CODEWITHSUNDEEP
common-lisp
Sati
Sati

Reputation: 726

LISP make list items positive

How do we loop through a list to get the absolute value of each list item using lambda or any other looping mechanism? 

(defun span (start end &key (step 1))
   (loop for n from start to end by step
      collect n))

(setf bce #'(lambda (x) (abs x)) (span -10 -1))

The result can be used, for example, as the BCE timeline.

Upvotes: 0

Views: 234

Answers (1)

Renzo
Renzo

Reputation: 27424

You can use mapcar (reference):

CL-USER> (mapcar #'(lambda (x) (abs x)) (span -10 -1))
(10 9 8 7 6 5 4 3 2 1)
   ;; can be written also as:
CL-USER> (mapcar (lambda (x) (abs x)) (span -10 -1))
(10 9 8 7 6 5 4 3 2 1)
    ;; better yet:
CL-USER> (mapcar #'abs (span -10 -1))
(10 9 8 7 6 5 4 3 2 1)

As loop:

CL-USER> (loop for x in (span -10 -1) collect (abs x))
(10 9 8 7 6 5 4 3 2 1)

Combining this in a single function:

CL-USER> (defun span (start end &key (step 1) (key #'identity))
           (loop for n from start to end by step
                 collect (funcall key n)))
SPAN
CL-USER> (span -10 -1)
(-10 -9 -8 -7 -6 -5 -4 -3 -2 -1)
CL-USER> (span -10 -1 :key #'abs)
(10 9 8 7 6 5 4 3 2 1)

Upvotes: 5

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