Cumulative standard deviation: Difference between revisions

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 =={{header|Scheme}}==
-{{works with|Racket}}
-{{incorrect|Scheme|running-stddev procedure only in Racket? Usage example missing, and not evident.}}
 <lang scheme>
-(define ((running-stddev . nums) num)
+(define (standart-deviation-generator)
-  (set! nums (cons num nums))
+  (let ((nums '()))
+    (lambda (x)
-  (sqrt (- (/ (apply + (map (lambda (i) (* i i)) nums)) (length nums)) (expt (/ (apply + nums) (length nums)) 2))))
+      (set! nums (cons x nums))
+      (let* ((mean (/ (apply + nums) (length nums)))
+      (mean-sqr (lambda (y) (expt (- y mean) 2)))
+      (variance (/ (apply + (map mean-sqr nums)) (length nums))))
+    (sqrt variance)))))
+(let loop ((f (standart-deviation-generator))
+           (input '(2 4 4 4 5 5 7 9)))
+  (if (not (null? input))
+    (begin
+      (display (f (car input)))
+      (newline)
+      (loop f (cdr input)))))
 </lang>