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newLISP Fan Club

Forum => newLISP in the real world => Topic started by: jopython on December 02, 2012, 06:18:05 PM

Title: formatting 'nil'
Post by: jopython on December 02, 2012, 06:18:05 PM
I want  the formatted output to look like  follows for a csv file
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apple,,,peach


But I get an error.
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> (setq fruits '("apple" nil nil "peach"))
("apple" nil nil "peach")
> (format "%s,%s,%s,%s" fruits)

ERR: data type and format don't match in function format : nil

How to tell format to deal with an empty data type like nil
Title: Re: formatting 'nil'
Post by: bairui on December 03, 2012, 12:16:04 AM
There might be a shorter way to express this, but my limited (new)lisp-fu offers you:


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(format "%s,%s,%s,%s" (map (fn (x) (or x "")) fruits))
Title: Re: formatting 'nil'
Post by: cormullion on December 03, 2012, 04:59:29 AM
That's good, bairui. Similar is:


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(format "%s,%s,%s,%s" (replace nil fruits ""))
Title: Re: formatting 'nil'
Post by: jopython on December 03, 2012, 05:05:42 AM
Thanks bairui and cormullion.
Title: Re: formatting 'nil'
Post by: m i c h a e l on December 03, 2012, 09:38:45 AM
cormullion's solution is considerably faster:


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> (time (replace nil fruits "") 1000000)
173.114
> (time (map (fn (x) (or x "")) fruits) 1000000)
3435.875
> _
>

m i c h a e l
Title: Re: formatting 'nil'
Post by: cormullion on December 03, 2012, 09:45:14 AM
Hey michael! Your second post this year - glad you still visit!



Yes, mapping anonymous functions is always going to be slower.
Title: Re: formatting 'nil'
Post by: m i c h a e l on December 03, 2012, 09:53:41 AM
cormullion,



Yes, I'm still here, lurking in the shadows. I don't do much programming lately, but I'm still an enthusiastic newlisper!
Title: Re: formatting 'nil'
Post by: bairui on December 03, 2012, 01:27:43 PM
W00t! Learned about replace() today. Yay. :-) Thanks, cormullion.



It was interesting to see Michael's speed test. I hadn't thought about how slow map() might be. And what a speed difference!



Another thing to note about the two different solutions, though, is that the map() is non-destructive, whereas replace() alters the original list.
Title: Re: formatting 'nil'
Post by: cormullion on December 04, 2012, 01:00:35 AM
since replace is so useful, you might sometimes want to use it non-destructively:


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(time (replace nil fruits "") 100000)
11.262

(time (replace nil (copy fruits) "") 100000)
63.958

(time (map (lambda (x) (or x "")) fruits) 100000)
167.338
Title: Re: formatting 'nil'
Post by: bairui on December 04, 2012, 04:41:39 AM
I should have thought about copy(). I have to do that all the time in Vim because its map() function is destructive. :-/



Thanks for the reminder, and speed test, Cormullion. Completed this thread nicely, I'd say. :-)
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