my attempt to do the exercises in sicp.

Sunday, June 29, 2008

sicp exercise 1.26

;  Exercise 1.26. Louis Reasoner is having great difficulty doing exercise 1.24. His fast-prime? test
;  seems to run more slowly than his prime? test. Louis calls his friend Eva Lu Ator over to help.
;  When they examine Louis's code, they find that he has rewritten the expmod procedure to use an
;  explicit multiplication, rather than calling square:
;     (define (expmod base exp m)
;        (cond ((= exp 0) 1)
;              ((even? exp)
;                  (remainder (* (expmod base (/ exp 2) m)
;                                (expmod base (/ exp 2) m))
;                    m))
;              (else
;                  (remainder (* base (expmod base (- exp 1) m))
;                    m))))
;  ``I don't see what difference that could make,'' says Louis. ``I do.'' says Eva. ``By writing the
;  procedure like that, you have transformed the O(log n) process into a O(n) process.'' Explain.

;  Answer: if the procedure square is used, then the expression (expmod base (/ exp 2) m) needs
;  to be calculated once instead of twice when * is used.

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