Pearls in the Box Pearls in the Box
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To start the game they put several black and white pearls in one of the boxes. Each robot has N moves, after which the initial set is being restored for the next game. Each turn, the robot takes a pearl out of the box and puts one of the opposite color back. The winner is the one who takes the white pearl on the Nth move.

Our robots don't like uncertainty, that's why they want to know the probability of drawing a white pearl on the Nth move. The probability is a value between 0 (0% chance or will not happen) and 1 (100% chance or will happen). The result is a float from 0 to 1 with two decimal digits of precision (±0.01).

You are given a start set of pearls as a string that contains "b" (black) and "w" (white) and the number of the move (N). The order of the pearls does not matter.


Input: The start sequence of the pearls as a string and the move number as an integer.

Output: The probability for a white pearl as a float.


checkio('bbw', 3) == 0.48
checkio('wwb', 3) == 0.52
checkio('www', 3) == 0.56
checkio('bbbb', 1) == 0
checkio('wwbb', 4) == 0.5
checkio('bwbwbwb', 5) == 0.48

How it is used: This task introduces you to the basics of probability theory and statistics. Fun fact: regardless of the initial state, as the number of moves increases, the probability approaches 0.5!

Precondition: 0 < N ≤ 20
0 < |pearls| ≤ 20

CheckiO Extensions

CheckiO Extensions allow you to use local files to solve missions. More info in a blog post.

In order to install CheckiO client you'll need installed Python (version at least 3.8)

Install CheckiO Client first:

pip3 install checkio_client

Configure your tool

checkio --domain=py config --key=

Sync solutions into your local folder

checkio sync

(in beta testing) Launch local server so your browser can use it and sync solution between local file end extension on the fly. (doesn't work for safari)

checkio serv -d

Alternatevly, you can install Chrome extension or FF addon

checkio install-plugin
checkio install-plugin --ff
checkio install-plugin --chromium

Read more here about other functionality that the checkio client provides. Feel free to submit an issue in case of any difficulties.

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