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# Concat function

Posted by annitaz
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 Concat function May 08, 2010 09:12AM Registered: 9 years ago Posts: 17 Rating: 0
Chapter 3: How does this function work? How do I know which word to concatenate with w?
 Re: Concat function May 08, 2010 10:08AM Registered: 10 years ago Posts: 3,496 Rating: 1
Take two concrete strings: "anni" and "taz".

Feed them to CONCAT: CONCAT ("anni", "taz".

Concat maps this pair of points to the string "annitaz".

(Let's just say for arguments sake that I've got it back to front, though. ie. The mapping is actually to "tazanni". Even in this case, if you misused it consistently I'm pretty sure the examiners would be reasonable and agree to notice that you'd redefined the function to suit yourself - as long as you misused it consistently).

OK. Now we have a recursive definition, and lets say the generators are ab and ba.

Then further words could be made up using the function CONCAT. So you could have a word that's in your L like say "ababababab", but to generalise, let's call w any such word.

CONCAT(w,ba) could build you a new word. 'w'ab

If your string had been the one above, the new word would be "abababababba".

You could also make CONCAT(ba,w) a way to generate new words. It all depends on how you want your words to look. Similarly (if necessary), CONCAT(w, some-other-concrete-generator) could be brought in (or vice versa.)

This isn't really any clearer than the answers we got back from the assignments, so it's probably not helping?
 Re: Concat function May 08, 2010 10:08AM Registered: 10 years ago Posts: 3,496 Rating: 1
Oh hell! I got attacked by an evil smiley there!
 Re: Concat function May 08, 2010 10:54AM Registered: 9 years ago Posts: 17 Rating: 0
it is clearer now thank u
 Re: Concat function May 08, 2010 10:59AM Registered: 10 years ago Posts: 3,496 Rating: 1
No problem.

Looks like we'll also be asked questions on the last few chapters prescribed. (If one looks at the old exam paper Despatch hasn't quite gotten around to sending to me this year).

Using the Pumping Lemma with length, and showing languages are equivalent are two things I remember skrikking a bit at when I tried them. Thought I had it sorted, but oh no I didn't.
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