Count the dots on the cards

Python Intermediate alien ages 11-13 From CS Unplugged

The problem

A row of binary cards is written as letters: W means the card shows its dots, B means it is turned over. The last card is worth 1, the one before it 2, then 4, and so on. Write a function count_dots(cards) that gives back the number the row shows. Example: "WBB" is 4.

You will practice: binary to decimal, loops, doubling

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CC BY-SA 4.0

Adapted from a CS Unplugged programming challenge by the University of Canterbury Computer Science Education Research Group, licensed CC BY-SA 4.0.

Original challenge · License: CC BY-SA 4.0

Changes: Rewritten in our own words as a function with tests, hints and steps, in Python and JavaScript, by Open Cosmos. This version is also CC BY-SA 4.0.