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:
Did you get the tests passing and the code clean? If you want to, these are some additional things you could try:
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?
Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include a message.
To raise a message with an exception, just write it as an argument to the exception type. For example, instead of
raise Exception, you should write:
raise Exception("Meaningful message indicating the source of the error")
To run the tests, run the appropriate command below (why they are different):
Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version):
python -m pytest grains_test.py
-v: enable verbose output
-x: stop running tests on first failure
--ff: run failures from previous test before running other test cases
For other options, see
python -m pytest -h
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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.
import unittest from grains import ( on_square, total_after, ) # Tests adapted from `problem-specifications//canonical-data.json` @ v1.1.0 class GrainsTest(unittest.TestCase): def test_square_1(self): self.assertEqual(on_square(1), 1) def test_square_2(self): self.assertEqual(on_square(2), 2) def test_square_3(self): self.assertEqual(on_square(3), 4) def test_square_4(self): self.assertEqual(on_square(4), 8) def test_square_16(self): self.assertEqual(on_square(16), 32768) def test_square_32(self): self.assertEqual(on_square(32), 2147483648) def test_square_64(self): self.assertEqual(on_square(64), 9223372036854775808) def test_square_0_raises_exception(self): with self.assertRaisesWithMessage(ValueError): on_square(0) with self.assertRaisesWithMessage(ValueError): total_after(0) def test_square_negative_raises_exception(self): with self.assertRaisesWithMessage(ValueError): on_square(-1) with self.assertRaisesWithMessage(ValueError): total_after(-1) def test_square_gt_64_raises_exception(self): with self.assertRaisesWithMessage(ValueError): on_square(65) with self.assertRaisesWithMessage(ValueError): total_after(65) def test_total(self): self.assertEqual(total_after(64), 18446744073709551615) # Utility functions def setUp(self): try: self.assertRaisesRegex except AttributeError: self.assertRaisesRegex = self.assertRaisesRegexp def assertRaisesWithMessage(self, exception): return self.assertRaisesRegex(exception, r".+") if __name__ == '__main__': unittest.main()
def on_square(x): return 2 ** (x - 1) def total_after(x): return 1 if x == 1 else on_square(x) + total_after(x - 1)
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.