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Giving nil a value

PostPosted: Tue Nov 07, 2017 1:02 am
by tomtoo
Hi guys,

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

Code: Select all
(set 'a '(1 2 nil 4))


I can do
Code: Select all
(setf (a 2) "none")


but
Code: Select all
(dolist (i a)(when (null? i)(setf i "none")))


returns nil. and
Code: Select all
(null? (a 2))

returns true.

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

Re: Giving nil a value

PostPosted: Tue Nov 07, 2017 12:42 pm
by fdb
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.
Code: Select all
(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:
Code: Select all
(set 'a '(1 2 nil 4))

(replace nil a "none")

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