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xenchu (4746)

xenchu
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The vapid tale of a boring layabout. I mean I don't even drink. I like women, books and Perl (hmmm, yeah, that's about the right order).

Journal of xenchu (4746)

Friday January 23, 2004
11:07 AM

Problem Learning Perl

[ #16976 ]

One of my problems with learning Perl is -- I don't know enough Perl. A common but frustrating problem. Let me explain my learning situation.

Here is an example from one of my Perl books (not Camel or Llama, by the way):

@numerics = map {/^\d + /?$_:()} ;

Now, since the book tells me what this construct is doing (it calculates lines starting with numbers, and then builds a list of the lines input), most of operation is clear enough. Map reads a line from input, checks to find out if the line starts with a digit and then does -- what? I see $_ which is the line input but what does the ? mean? Is it part of the regex or does it operate on $_? What is the purpose of :()? Is :() one operation or two? And why '+ ' instead of \b?

I checked the index and found nothing I thought applied. I doubt that means the answer is not in the book, just that if I saw the answer I didn't recognize it. And if I have come across what I want to know in earlier reading I have either forgotten it or I wasn't paying sufficient attention. Either option has a 50% chance of being true.

Nothing as concise as map is present in any other language I program in. The ease with which it can be used is fascinating. I know I will find out what I want to know eventually. I am sure all of you who read this see my problem (he's an idiot) and understand what I am trying to say. The more you have of a thing the easier it is to get more. It works for Perl as with everything else.

The solution is obvious; keep studying Perl. Most things that frustrate me seem easy when I look back on them knowing the answer. This will probably not be an exception. And as they say, 'a good learning experience'. As I gain knowledge of Perl the pieces will start to come together.

Hello to chaoticset who has signed on as a Fan. Astounding, just astounding. And hello to merlyn and Louis_Wu as well. I am certain that all three know exactly what the example above means. Inspires me to keep trying to catch up with them to know others have learned before me.

And a public thanks to Corion who answered a Perl Monk question I asked in an earlier journal entry.


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  • @numerics = map {/^\d + /?$_:()} ;

    The ? and : are working together here.

    For teh full details look up the perlvar man page and search for "Conditional Operator" and then Ternary operator "?:"

    The expression /^\d + / ? $_ : ()

    is equivalent to:

    if (/^\d + /){
          $_
    } else {
          ()
    }

    Translated to english:

    "If $_ starts with a digit give me $_ otherwise give me the empty list ()"

    • Thank you. Another thing I am not used to yet is that Perl allows operations to be jammed together. I would have expected /^d + / ? $_ : (); In the languages I am familiar with that spacing makes a difference. It would be an error to write like that!

  • What you have is an emulated grep. I wonder why the author didn't just use that. Your snippet is equivalent to
    @numerics = grep { /^\d + / } LIST;

    • Actually he did list the grep you give as another way it would work along with a 'while' example. The reason I listed the map example is I didn't understand how it worked. I was displaying my ignorance, not the author's.

  • Your main question has already been answered, but you also asked why the + was there and why it wasn't \b.

    The + is actually irrelevant - it could be removed. /^\d+/ looks at the beginning of the matched text (^) for one digit (\d) or more digits (+). If the plus were changed to \b, it would require exactly one digit and disallow the line if that digit was immediately followed by an alphanumeric. If the + were removed, it would look for one digit at the start of a line (and if a line starts with one or more

    • Another visual disfunction. Usually I see that written as d+ and not 'd +'. In the languages I usually program you have to be exact. Somehow I equated that with \b. No good reason why.

      I am a little puzzled by your comment on <N>. I don't recall using the term here. Was it in another journal entry? Other special characters appear in Preview as they are. But looking at it in Preview I see the angle characters vanish. Now I see. Thanks for the tip.

      • Oops, I didn't see the space in your message. That means the pattern is requiring exactly one digit, followed by one or more spaces. (The + is still pointless in this case.) If you use the x modifier on a regular expression, the space would be for formatting purposes and not matching, so /^\d+/ and /^ \d +/x are the same, but when the x is not there, spaces and other whitespace characters match themselves.

        When it is <SPACE>+, it is still not the same as \b. The \b pattern matches a "word boundary",

    • Just wrap your code examples with <ecode></ecode> and all the escaping is taken care of :)
  • That threw me too, don't feel bad. I had never seen it in C before I started Perling.
    --

    ------------------------------
    You are what you think.
  • It just gets worse, and worse, and then more worse. :)
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