Print Page - Giving nil a value

Title: Giving nil a value
Post by: tomtoo on November 06, 2017, 05:02:24 PM

Hi guys,

I would like to replace any and all nils in a list with "none" . If I can see the nil

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(set 'a '(1 2 nil 4))

I can do

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(setf (a 2) "none")

but

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(dolist (i a)(when (null? i)(setf i "none")))

returns nil. and

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(null? (a 2))

returns true.

How might I fix this? I don't know where a nil might pop up.

thank you.

Title: Re: Giving nil a value
Post by: fdb on November 07, 2017, 04:42:36 AM

Hi tomtoo,

you'll have to refer to an index in the list to set the correct value in the list, i is just a temporary loop variable, see below.

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(set 'a '(1 2 nil 4))

(dolist (i a)(when (null? i)(setf (a $idx) "none")))

This works but doesn't return the changed list. An easier way is to use the replace function:

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(set 'a '(1 2 nil 4))

(replace nil a "none")

> 
(1 2 nil 4)
(1 2 "none" 4)

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