Lisp loop do let when

Question

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?

See how to compute intermediate variables: lispcookbook.github.io/cl-cookbook/… — Ehvince
– Ehvince, Commented Jun 19, 2021 at 9:18

Rainer Joswig · Accepted Answer · 2021-06-19 06:27:18Z

4

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.

answered Jun 18, 2021 at 18:21

Rainer Joswig

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