The Curta calculator was developed in 1948 and, although costly, became popular for its portability. This purely mechanical hand-held device could do addition, subtraction, multiplication and division. By the early 1970s electronic pocket calculators ended manufacture of mechanical calculators, although the Curta remains a popular collectable item. Find more on wikipedia page...

In this series of missions you are going to build an elementary calculator.

As an input, you get a sequence of keys pressed, and, as the result of that function, you should show what will be shown on the screen when the last key is pressed. Be attentive, it's not always the result of the expression. Actually, playing with the physical calculator or app will really helps you to catch edge cases.

In the fifth mission your function should work properly with additional operations: *, /, // (integer division), % (modulo) and ** (power) and their combinations with "=".

Expected behavior:

  • beginning zeros should be removed, only-zeros number - converted to single zero;
  • among +- signs between numbers, the last one should be taken;
  • "==" means repeating the last operation;
  • "+=" or "-=" - adding/subtracting the number (or operations result) before the combination (doubling the number/subtracting itself);
  • the calculator ignores digit you enter after 5th;
  • "-" for numbers < 0 is NOT taking digit place;
  • if the abs(result) is more than 99999 - "error" is shown as a result;
  • for float, if the integer part of abs(result) is more then 9999 - "error" is shown as a result;
  • for float, in case the total length of number is more than 5 digits, it should be rounded to 5 digits (1.23456 -> 1.235);
  • for float, beginning and trailing zeros should be removed (until the "." if possible): 0.1200 -> .12 , 123.00 -> 123. . It should be done after the rounding: 1.000123 -> 1. . Stripping of trailing zeros should only be done after entering a number has concluded (non-digit character pressed).

Input: String.

Output: String.


assert calculator("10/2*2=") == "10."
assert calculator("10/=*=-=") == "0."
assert calculator("100//33**3=") == "27"
assert calculator("10%10=") == "0"

How it’s used: Calculators are widely used. Understanding the principles of its input and output are interesting and useful.

Precondition: Allowed characters: digits (0-9), dot (.), signs +, -, =, *, /, //, %, ** and their combinations (+=, +-, ==, %= etc.). The integer number may contain no more than 5 digits, float - 4.

Invalid hot key. Each hot key should be unique and valid
Hot keys:
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.

Pair Programming (Beta-version)

Welcome to Pair Programming! Engage in real-time collaboration on coding projects by starting a session and sharing the provided unique URL with friends or colleagues. This feature is perfect for joint project development, debugging, or learning new skills together. Simply click 'Start Session' to begin your collaborative coding journey!

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Please wait for the session creator to join.

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It might happen accidentally, so that you can wait for reconnection.

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