formatting 'nil'

[jopython]

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

[bairui]

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))

[cormullion]

That's good, bairui. Similar is:

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(format "%s,%s,%s,%s" (replace nil fruits ""))

[jopython]

Thanks bairui and cormullion.

[m i c h a e l]

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

[cormullion]

Hey michael! Your second post this year - glad you still visit!



Yes, mapping anonymous functions is always going to be slower.

[m i c h a e l]

cormullion,



Yes, I'm still here, lurking in the shadows. I don't do much programming lately, but I'm still an enthusiastic newlisper!

[bairui]

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.

[cormullion]

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

[bairui]

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. :-)