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Defense of nested ifs

by drinkd (Pilgrim)
on Jan 04, 2002 at 21:27 UTC ( #136315=note: print w/ replies, xml ) Need Help??


in reply to Why I Hate Nested If-Else blocks

While nested if statements look a mess, the unfortunate truth is that this is really the way decisions are made by humans.

If my car starts, I'll go to work, If net nets up, I'll check email and read PM, If there's a good thread, I'll read it, etc. . . . ..

The problem with a one-dimensional dispatch table, is that the operations in the outermost nests need to be replicated into much of the table entries, which require programatically building complicated hashes, futhur sacraficing readability.

I'm not sure how to build a "multi-dimensional" dispatch table where say the "rows" have one set of functions and the "columns" have another, without programatically building the subroutines.

say $daysactions{$morning_situation}{$afternoon_situation}=\&listofstufftodotodaygivenmorningandafternoonsituations;

Any ideas?

drinkd


Comment on Defense of nested ifs
How to build a real multi-level dispatch table
by dragonchild (Archbishop) on Jan 04, 2002 at 21:40 UTC
    Using Ovid's example above ...
    sub whatFoo { if ($_[0] > 7) { return 'ONE' } else { return 'TWO' } } sub whatBarBaz { my ($bar, $baz) = @_; return 'BAR' if $bar; return 'BAZ' if $baz; return 'DEFAULT'; } sub no_op { } my %Dispatch = ( ONE => { BAR => \&firstBar, BAZ => \&firstBaz, }, TWO => { BAR => \&secondBar, BAZ => \&secondBaz, }, ); # Put the defaults in as NO-OPs, unless we already have a # default action foreach my $key (keys %Dispatch) { $Dispatch{$key}{DEFAULT} = \&no_op unless exists $Dispatch{$key}{DEFAULT}; } my $func = $Dispatch{whatFoo($foo)}{whatBarBaz($bar, $baz)}; $func->(@someArgs);
    Seems pretty clear-cut to me ... Plus, it's extensible. If you have a $quz you want to put in, add your QUZ entries to the dispatch and then change whatBarBaz() to allow for QUZ.

    ------
    We are the carpenters and bricklayers of the Information Age.

    Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.

      I've been following this thread with some interest, as I just finished writing a CGI with a very unwieldy set of if / elsif / else statements, and I'm interested in how to put it on a more rational footing (posted below for anyone who wants a laugh). In simpler situations I've used a dispatch table. But here I couldn't work out how to pass (the right) arguments. As far as I can see, this is not handled explicitly above. I'd be interested in how to deal with the need (A) to pass different arguments to different subs; and (B)to pass different args to the *same* sub under different conditions ?

      My baobab-style if tree, so we can all see what I'm up against:
      if ($g{'Action'} eq 'MainPage') { MainPage($Persist, $UserName); } if (($g{'Action'} eq 'Course') or ($g{'Action'} eq 'EditCourse')) +{ $g{'CourseID'} = GetNewCourse() unless defined $g{'CourseID'}; UpdateTempData($g{'CourseID'}); EditCourse($Persist, $g{'CourseID'}, $g{'Return'}); } elsif ($g{'Action'} eq 'Create Course') { my $CourseID = GetNewCourse($UserName); UpdateTempData($CourseID); EditCourse($Persist, $CourseID, $g{'Return'}); } elsif ($g{'Action'} eq 'EditEvent') { EditEvent($Persist, $g{'CourseID'}, $g{'Return'}); } elsif ($g{'Action'} eq 'New Venue') { my $VenueID = GetNewVenue($Persist,$UserName); EditVenue($Persist, $VenueID, $g{'Return'}); } elsif (($g{'Action'} eq 'Venue') or ($g{'Action'} eq 'EditVenue')) + { EditVenue($Persist, $g{'VenueID'}, $g{'Return'}); } elsif ($g{'Action'} eq 'Course Overview') { CourseView($Persist, $UserName); } elsif ($g{'Action'} eq 'Venue Overview') { VenueView($Persist,$UserName); } elsif ($g{'Action'} eq 'DeleteCourse') { DeleteCourse($Persist, $g{'CourseID'}, $g{'Return'}); } elsif ($g{'Action'} eq 'Delete Course') { DeleteCourseYes($Persist, $g{'CourseID'}, $g{'Return'}); if ($g{'Return'} eq 'CourseView') { CourseView($Persist, $UserName); } else { MainPage($Persist, $UserName); } } elsif ($g{'Action'} eq 'Keep Course') { if ($g{'Return'} eq 'CourseView') { CourseView($Persist, $UserName); } else { MainPage($Persist, $UserName); } } elsif ($g{'Action'} eq 'DeleteEvent') { DeleteEvent( $Persist, $g{'EventID'}, $g{'Return'}); } elsif ($g{'Action'} eq 'Delete Event') { DeleteEventYes( $Persist, $g{'EventID'}); if ($g{'Return'} eq 'CourseView') { CourseView($Persist, $UserName); } elsif ($g{'Return'} eq 'EditEvent') { EditEvent($Persist, $g{'CourseID'},0,$g{'CourseTitle'}); } else { MainPage($Persist, $UserName); } } elsif ($g{'Action'} eq 'Keep Event') { if ($g{'Return'} eq 'CourseView') { CourseView($Persist, $UserName); } elsif ($g{'Return'} eq 'EditEvent') { EditEvent($Persist, $g{'CourseID'},0,$g{'CourseTitle'}); } else { MainPage($Persist, $UserName); } } elsif ($g{'Action'} eq 'DeleteVenue') { DeleteVenue($Persist, $g{'VenueID'}, $g{'Return'}); } elsif ($g{'Action'} eq 'Delete Venue') { DeleteVenueYes($Persist, $g{'VenueID'}, $g{'Return'}); if ($g{'Return'} eq 'VenueView') { VenueView($Persist, $UserName); } else { MainPage($Persist, $UserName); } } elsif ($g{'Action'} eq 'Keep Venue') { if ($g{'Return'} eq 'VenueView') { VenueView($Persist, $UserName); } else { MainPage($Persist, $UserName); } } elsif ($g{'Action'} eq 'Save Venue') { if (my $Errors = SaveVenue($Persist, $UserName)) { EditVenue($Persist, $g{'VenueID'}, $g{'Return'}, $Errors); } else { WriteVenue(); if ($g{'Return'} eq 'ReturnToEditEvent') { SaveVenue($Persist, $UserName); EditEvent($Persist, $g{'CourseID'}, $g{'Return'},$g{'C +ourseTitle'},0); } elsif ($g{'Return'} eq 'VenueView') { SaveVenue($Persist, $UserName); VenueView($Persist, $UserName); } else { SaveVenue($Persist, $UserName); MainPage($Persist, $UserName); } } } elsif ($g{'Action'} eq 'More Keywords') { EditCourse($Persist, $g{'CourseID'}, $g{'Return'}); } elsif ($g{'Action'} eq 'Next Step') { if (my @Errors = CourseErrorCheck()) { EditCourse($Persist, $g{'CourseID'}, $g{'Return'}, \@Error +s); } elsif (my $OldTitle = TitleCheck()) { UpdateTempData($g{'CourseID'}); QueryTitle($Persist, $g{'CourseID'}, $g{'Return'}, $OldTit +le); } else { UpdateTempData($g{'CourseID'}); EditCourseInfo($Persist, $g{'CourseID'}, $g{'Return'}); } } elsif ($g{'Action'} eq 'New Course') { my $g = GetFromFile($g{'CourseID'},\%g); %g = %$g; DestroyTempFile($g{'CourseID'}); $g{'CourseID'} = GetNewCourse(); MakeTempFile($g{'CourseID'}); UpdateTempData($g{'CourseID'}); EditCourseInfo($Persist, $g{'CourseID'}, $g{'Return'}); } elsif ($g{'Action'} eq 'Change Name') { UpdateTempData($g{'CourseID'}); EditCourseInfo($Persist, $g{'CourseID'}, $g{'Return'}); } elsif ($g{'Action'} eq 'More Links') { UpdateTempData($g{'CourseID'}); EditCourseInfo($Persist, $g{'CourseID'}, $g{'Return'}); } elsif ($g{'Action'} eq 'Another Page') { SaveUserPage($Persist, $g{'CourseID'},$g{'PageTitle'},$g{'Page +Text'},$g{'WeFormat'},$g{'CurrentPage'}); $g{'PageTitle'} = undef; $g{'PageText'} = undef; EditCourseInfo($Persist, $g{'CourseID'}, $g{'Return'},0,1); } elsif ($g{'Action'} =~ /^Edit Page (\d*)$/) { SaveUserPage($Persist, $g{'CourseID'},$g{'PageTitle'},$g{'Page +Text'},$g{'WeFormat'},$g{'CurrentPage'}); EditCourseInfo($Persist, $g{'CourseID'}, $g{'Return'}, $g{"Pag +eID_$1"},1); } elsif ($g{'Action'} =~ /^Delete Page (\d*)$/) { UpdateTempData($g{'CourseID'}); SaveUserPage($Persist, $g{'CourseID'},$g{'PageTitle'},$g{'Page +Text'},$g{'WeFormat'},$g{'CurrentPage'}); QueryDeleteUserPage($Persist, $g{'CourseID'}, $g{'Return'},$g{ +"PageID_$1"},$g{'CurrentPage'}); } elsif ($g{'Action'} eq 'Yes, Delete Page') { DeleteUserPage($g{'PageID'}); UpdateTempData($g{'CourseID'}); EditCourseInfo($Persist, $g{'CourseID'}, $g{'Return'},$g{'Curr +entPage'},1); } elsif ($g{'Action'} eq 'No, Keep Page') { UpdateTempData($g{'CourseID'}); EditCourseInfo($Persist, $g{'CourseID'}, $g{'Return'},$g{'Curr +entPage'},1); } elsif ($g{'Action'} eq 'No, Keep Page') { UpdateTempData($g{'CourseID'}); EditCourseInfo($Persist, $g{'CourseID'}, $g{'Return'},$g{'Curr +entPage'},1); } elsif ($g{'Action'} eq 'Proceed') { SaveUserPage($Persist, $g{'CourseID'},$g{'PageTitle'},$g{'Page +Text'},$g{'WeFormat'},$g{'CurrentPage'}); UpdateTempData($g{'CourseID'}); if (my $TearSheetError = CourseInfoErrorCheck()) { EditCourseInfo($Persist, $g{'CourseID'}, $g{'Return'},0,0, +$TearSheetError); } else { EditEvent($Persist, $g{'CourseID'}, $g{'Return'},$g{'Cours +eTitle'},0); } } elsif ($g{'Action'} eq 'Save Course') { SaveUserPage($Persist, $g{'CourseID'},$g{'PageTitle'},$g{'Page +Text'},$g{'WeFormat'},$g{'CurrentPage'}); UpdateTempData($g{'CourseID'}); if (my $TearSheetError = CourseInfoErrorCheck()) { EditCourseInfo($Persist, $g{'CourseID'}, $g{'Return'},0,0, +$TearSheetError); } else { SaveCourse($Persist, $g{'CourseID'}, $g{'Return'}); if ($g{'Return'} eq 'CourseView') { CourseView($Persist, $UserName); } else { MainPage($Persist, $UserName); } } } elsif ($g{'Action'} eq 'Make Venue') { my $VenueID = GetNewVenue($Persist,$UserName); EditVenue($Persist, $VenueID,'ReturnToEditEvent',0,$g{'CourseI +D'},$g{'CourseTitle'}); } elsif ($g{'Action'} eq 'Add Event') { if (my $Errors = CheckEvent($g{'CourseID'})) { EditEvent($Persist, $g{'CourseID'}, $g{'Return'},$g{'Cours +eTitle'},$Errors); } else { AddEvent($Persist, $g{'CourseID'}, $g{'CourseTitle'}); EditEvent($Persist, $g{'CourseID'}, $g{'Return'},$g{'Cours +eTitle'},0); } } elsif ($g{'Action'} eq 'Save All') { my $Errors = CheckEvent($g{'CourseID'}); my $DateError = 0; my $VenueError = 0; my $DuplicateError = 0; if ($Errors) { $DateError = 1 if grep {$_ eq 'Date'} @$Errors; $VenueError = 1 if grep {$_ eq 'Venue'} @$Errors; $DuplicateError = 1 if grep {$_ eq 'Duplicate'} @$Errors; } if ($DateError + $VenueError == 1) { EditEvent($Persist, $g{'CourseID'}, $g{'Return'},$g{'Cours +eTitle'},$Errors); } else { AddEvent($Persist, $g{'CourseID'}, $g{'CourseTitle'}) unle +ss ($DuplicateError == 1 or $DateError + $VenueError == 2); if (stat "$filepath/tempfiles/$g{'CourseID'}") { UpdateTempData($g{'CourseID'}); SaveCourse($Persist, $g{'CourseID'}, $g{'Return'}); } if (defined $g{'Return'} and $g{'Return'} eq 'CourseView') + { CourseView($Persist, $UserName); } else { MainPage($Persist, $UserName); } } } else { MainPage($Persist, $UserName, $UserCookie); }


      George Sherston

        That is what closures are for:

        my %actions= ( MainPage => sub { MainPage( $Persis, $UserName ) }, Course => sub { EditCourse( $Persist, def(\$g{CourseID},sub{GetNewCourse()}), $g{Return} ) }, EditCourse => 'Course', 'Create Course' => sub { EditCourse( $Persist, GetNewCourse($UserName), $g{Return} ) }, EditEvent => sub { EditEvent( $Persist, $g{CourseID}, $g{Return} ) }, 'New Venue' => sub { EditVenue( $Persist, GetNewVenue($Persist,$UserName), $g{Return} ) }, Venue => sub { EditVenue( $Persist, $g{'VenueID'}, $g{'Return'} ) }, EditVenue => 'Venue', 'Delete Course' => \&DeleteCoursePage, 'Keep Course' => sub { if ($g{'Return'} eq 'CourseView') { CourseView($Persist, $UserName); } else { MainPage($Persist, $UserName); } }, # ... ); # ... my $action= $actions{ $g{Action} }; if( ! $action ) { MainPage( $Persist, $UserName, $UserCookie ); } else { $action= $actions{$action} if ! ref($action); $action->(); }
        where you add more subroutines as needed to keep the hash declaration from being too complex.

                - tye (but my friends call me "Tye")

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