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4d47's solution

to Grains in the Scheme Track

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Instructions
Test suite
Solution

Calculate the number of grains of wheat on a chessboard given that the number on each square doubles.

There once was a wise servant who saved the life of a prince. The king promised to pay whatever the servant could dream up. Knowing that the king loved chess, the servant told the king he would like to have grains of wheat. One grain on the first square of a chess board. Two grains on the next. Four on the third, and so on.

There are 64 squares on a chessboard.

Write code that shows:

  • how many grains were on each square, and
  • the total number of grains

For bonus points

Did you get the tests passing and the code clean? If you want to, these are some additional things you could try:

  • Optimize for speed.
  • Optimize for readability.

Then please share your thoughts in a comment on the submission. Did this experiment make the code better? Worse? Did you learn anything from it?

Source

JavaRanch Cattle Drive, exercise 6 http://www.javaranch.com/grains.jsp

Submitting Incomplete Solutions

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

grains-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 (grains))

 (test-begin "grains")

(test-eqv "square 1"
          1
          (square 1))

(test-eqv "square 2"
          2
          (square 2))

(test-eqv "square 3"
          4
          (square 3))

(test-eqv "square 4"
          8
          (square 4))

(test-eqv "square 16"
          32768
          (square 16))

(test-eqv "square 32"
          2147483648
          (square 32))

(test-eqv "square 64"
          9223372036854775808
          (square 64))

(test-eqv "total grains"
          18446744073709551615
          (total))

 ;; Tests go here

 (test-end "grains")
(define-module (grains)
  #:export (square total)
  #:autoload (srfi srfi-1) (iota))

(define (square n)
  (expt 2 (- n 1)))

(define (total)
  (- (square 65) 1))

What can you learn from this solution?

A huge amount can be learnt 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 I could read more about to develop my understanding?

Community comments

See what others have said about this solution
about 2 years ago
4d47 says

did not recognize expt first time