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WammKD's solution

to Difference Of Squares in the Scheme Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Find the difference between the square of the sum and the sum of the squares of the first N natural numbers.

The square of the sum of the first ten natural numbers is (1 + 2 + ... + 10)² = 55² = 3025.

The sum of the squares of the first ten natural numbers is 1² + 2² + ... + 10² = 385.

Hence the difference between the square of the sum of the first ten natural numbers and the sum of the squares of the first ten natural numbers is 3025 - 385 = 2640.

Source

Problem 6 at Project Euler http://projecteuler.net/problem=6

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

difference-of-squares-test.scm

;; Load SRFI-64 lightweight testing specification
(use-modules (srfi srfi-64))

;; Suppress log file output. To write logs, comment out the following line:
(module-define! (resolve-module '(srfi srfi-64)) 'test-log-to-file #f)

;; Require module
(add-to-load-path (dirname (current-filename)))
(use-modules (squares))

(test-begin "difference-of-squares")

(test-eqv "square-of-sums-to-5"
          225
          (square-of-sums 5))
(test-eqv "sum-of-squares-to-5"
          55
          (sum-of-squares 5))
(test-eqv "difference of-sums-to-5"
          170
          (difference 5))

(test-eqv "square-of-sums-to-10"
          3025
          (square-of-sums 10))
(test-eqv "sum-of-squares-to-10"
          385
          (sum-of-squares 10))
(test-eqv "difference of-sums-to-10"
          2640
          (difference 10))

(test-eqv "square-of-sums-to-100"
          25502500
          (square-of-sums 100))
(test-eqv "sum-of-squares-to-100"
          338350
          (sum-of-squares 100))
(test-eqv "difference of-sums-to-100"
          25164150
          (difference 100))



(test-end "difference-of-squares")
(define-module (squares)
  #:export (sum-of-squares
            square-of-sums
            difference)
  #:autoload (srfi srfi-1) (fold iota))

(define (square-of-sums n)
  (expt (fold + 0 (iota n 1)) 2))

(define (sum-of-squares n)
  (fold (lambda (x y)
          (+ (expt x 2) y)) 0 (iota n 1)))

(define (difference n)
  (- (square-of-sums n) (sum-of-squares n)))

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