Question

Sometimes I need to loop through consecutive pairs in a list. The way I do it right now is

(loop for x on lst while (not (null (cdr x)))
       (do something on (car x) and (cadr x)))

I’m wondering if there is a better/built-in way to do this.

The reason I need this is sometimes I want, e.g. some function that add consecutive pairs

(1 2 3 4 5) ----> (3 5 7 9)

Is there any built-in function like reduce which allow me to get this?

Answer 1

AFAIK, there isn’t a built-in function to do what you want. You could try to put something together with maplist, but my first instinct would be to reach for loop too.

Just a couple of notes on what you’ve got there though. First, (not (null foo)) is equivalent to foo in CL, since a non-NIL value is treated as t by boolean operations. Second, loop can destructure its arguments, meaning you can write this more elegantly as

(loop for (a b) on lst while b
      collect (+ a b))

The maplist version would look something like

(maplist 
   (lambda (rest) 
     (when (cdr rest) 
        (+ (first rest) (second rest)))
   lst)

which I consider less readable (this would also return NIL as the last element of its result, rather than just ending before that).