Training, Open Source computer languages
PerlPHPPythonMySQLApache / TomcatTclRubyJavaC and C++LinuxCSS 
Search for:
Home Accessibility Courses Diary The Mouth Forum Resources Site Map About Us Contact
This week, we're updating our course layouts and descriptions. Presentation and materials always gently change over time, but just occasionally there's a need to make a step change to clear out some of the old and roll in the new. That's now happening - but over a long and complex site it's not instant and you'll see sections of the site changing up to and including 19th September.

See also [here] for status update
Passing several lists to a sub

Posted by enquirer (enquirer), 17 March 2005
How do I pass in several lists to a sub in Perl and avoid the contents of them all getting lumped together?

Posted by admin (Graham Ellis), 17 March 2005
In Perl,  you call subs with a list as their parameter - so if you simply provide a number of comma separated lists in the call then they'll all get combined into a single list ... not what you want much of the time.

The solution is to use the \ operator to pass the ADDRESS of each list into the sub; it makes the internal code in the sub slightly more complex, but it cures the problem.


#%% Passing several lists into a sub

# Passing several lists to a Perl sub

@breakfast = qw(sausage bacon eggs);
@lunch = qw(kurma, nan, rice, dhansak);
@teatime = qw(water rest);

# WRONG - The lists get combined into one list
# Then there's no way of keeping them apart within

printmen1 (@breakfast,@lunch,@teatime,("pizza","McDonalds"));

sub printmen1 {
       print "FIRST call - one long list\n";
       @igot = @_;
       print $#igot," @igot\n";

# RIGHT - Passing references to lists
# Then they're separated internally

printmen2 (\@breakfast,\@lunch,\@teatime,["pizza","McDonalds"]);

sub printmen2 {
       print "SECOND call - a list of lists\n";
       @igot = @_;
       for ($k=0; $k<@igot; $k++) {
               @current = @{$igot[$k]};
               print "$k:... @current\n";


[localhost:~/marpl] graham% perl 2lp
FIRST call - one long list
10 sausage bacon eggs kurma, nan, rice, dhansak water rest pizza McDonalds
SECOND call - a list of lists
0:... sausage bacon eggs
1:... kurma, nan, rice, dhansak
2:... water rest
3:... pizza McDonalds
[localhost:~/marpl] graham%

Note too how I've used square rather than round brackets to pass in the address of anonymous list(s) ...

This page is a thread posted to the opentalk forum at and archived here for reference. To jump to the archive index please follow this link.

You can Add a comment or ranking to this page

© WELL HOUSE CONSULTANTS LTD., 2014: Well House Manor • 48 Spa Road • Melksham, Wiltshire • United Kingdom • SN12 7NY
PH: 01144 1225 708225 • FAX: 01144 1225 899360 • EMAIL: • WEB: • SKYPE: wellho