🎉 Exercism Research is now launched. Help Exercism, help science and have some fun at research.exercism.io 🎉
Avatar of marchukov

marchukov's solution

to Difference Of Squares in the Scheme Track

Published at Jan 27 2020 · 0 comments
Test suite


This exercise has changed since this solution 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.

You are not expected to discover an efficient solution to this yourself from first principles; research is allowed, indeed, encouraged. Finding the best algorithm for the problem is a key skill in software engineering.

Running and testing your solutions

From the command line

Simply type make chez if you're using ChezScheme or make guile if you're using GNU Guile. Sometimes the name for the scheme binary on your system will differ from the defaults. When this is the case, you'll need to tell make by running make chez chez=your-chez-binary or make guile guile=your-guile-binary.

From a REPL

  • Enter (load "test.scm") at the repl prompt.
  • Develop your solution in difference-of-squares.scm reloading as you go.
  • Run (test) to check your solution.

Failed Test Cases

If some of the test cases fail, you should see the failing input and the expected output. The failing input is presented as a list because the tests call your solution by (apply difference-of-squares input-list). To learn more about apply see The Scheme Programming Language -- Chapter 5


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.


(import (rnrs))

(define test-fields '(input output who))

(define (test-run-solution solution input)
  (if (procedure? solution) (apply solution input) solution))

(define (test-success description success-predicate
         procedure input output)
    (lambda (k)
        (lambda (e)
          (k `(fail
                (description . ,description)
                (input . ,input)
                (output . ,output)
                (who . ,procedure))))
        (lambda ()
          (let ([result (test-run-solution procedure input)])
            (unless (success-predicate result output)
              (error 'exercism-test
                "test fails"
          `(pass . ,description))))))

(define (test-error description procedure input)
    (lambda (k)
        (lambda (e) (k `(pass . ,description)))
        (lambda ()
          (test-run-solution procedure input)
             (description . ,description)
             (input . ,input)
             (output . error)
             (who . ,procedure)))))))

(define (run-test-suite tests . query)
    (lambda (field)
      (unless (and (symbol? field) (memq field test-fields))
        (error 'run-test-suite
          (format #t "~a not in ~a" field test-fields))))
  (let-values ([(passes failures)
                  (lambda (result) (eq? 'pass (car result)))
                  (map (lambda (test) (test)) tests))])
      [(null? failures) (format #t "~%Well done!~%~%")]
         "~%Passed ~a/~a tests.~%~%The following test cases failed:~%~%"
         (length passes)
         (length tests))
         (lambda (failure)
             "* ~a~%"
               [(assoc 'description (cdr failure)) => cdr]
               [else (cdr failure)]))
             (lambda (field)
               (let ([info (assoc field (cdr failure))])
                 (display "  - ")
                 (write (car info))
                 (display ": ")
                 (write (cdr info))
       (error 'test "incorrect solution")])))

(define sum-of-squares)

(define difference-of-squares)

(define square-of-sum)

(define test-cases
    (lambda ()
      (test-success "square of sum 1" = square-of-sum '(1) 1))
    (lambda ()
      (test-success "square of sum 5" = square-of-sum '(5) 225))
    (lambda ()
      (test-success "square of sum 100" = square-of-sum '(100)
    (lambda ()
      (test-success "sum of squares 1" = sum-of-squares '(1) 1))
    (lambda ()
      (test-success "sum of squares 5" = sum-of-squares '(5) 55))
    (lambda ()
      (test-success "sum of squares 100" = sum-of-squares '(100)
    (lambda ()
      (test-success "difference of squares 1" =
        difference-of-squares '(1) 0))
    (lambda ()
      (test-success "difference of squares 5" =
        difference-of-squares '(5) 170))
    (lambda ()
      (test-success "difference of squares 100" =
        difference-of-squares '(100) 25164150))))

(define (test . query)
  (apply run-test-suite test-cases query))

(let ([args (command-line)])
  (if (null? (cdr args))
      (load "difference-of-squares.scm")
      (load (cadr args)))
  (test 'input 'output))
(import (rnrs))

(define (square n)
  (* n n))

(define (square-of-sum n)
  (square (/ (* n (+ n 1)) 2)))

(define (sum-of-squares n)
  (/ (* n (+ n 1) (+ (* 2 n) 1)) 6))

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

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?