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AndrewNelis's solution

to Grains in the Clojure Track

Published at Feb 02 2021 · 0 comments
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.clj

(ns grains-test
  (:require [clojure.test :refer [deftest is]]
            grains))

(deftest square-1
  (is (= 1 (grains/square 1))))

(deftest square-2
  (is (= 2 (grains/square 2))))

(deftest square-3
  (is (= 4 (grains/square 3))))

(deftest square-4
  (is (= 8 (grains/square 4))))

(deftest square-16
  (is (= 32768 (grains/square 16))))

(deftest square-32
  (is (= 2147483648 (grains/square 32))))

(deftest square-64
  (is (= 9223372036854775808 (grains/square 64))))

(deftest total-grains
  (is (= 18446744073709551615  (grains/total))))
(ns grains)

(def big-two (BigInteger. "2"))

(defn square [n]
  (->> (dec n)
       (.pow big-two)))

(defn total []
  (->> (range 1 65)
       (map square)
       (reduce +)))

Community comments

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AndrewNelis's Reflection

The biggest challenge is finding the right data type to hold the large integers which match what the tests are expecting.

There's likely a good solution using memoize also that runs faster...