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Re: Homework question list

by cLive ;-) (Prior)
on May 21, 2005 at 08:38 UTC ( #459217=note: print w/replies, xml ) Need Help??


in reply to Homework question list

Slightly OT, but I've been interviewing long distance recently, and I used the following 3 questions tasks (ffs). Bear in mind these are for web application development, so front end is important too.

  1. create a calculator using HTML/Javascript, that processes the entered equation through a Perl CGI script and displays the result. This tests JS usage and, the biggie, whether they untaint user input in the CGI before 'eval'ing. Also, use of CGI/Mason/Template Toolkit is examined (and mentioned in the original question).
  2. Compare two arrays and show the elements that exist in both (I actually screwed up in my live question, but the answers were still informative). This is a great question because it shows how someone thinks. Lower level devs use nested loops (I did, when this was thrown at me a few years ago). Smart people use hash slices. Really smart people use 'undef'd hash slices to save memory (or, rather, time - ty runrig).
  3. Search google for a particular search term and present the first ten results in a web page. Another good one to see how someone thinks. LWP is quick and dirty, the Google SOAP Perl API is elegant, yet non-intuitive if you haven't heard of it.

Maybe we should start a quiz section where these sorts of questions can be dumped? I'm sure there are many more that push you to think laterally - or use aspects of Perl that involve some interesting thinking.

cLive ;-)

Replies are listed 'Best First'.
Re^2: Homework question list
by Aragorn (Curate) on May 22, 2005 at 15:06 UTC
    2. Compare two arrays and show the elements that exist in both (I actually screwed up in my live question, but the answers were still informative). This is a great question because it shows how someone thinks. Lower level devs use nested loops (I did, when this was thrown at me a few years ago). Smart people use hash slices. Really smart people use 'undef'd hash slices to save memory.
    Could you give an example of the "hash slice" method? Using hashes is a good way (something like my %temp; my @doubles = grep { ++$temp{$_} == 2 } @a1, @a2; works nicely), but I can't seem to figure out the "(undef'd) hash slice" method. Please enlighten me :-)

    Arjen

      Your example fails if @a1 or @a2 have a value more than once. For example, @a1 = (1, 2, 3, 2, 1) and @a2 = (4, 5, 6, 5, 4). Your grep would show 1, 2, 4, and 5 as elements existing in both arrays, when the right answer is obviously zero.

      I will also disagree with cLive ;-) that "really smart people" use the undef'd hash slices to save memory - if memory is a concern, sure. But in general, the difference is not going to be significant. Premature optimisation and all that. I've even found sometimes where using the standard "++$hash{$value}" turns out to be handy three months later when the number of times a value shows up becomes relevant. I didn't need to change nearly as much code because I wasn't "really smart" according to cLive's definition.

      Anyway, as always, TIMTOWTDI, so being able to use the undef'd hash slice is still a tool to keep handy:

      my %a1; undef @a1{@a1}; my @in_both = grep { exists $a1{$_} } @a2;
      Unfortunately, this has the side effect of showing duplicates if @a1 has a value, and @a2 has that value multiple times. Which, of course, may be what you want, but it's not explicit in the original requirement.
      my (%a1, %a2); undef @a1{@a1}; undef @a2{@a2}; my @in_both = grep { exists $a1{$_} } keys %a2;
      Oh, and I also recommend better variable names than what I'm using here. :-)

      Update: Added the italicised part in the last line.

        The example isn't a real good one, I know. I was just playing around with variations on a theme, not trying to get a bullet-proof solution. But still, good point about the example :-)

        cLive ;-)'s solution is a variation on a theme explained in the Cookbook. I was under the impression that there existed some really nifty hash slice trick to get the results that I didn't know about, hence the question.

        In my production code, I use good variable names. When experimenting, I'm not as picky :-) I probably should be when posting code here.

        Thanks, Arjen

      Interestingly, you made the same mistake an applicant made when doing the compare - as Tanktalus has pointed out :)

      By the way, Tanktalus, by "really smart people", I was referring to tilly here ;-)

      cLive ;-)

        It is more accurate to say that smart people know that they can use the undef trick - then don't. Because it is an unnecessary micro-optimization that could easily confuse others.
Re^2: Homework question list
by runrig (Abbot) on May 23, 2005 at 22:59 UTC
    Smart people use hash slices. Really smart people use 'undef'd hash slices to save memory.

    undef'd hash slices save time not memory.

    Update: Also, as pointed out to me in a previous post, only use this method if you don't care about the values of the hash, as the values do not necessarily get set to undef.

      I notice that in five years no one has filled in the details of the concise implementation.

      # given two arrays you want to compare for common elements, @a1 & @a2 +... my %common; undef @common{@a1}; delete @common{@a2}; my @common = keys %common; #for example: DB<1> @a1 = qw(a b c d e) DB<2> @a2 = qw(a c e ) DB<3> undef @common{@a1} DB<4> x \%common 0 HASH(0x8408418) 'a' => undef 'b' => undef 'c' => undef 'd' => undef 'e' => undef DB<5> delete @common{@a2} DB<6> x \%common 0 HASH(0x8408418) 'b' => undef 'd' => undef DB<7> print join " ", keys %common b d

      As Occam said: Entia non sunt multiplicanda praeter necessitatem.

Re^2: Homework question list
by Anonymous Monk on May 25, 2005 at 13:55 UTC
    Those aren't questions.

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