in reply to Optional Arguments..?

The array, @ARGV contains the various arguments passed on the command line. To access the first element by index, it would be $ARGV[0]. You're using that first element to handle the command line switch, L, U or D. The second arg is $ARGV[1], and that's where you're passing the key. The third (optional) parameter is $ARGV[2]. That element will either have a value, or it will be undefined. To detect which, you might do something like this: 

if( defined $ARGV[2] ) {
    # Handle the search string.
}
else {
    # Perform whatever default preparations are necessary in
    # the absence of a defined search string.
}

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So the key is to test $ARGV[2] for definedness. There are a couple of useful ways to do that. The most common by far is the defined built-in function. But there's also the // and //= operators, described in perlop. With //=, you could do something like this: 

$ARGV[2] //= 'Default value';

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In this case, if $ARGV[2] already contains a value, the line has no effect.  If it doesn't contain a value, '<c>Default value' is assigned.

Using the // operator might look like this: 

my $pattern = $ARGV[2] // 'Default value';

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Here, $pattern receives the contents of $ARGV[2] if $ARGV[2] is defined. If it's undefined, then 'Default value' is assigned to $pattern.

The // and //= operators were introduced to Perl in version 5.10.0 (I think). So they won't work if you're stuck using older versions. defined works all the way back to the earliest Perl 5 versions, and possibly more.

Welcome to Perl! :)


Dave

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Re^2: Optional Arguments..?
by Watergun (Novice) on Jun 03, 2012 at 03:52 UTC

    Hi Dave!

    Firstly I wanna say I almost burst into happiness when you explained this.. "defined" to me. It's the one thing that my lecturer told us to google for, to implement in our codes but I just couldn't find a simple explanation to use this. Thanks a lot. Right now after using defined, my code looks like this 

    #!/usr/bin/perl -w
use strict;
use warnings;
use ST2614 1.0;

#subroutine guide provides user a simple documentation on how to use t
+he password manager
sub guide {
    my $pwdmgr = shift;
    print "usage: $pwdmgr L key [site_pattern]\n";
    print "-"x5, " takes the encoding key and optional site_pattern to
+ retrieve 
    all saved entries that match.\n If no site_pattern, retrieve all s
+aved entries\n\n";
    print "usage: $pwdmgr U key site_name login_id password [URL]\n";
    print "-"x5, " Using the encoding key, add new or update existing 
+entry.\n Complete set of 
    entry info required: Site name, login id, password, URL(Optional)\
+n\n";
    print "usage: $pwdmgr D key site_name\n";
    print "-"x5, " Takes in encoding key and site_name to delete entry
+; will only work if 
    encoding key matches and entry found matching with site_name.\n";
    exit;
}

#an if-elsif loop to check which option user has selected
if ($ARGV[0] eq "L") { 
    sub listFunct ($;$) {
        open(FILEHANDLE, "passmgr.dat") or die ("The file cannot be op
+ened!");
            my $decode = ST2614::decode(FILEHANDLE, $ARGV[1]); 
                if ( defined $ARGV[2] ) {
                    for $decode (keys =~ m/$ARGV[2]/) {
                        print "$decode: @{ m/$ARGV[2]/ {$decode} }\n";
+ 
                        }
                    } else { 
                    for $decode () {
                        print 
                        }
                    
                }
            
    }
    

} elsif ($ARGV[0] eq "U") { 

    #declare the individual variables that are to be stored
    my $sitename = $ARGV[2]; #sitename will be the key in the hash of 
+array
    my $loginid = $ARGV[3];
    my $password = $ARGV[4];
    my $url = $ARGV[5];
    
    #declare a hash of array to prepare storage of all variables to be
+ encoded together
    my HoA {$sitename} = ["$loginid", "$password", "$url"];;
    open(FILEHANDLE, ">>passmgr.dat") or die("The file cannot be opene
+d!");
        my $encode = ST2614::encode(HoA, $ARGV[1]);
        print FILEHANDLE "$encode";
        close FILEHANDLE;
    
    print "Data successfully encoded. Please remember your key.\n Your
+ key is $ARGV[1].";
    
} elsif ($ARGV[0] eq "D") { 

    

} else {
    guide($0);
    }

    [download]

    What I want to achieve is to be able to print all password and related data by searching for the sitename, so for example searching for 'a' might give me yahoo and facebook, and list the passwords accordingly. Is my following code correct?

    Also, I would like to be able to print all hashes of arrays in my decoded file, how do I do that?

    With thanks Junming
[reply]
[d/l]

      It's the one thing that my lecturer told us to google for, to implement in our codes but I just couldn't find a simple explanation to use this.

      Google is great, but I'm surprised your professor didn't mention the Perl documentation. It's included free of charge with every Perl distribution, and also at http://perldoc.perl.org. You can read about defined on your own terminal by typing "perldoc -f defined".

      Is my following code correct?

      No.

      Well... your use of defined is reasonable, but besides that, NO. It doesn't compile.

      Why would you post code that doesn't compile except to possibly ask why it doesn't? You can check yourself, at which point you can begin working through the error messages. For example, "Not enough arguments for keys at mytest.pl line 27, near "keys =~"" Your line numbers will probably be different, but that error message will still be there, unless the code you posted isn't what's in your editor.

      There's no need for me to enumerate the error messages you're getting; Perl will do it for you when you type perl -c mytest.pl, where "mytest.pl" is the name of your script. Once you resolve the issues that perl -c tells you about, you'll run into other problems as well, such as your strange for loop around line 31. It seems to set a topical variable, but the list of items it iterates over is empty, and the print statement on the next line tries to print $_, which isn't going to contain anything meaningful.

      Really, there are enough problems you probably should sit down with your instructor for a half hour and go over them together.

      If you don't have it already, get yourself a copy of Learning Perl, and start reading. It will begin to flow before you know it. And remember this slightly gentler version of a famous quote within the Perl community: "You can't just make stuff up and expect the computer to know what you mean." Programming rewards attention to detail.


      Dave

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[d/l]
[select]
        Thanks for the reply again. My lecturer doesn't really helps us with assignments, we are expected to complete it without his help. How do I print the hashes of arrays that I encoded, with or without the specified search string (site name)? I don't know how to tell the computer what I want to convey, I need a little help here :(
[reply]

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