Week11-Accumulators

Week11 - Context-Preserving, Result-so-far and Worklist Accumulators.
Optimizing Recursion into Tail-Recursion using accumulators.

Author: divx89

Week11-Accumulators/01-Context_Preserving_Accumulators/Practice01_String-concat-with-position.rkt

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+;; The first three lines of this file were inserted by DrRacket. They record metadata
+;; about the language level of this file in a form that our tools can easily process.
+#reader(lib "htdp-intermediate-lambda-reader.ss" "lang")((modname Practice01_String-concat-with-position) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f () #f)))
+;; You are asked to design a function that numbers a list of strings by inserting a number
+;; and a colon before each element of the list based on its position.
+
+
+
+;; (listof String) -> (listof String)
+;; append each string's position in the list to the front of the string to number the list
+
+(check-expect (number-list empty) empty)
+(check-expect (number-list (list "first" "second" "third"))
+              (list "1: first" "2: second" "3: third"))
+
+;; (define (number-list los) los)   ;stub
+
+(define (number-list lon0)
+  ;acc: 1-based position of the (first lon) of lon0;
+  ;(number-list (list "first" "second" "third") 1)
+  ;(number-list (list         "second" "third") 2)
+  ;(number-list (list                  "third") 3)
+  (local [(define (number-list lon acc)
+            (cond [(empty? lon) empty]
+                  [else
+                   (cons (string-append (number->string acc) ": " (first lon))
+                         (number-list (rest lon) (add1 acc)))]))]
+    (number-list lon0 1)))