confused by match semantics and $0 var

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confused by match semantics and $0 var



Postby bairui » Mon Jul 23, 2012 9:10 am

I've either found a bug in NL (10.4.3) or, more likely, a bug in my understanding of how its match semantics and the $0 variable work. In particular, I am having trouble with an example on the NL wikibooks site:



http://en.wikibooks.org/wiki/Introducti">http://en.wikibooks.org/wiki/Introducti ... et-ref-all



Given a sample list:



Code: Select all

    (set 'planets '(("Mercury" (p-name "Mercury") (diameter 0.382) (moons 0))("Venus" (p-name "Venus")(diameter 0.949)(moons 10))("Earth" (p-name "Earth")(diameter 1)(moons 1))))







The example on the wiki page is:



Code: Select all

    (set-ref-all '(moons ?) planets (if (> (last $0) 9) "lots" (last $0)) match)







But that doesn't seem to work for me. This seems to instead:



Code: Select all

    (set-ref-all '(moons ?) planets (if (> (last $it) 9) '(moons "lots") $it) match)







I am confused with what is supposed to be stored in the $0 variable. It seems that the [*+?] capture is being ignored and $0 is being set to 1 regardless of the actual value in the list.



Code: Select all

    (set-ref-all '(moons ?) planets $0 match)







Results in:



Code: Select all

    (("Mercury" (p-name "Mercury") (diameter 0.382) (moons 1)) ("Venus" (p-name "Venus") (diameter 0.949) (moons 1)) ("Earth" (p-name "Earth") (diameter 1) (moons 1)))







Where the (moons 1) sublists boggle me. What have I missed here?



bairui

     

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Re: confused by match semantics and $0 var



Postby cormullion » Mon Jul 23, 2012 10:53 am

I think the wikibook is wrong. In newLISP version 9, I think the code worked as written, but I can't remember when $it was introduced. However, there are still some lingering errors in the wikibook, and you've found one of them. Sorry - but congratulations anyway... :)



To preserve the spirit of the original, you could probably use:



Code: Select all

    (set-ref-all '(moons ?) planets (if (> (last $it) 9) '(moons "lots")  $it) match)







But perhaps the whole section should be rewritten.



cormullion

     

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Re: confused by match semantics and $0 var



Postby bairui » Mon Jul 23, 2012 12:34 pm

Thanks, cormullion,



I gather you did most of the NL wikibook work. Awesome job, mate. I suspected the few glitches I've found along the way were due to API/lib changes as NL marched along. That's fair. It would be good to keep the wikibook resource as up to date there as we can. I patched it in a few small places today, but as I'm new to NL and lisp in general, I am hesitant to make bold changes.



I am still confused about the differences between $0 and $it. It seems that $it is working the way $0 did for you in version 9 (I came to NL only after it was well into version 10). $0 on the other hand seems... broken to my mind. I am happy to assume the breakage there is all mine though, so I'd like an explanation of how it works. (Lutz?)



bairui

     

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Re: confused by match semantics and $0 var



Postby Lutz » Mon Jul 23, 2012 2:26 pm

In set-ref-all the $0 variable is only set when regular expressions are involved. But $it is always set. The following example works with both $0 and $it



Code: Select all

    > (set-ref-all ".*m.*" '("abmcd" "defg" "xymzw") (upper-case $0) regex)

    ("ABMCD" "defg" "XYMZW")



    > (set-ref-all ".*m.*" '("abmcd" "defg" "xymzw") (upper-case $it) regex)

    ("ABMCD" "defg" "XYMZW")

    >







All strings containing an "m" are upper-cased.



See also here for an explanation of anaphoric $it and other system variables:



http://www.newlisp.org/downloads/newlis">http://www.newlisp.org/downloads/newlis ... em_symbols



Lutz

     

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Re: confused by match semantics and $0 var



Postby bairui » Mon Jul 23, 2012 2:39 pm

Thanks, Lutz.



Interestingly, $0 and $it behave differently when the regex matches less than the element:



Code: Select all

    > (set-ref-all "m" '("abmcd" "defg" "xymzw") (upper-case $0) regex)

    ; ("M" "defg" "M")

    > (set-ref-all "m" '("abmcd" "defg" "xymzw") (upper-case $it) regex)

    ; ("ABMCD" "defg" "XYMZW")



bairui

     

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Re: confused by match semantics and $0 var



Postby Lutz » Mon Jul 23, 2012 3:01 pm

Yes, because $0 referes to the matched sub-expression part, and $it to the entire element.

[joejoe]

I think the two broken links in the post are meant to be these:



https://en.wikibooks.org/wiki/Introduction_to_newLISP/Lists#Find_and_replace_all_matching_elements:_set-ref-all">https://en.wikibooks.org/wiki/Introduct ... et-ref-all">https://en.wikibooks.org/wiki/Introduction_to_newLISP/Lists#Find_and_replace_all_matching_elements:_set-ref-all



http://www.newlisp.org/downloads/newlisp_manual.html#system_symbols">http://www.newlisp.org/downloads/newlis ... em_symbols">http://www.newlisp.org/downloads/newlisp_manual.html#system_symbols