CODEWITHSUNDEEP
lisp
Ayman
Ayman

Reputation: 1009

How can I add a value stored in a variable to a list?

For example:

(setf s 2)
    s => 2

(setf list1 '(1 s 3 4))
    list1 => (1 s 3 4)

How do i get it to add the value stored in s to the list? For this example I would want to use s to generate a list (1 2 3 4) I have a lisp book I'm reading and I can't seem to find any mention of how to do this so I thought i'd ask. Thanks

Upvotes: 1

Views: 70

Answers (2)

Rainer Joswig
Rainer Joswig

Reputation: 139411

I would want to use s to generate a list (1 2 3 4)

The function list can be handy:

CL-USER 14 > (let ((s '2))
               (list 1 s 3 4))
(1 2 3 4)

The function LIST creates a fresh new list from its arguments.

Upvotes: 1

Sylwester
Sylwester

Reputation: 48775

So quoted data in Scheme are like String constants.. If I wrote "1 s 3 4" in Java I wouldn't be able to get s replaced with the variable contents. I 'd have to write "1 " + s + " 3 4". In Lisp we have backquote to do this in list structures:

`(1 ,s 3 4)
; ==> (1 2 3 4)

Note that this is a trick.. It's like "1 $s 3 4" in PHP as it's representing code that creates the list with the unquoted variables evaluated and return a new list structure. Under the hood it's very similar to writing:

(list 1 s 3 4)
; ==> (1 2 3 4)

And of course list is not a primitive since it just uses cons. What it does is this:

(cons 1 (cons s (cons 3 (cons 4 '()))))
; ==> (1 2 3 4)

Upvotes: 1

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