June 2018 Update

Task/Knapsack-problem-0-1/Racket/knapsack-problem-0-1-3.rkt

@@ -0,0 +1,33 @@
+#lang racket
+
+(define items '((map 9 150) (compass 13 35) (water 153 200) (sandwich 50 160)
+      (glucose 15 60) (tin 68 45)(banana 27 60) (apple 39 40)
+      (cheese 23 30) (beer 52 10) (cream 11 70) (camera 32 30)
+      (T-shirt 24 15) (trousers 48 10) (umbrella 73 40)
+      (trousers 42 70) (overclothes 43 75) (notecase 22 80)
+      (glasses 7 20) (towel 18 12) (socks 4 50) (book 30 10)))
+
+(define max-weight 400)
+
+(define (item-value item)
+  (caddr item))
+
+(define (item-weight item)
+  (cadr item))
+
+(define (pack-weight pack)
+  (apply + (map item-weight pack)))
+
+(define (pack-value pack)
+  (apply + (map item-value pack)))
+
+(define (max-pack-value pack-with pack-without max-weight)
+  (if (and
+       (not (> (pack-weight pack-with) max-weight))
+       (> (pack-value pack-with) (pack-value pack-without)))
+      pack-with pack-without))
+
+(define (display-solution pack)
+    (displayln (list 'weight: (pack-weight pack)
+                     'value:  (pack-value pack)
+                     'items:  (map car pack))))

Task/Knapsack-problem-0-1/Racket/knapsack-problem-0-1-4.rkt

@@ -0,0 +1,17 @@
+(define (show-brute)
+
+  (define empty-accumulator '())
+
+  (define (knapsack-brute included items)
+    (cond
+      ((null? items) included)
+      (else
+       (max-pack-value
+        (knapsack-brute (cons (car items) included) (cdr items))
+        (knapsack-brute included (cdr items))
+        max-weight
+        ))))
+
+  (display-solution (reverse (knapsack-brute empty-accumulator items))))
+
+(show-brute); takes around five seconds on my machine

Task/Knapsack-problem-0-1/Racket/knapsack-problem-0-1-5.rkt

@@ -0,0 +1,24 @@
+(define (show-memoized)
+
+  (define (memoize func)
+    (let ([result-ht (make-hash)])
+      (lambda args ; this is the rest-id pattern
+        (when (not (hash-has-key? result-ht args))
+          (hash-set! result-ht args (apply func args)))
+        (hash-ref result-ht args))))
+
+  (define knapsack
+    (memoize
+     (lambda (max-weight items)
+       (cond
+         ((null? items) '())
+         (else
+          (let ([item (car items)] [items (cdr items)])
+            (max-pack-value
+             (cons item (knapsack (- max-weight (item-weight item)) items))
+             (knapsack max-weight items)
+             max-weight)))))))
+
+  (display-solution (knapsack max-weight items)))
+
+(show-memoized)

Task/Knapsack-problem-0-1/Racket/knapsack-problem-0-1-6.rkt

@@ -0,0 +1 @@
+(weight: 396 value: 1030 items: (map compass water sandwich glucose banana cream trousers overclothes notecase glasses socks))