SICP Exercise 3.33

Question

Using primitive multiplier, adder, and constant constraints, define a procedure averager that takes three connectors a, b, and c as inputs and establishes the constraint that the value of c is the average of the values of a and b.

Answer

Here’s the constraint network for this scenario:

And the equations: $$a=2c-b$$ $$b=2c-a$$ $$c=\frac{a+b}{2}$$

And finally the code:

copy(define (averager a b c)
  (let ((u (make-connector))
        (v (make-connector)))
    (adder a b u)
    (constant 2 v)
    (multiplier c v u)
    'ok))