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Unformatted text preview: Teach Yourself Scheme in Fixnum Days [Go to first , previous , next page; contents ; index ] Appendix C Numerical techniques Recursion (including iteration) combines well with Scheme's mathematical primitive procedures to implement various numerical techniques. As an example, let's implement Simpson's rule, a procedure for finding an approximation for a definite integral. C.1 Simpson's rule The definite integral of a function f ( x ) within an interval of integration [ a , b ] can be viewed as the area under the curve representing f ( x ) from the lower limit x = a to the upper limit x = b . In other words, we consider the graph of the curve for f ( x ) on the x , y-plane, and find the area enclosed between that curve, the x-axis, and the ordinates of f ( x ) at x = a and x = b . [t-y-scheme-Z-G-1.gif] According to Simpson's rule, we divide the interval of integration [ a , b ] into n evenly spaced intervals, where n is even. (The larger n is, the better the approximation.) The interval boundaries constitute n + 1 points on the x-axis, viz, x , x 1 , ... , x i , x i +1 , ... , x n , where x = a and x n = b . The length of each interval is h = ( b- a )/ n , so each x i = a + ih . We then calculate the ordinates of f ( x ) at the interval boundaries. There are n + 1 such ordinates, viz, y , ... , y i , ... , y n , where y i = f ( x i ) = f ( a + ih ). Simpson's rule approximates the definite integral of f ( x ) between a and b with the value 12 : [t-y-scheme-Z-G-2.gif] We define the procedure integrate-simpson to take four arguments: the integrand f ; the x-values at the limits a and b ; and the number of intervals n . (define integrate-simpson (lambda (f a b n) ;... file:///C|/Documents%20and%20Settings/Linda%20Grauer...tes/Teach%20Yourself%20Scheme/t-y-scheme-Z-H-22.html (1 of 9) [2/6/2008 11:43:05 AM] Teach Yourself Scheme in Fixnum Days The first thing we do in integrate-simpson 's body is ensure that n is even -- if it isn't, we simply bump its value by 1. ;... (unless (even? n) (set! n (+ n 1))) ;... Next, we put in the local variable h the length of the interval. We introduce two more local variables h*2 and n/2 to store the values of twice h and half n respectively, as we expect to use these values often in the ensuing calculations. ;... (let* ((h (/ (- b a) n)) (h*2 (* h 2)) (n/2 (/ n 2)) ;... We note that the sums y 1 + y 3 + + y n- 1 and y 2 + y 4 + + y n- 2 both involve adding every other ordinate. So let's define a local procedure sum-every-other-ordinate-starting-from that captures this common iteration. By abstracting this iteration into a procedure, we avoid having to repeat the iteration textually. This not only reduces clutter, but reduces the chance of error, since we have only one textual occurrence of the iteration to debug....
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