CODEWITHSUNDEEP
lispcommon-lisp
Garrett Badeau
Garrett Badeau

Reputation: 348

Lisp loop do let when

I'm trying to do something like this

(loop for i from 1 to 10
      do (let ((fi (funcall f i)))
           when (> fi 0)
           collect (list i fi)))

f is expensive to compute, so I want to only calculate it once.

This isn't working, so I introduce my fi local variable outside the loop scope:

(let ((fi nil))
  (loop for i from 1 to 10
        do (setf fi (funcall f i))
        when (> fi 0)
        collect (list i fi)))

Is there a way to do this (like the first way) that is completely contained within the loop macro?

Upvotes: 1

Views: 300

Answers (1)

Rainer Joswig
Rainer Joswig

Reputation: 139261

Two independent for clauses will do:

(loop for i from 1 to 10
      for fi = (funcall f i)
      when (> fi 0)
      collect (list i fi))

Both iteration clauses will be executed at every iteration. The second one will see each new value of i.

Upvotes: 4

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