CL-USER 2 > (defvar a (make-array 10 :initial-element 1))
A
CL-USER 3 > (defun increment-every-third-elt (arr)
(loop for elt across arr
for i upfrom 1 do
(when (= 0 (mod i 3))
(incf (aref arr (1- i))))))
INCREMENT-EVERY-THIRD-ELT
CL-USER 4 > (increment-every-third-elt a)
NIL
CL-USER 5 > a
#(1 1 2 1 1 2 1 1 2 1)
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