7

How can I exchange two variables in LISP without using a third variable?

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(rotatef a b)
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1

Also:

(let ((a b) (b a)) ...)

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That does not change the variables' values, it only creates new lexical bindings.
@dmitry-vk: That's often all you need. It depends on what you're doing, of course.
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Another alternative, "parallel setf":

(psetf a b b a)

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A difference between rotatef and psetf is that if a and b are really more complicated forms, then psetf will evaluate any subforms twice, but rotatef will evaluate them only once, which is what one would normally want.
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Yet another approach:

(setf (values a b) (values b a))
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-3

Rather gruesome method and it works only for numerical values, but it's more general and not syntax dependent:

a = a^b

b = a^b

a = a^b

Assuming that a and b were assigned before, the ^ means logical exclusive alternative.

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Fascinating, the two binary numbers really pass through each other without additional space requirements.

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