Exercise 1.20

src/sicp/chapter_1/ex_1_20.clj

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+(ns sicp.chapter-1.ex-1-20)
+
+; Exercise 1.20
+; The process that a procedure generates is of course dependent on the rules used by the interpreter.
+; As an example, consider the iterative gcd procedure given above.
+; Suppose we were to interpret this procedure using normal-order evaluation, as discussed in 1.1.5.
+; (The normal-order-evaluation rule for if is described in Exercise 1.5.)
+; Using the substitution method (for normal order), illustrate the process generated in evaluating
+; (gcd 206 40) and indicate the remainder operations that are actually performed.
+;
+; How many remainder operations are actually performed in the normal-order evaluation of (gcd 206 40)?
+; In the applicative-order evaluation?
+
+(defn gcd [a b]
+  (if (= b 0)
+    a
+    (gcd b (mod a b))))

test/sicp/chapter_1/ex_1_20_test.clj

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+(ns sicp.chapter-1.ex-1-20-test
+  (:require [clojure.test :refer :all])
+  (:require [sicp.chapter-1.ex-1-20 :refer [gcd]]))
+
+(deftest gcd-test
+  (is (= 2 (gcd 206 40)))                                   ; 4 times
+)