47 lines
1.2 KiB
Racket
47 lines
1.2 KiB
Racket
#lang racket
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;; for n=100
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;; iterative version takes roughly 0.008 milliseconds.
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;; recursive version does not seem like it will finish any time soon.
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;; for n=50
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;; iterative version takes roughly 0.003 milliseconds in drracket
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;; recursive version has not yet finished executing after a couple minutes, and I got bored.
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;; for n=25.
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;; iterative version takes 0.002ms
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;; recursive version takes 46.5 ms
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;; yeah. that's big. okay. this shit's important.
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;; I wonder how it would be with memoization.
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(define-syntax-rule (meas form)
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(let ([my-time (real-time)])
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(let ([res form])
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(- (real-time) my-time))))
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(define (recursive n)
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(if (< n 3)
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n
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(+ (recursive (- n 1))
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(* 2 (recursive (- n 2)))
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(* 3 (recursive (- n 3))))))
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(define (iter a b c n)
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(if (<= n 0)
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c
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(iter b c (+ c (* 2 b) (* 3 a)) (- n 1))))
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(define (iter-start n)
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(iter 0 1 2 (- n 2)))
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;; note, as the book says,
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;; this generates an *iterative process*, despite being
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;; fully recursive
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(define (ex-1-11 n)
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(let loop ((i n))
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(if (< i 2)
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#t
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(if (= (recursive i) (iter-start i))
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(loop (- i 1))
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(begin
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(display "UNEQUAL!")
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(displayln i))))))
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