your interpretation. After operating on the first input @in the @db should contain 
        my @db = (1, 3, 5, 7, 9, 11);
my @in = (1, 2, 5, 8, 9, 10, 13);
# results in: my @db = (3,  7, 11);

        [download]
        operating on the second example should give: 
        my @db = (1, 3, 5, 7, 9, 11);
my @in = (21, 22, 25, 28, 29, 210, 213);
# results in: my @db = (1, 3, 5, 7, 9, 11, 21, 22, 25, 28, 29, 210, 21
+3);

        [download]
[reply]
[d/l]
[select]
      This is actually a reply to fever.

      The reference to $count is actually pure guesswork on my part. The problem is that I cannot find a way to say "if the input exists - take it out, if it doesn't - put it in". Obviously inclusion of the $count mechanism is more problematical than I'd anticipated but I can't find another approach which comes close to letting me say, "while values of @in match values of @db or the converse".

      That, I think is the crux of the problem!
[reply]

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