Perl

fusion1275 · 07-03-2011, 12:16 PM

Hi all,

I am a total newbie with Perl but I am trying very hard to get things done. Anyway I have this small piece of code from a script I created but feel I am doing it the "hard way" and wondered if anyone could maybe simplify it. It would certainly help me by example.

Here is the code:

Code:

        if ($#ARGV == -1) {
        usage();
        }else{
        if ($ARGV[0] !~ /\.txt$/) {
        usage();
         }
        }

I tried with || and && in the if statements to join them (so to speak) but I was getting all manner of compilation errors. So managed to get the above working and have just left it. But now would like to see other ways of how to code this.

Your help is greatly appreciated.

Rgds

theNbomr · 07-03-2011, 12:31 PM

It is always a bit of a risk commenting on matters of style, especially with Perl, but...
Other than your indenting style, which may have been a victim of copy/paste, my main point would be on the use of '$#ARGV'. While it is technically fine, it is somewhat less conventional than the simpler '@ARGV'. When an array is used in scalar context, it evaluates to the length of the array. Since you evidently want to test whether the user gave any commandline arguments:

Code:

    if( ! @ARGV ){
        usage();
    }
    elsif( $ARGV[0] !~ m/\.txt$/ ){
        usage();
    }

It is a matter of personal preference, but I prefer not to combine various tests when parsing the commandline. In my experience, the commandline parser is one place most likely to be modified over time, and keeping all of the various tests separate seems to make that part cleaner. Other will have different opinions, no doubt.

--- rod.

fusion1275 · 07-03-2011, 12:34 PM

Rod,

Thank you very much for your help and comments. It has helped me a lot.

It's nice to know that I wasn't too far away from it being correct. Progress at last

Again thanks.

markush · 07-03-2011, 12:39 PM

Hello,

I would use

Code:

if (shift !~ m/\.txt$/) {
       usage();
}

which works but with a warning

The reason is that if no arguments are there, they cannot match *.txt and therefore I do the test in one step.

Markus

anomie · 07-03-2011, 12:40 PM

I'm a total Perl novice (so this sort of thread is fun for me), but I will say: those tests seem excessively redundant to me, especially since you're calling usage() in both cases.

If scalar $ARGV[0] does not match the pattern you specified, then testing @ARGV for undef seems completely superfluous. (i.e. The pattern test is the one that really counts. Who cares if @ARGV is undef?)

-------

edit: I was one minute too slow, but my comments are intended to be in the same vein as markush's.

theNbomr · 07-03-2011, 12:43 PM

Quote:

Code:

if (shift !~ m/\.txt$/) {
       usage();
}

Yabbut...
If there was an argument passed on the commandline, and you want to make use of it, you've just blown it away.
--- rod.

fusion1275 · 07-03-2011, 12:56 PM

Hi all, thanks for your comments.

Unfortunately I do need both tests in place for the rest of the code but do see what you meant with your example markush.

Cheers.

markush · 07-03-2011, 12:59 PM

Well, the Perl people say TMTOWTDI (there's more than one way to do it)....

I did not understand that you need the arguments even if they don't match *.txt

Markus

anomie · 07-03-2011, 02:40 PM

Quote:

Originally Posted by theNbomr

Yabbut...
If there was an argument passed on the commandline, and you want to make use of it, you've just blown it away.
--- rod.

Good eye. Could be corrected by using shift to assign it to a variable (while doing the test).

Does usage() also die? If so, I still don't see the point of both tests. I may be missing some important context.

markush · 07-03-2011, 02:49 PM

Quote:

Originally Posted by anomie

Good eye. Could be corrected by using shift to assign it to a variable (while doing the test)...

I've checked this, but one gets a warning about "use of uninitialized value..." if @ARGV is empty.

Quote:

...Does usage() also die? If so, I still don't see the point of both tests. I may be missing some important context.

this was what I thought, most programs die with a "usage" message if one executes them with wrong arguments. But it is possible to let the program run and print the usage-message anyway, which the OP wants to achieve.

Markus