SICP - Solution: Exercise 1.4

September 26, 2018

Exercise 1.4 #

Observe that our model of evaluation allows for combinations whose operators are compound expressions. Use this observation to describe the behavior of the following procedure:

(define (a-plus-abs-b a b)
  ((if (> b 0) + -) a b))

Solution #

If $b$ is strictly a positive number, the operator expression (if (> b 0) + -) will evaluate to the primitive procedure +, the expression then becomes (+ a b) and the result will be $a+b$.

In all other cases, when $b$ is a negative number, (if (> b 0) + -) will evaluate to the primitive procedure -, the expression then becomes (- a b) and the result will evaluate to $a-b$.

This function compute $a+\left|b\right|$.