solution to 2.34, 2.35, 2.36 and 2.37

diff --git a/src/sicp/ex2_34.rkt b/src/sicp/ex2_34.rkt
new file mode 100644 (file)
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--- /dev/null
+++ b/src/sicp/ex2_34.rkt
@@ -0,0 +1,10 @@
+#lang racket
+
+(require "utils.rkt")
+
+(define (horner-eval x coefficient-sequence)
+  (accumulate (lambda (this-coeff higher-terms) (+ (* x this-coeff) higher-terms))
+              0
+              coefficient-sequence))
+
+(horner-eval 2 '(1 3 0 5 0 1))
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diff --git a/src/sicp/ex2_35.rkt b/src/sicp/ex2_35.rkt
new file mode 100644 (file)
index 0000000..23e7e09
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+++ b/src/sicp/ex2_35.rkt
@@ -0,0 +1,8 @@
+#lang racket
+
+(require "utils.rkt")
+
+(define (count-leaves t)
+  (accumulate +
+              0
+              (map (lambda (x) (if (pair? x) (count-leaves x) 1)) t)))
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diff --git a/src/sicp/ex2_36.rkt b/src/sicp/ex2_36.rkt
new file mode 100644 (file)
index 0000000..658b860
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+++ b/src/sicp/ex2_36.rkt
@@ -0,0 +1,13 @@
+#lang racket
+
+(require "utils.rkt")
+
+(define (accumulate-n op init seqs)
+  (if (null? (car seqs))
+      '()
+      (cons (accumulate op init (map car seqs))
+            (accumulate-n op init (map cdr seqs)))))
+
+(provide accumulate-n)
+
+;; (accumulate-n + 0 '((1 2 3) (4 5 6) (7 8 9) (10 11 12)))
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diff --git a/src/sicp/ex2_37.rkt b/src/sicp/ex2_37.rkt
new file mode 100644 (file)
index 0000000..841e6c2
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+++ b/src/sicp/ex2_37.rkt
@@ -0,0 +1,24 @@
+#lang racket
+
+(require "utils.rkt"
+         "ex2_36.rkt")
+
+;; dot product
+(define (dot-product v w)
+  (accumulate + 0 (map * v w)))
+
+(define (matrix-*-vector m v)
+  (map (lambda (r) (dot-product r v)) m))
+
+;; (matrix-*-vector '((1 2 3 4) (4 5 6 6) (6 7 8 9)) '(1 2 3 4))
+
+(define (transpose m)
+  (accumulate-n cons '() m))
+
+;; (transpose '((1 2 3) (4 5 6) (7 8 9)))
+
+(define (matrix-*-matrix m n)
+  (let ([cols (transpose n)])
+    (map (lambda (v) (matrix-*-vector cols v)) m))) 
+
+;; (matrix-*-matrix '((0 -1 2) (4 11 2)) '((3 -1) (1 2) (6 1)))
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