Exercism v3 launches on Sept 1st 2021. Learn more! ðŸš€ðŸš€ðŸš€

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

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?

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

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

```
(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 +)))
```

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...

Level up your programming skills with 3,450 exercises across 52 languages, and insightful discussion with our volunteer team of welcoming mentors.
Exercism is
**100% free forever**.

## Community comments