LISP variable exchange

Question

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

Selwin

Answer 1

(rotatef a b)

Tantalum

Answer 2

Also:

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

Shirring

That does not change the variables' values, it only creates new lexical bindings. – Nonunionism

@dmitry-vk: That's often all you need. It depends on what you're doing, of course. – Shirring

Answer 3

Another alternative, "parallel setf":

(psetf a b b a)

Lepido

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. – Barroom

Answer 4

Yet another approach:

(setf (values a b) (values b a))

Sequestrate

Answer 5

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.

Midpoint

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