2 days ago

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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