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Cover image for H9A: Count How Many Times a Digit Appears in Any Range
RK Riad Khan
RK Riad Khan

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H9A: Count How Many Times a Digit Appears in Any Range

#python #cli #devops #rkriad585

A classic trick question: how many times does the digit 9 appear between 1 and 100? Most people answer 19. The correct answer is 20 — 9, 19, 29, ..., 99 give 10 in the ones place, and 90–99 give 10 in the tens place. The number 99 is counted twice.

I turned this into a small Python package, H9A.

Install and use 

pip install h9a
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h9a                       # 9 in 1..100 -> 20
h9a --digit 0 --start 0 --end 9
h9a --json                # machine-readable output
h9a --screenshot          # terminal-style PNG
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Why it's fast

The first version iterated the range. A range of a billion numbers meant a billion iterations. I replaced it with a closed-form, per-place formula: for each decimal position, the count comes from division and modulo on the endpoints. Ranges up to 1,000,000,000,000 now resolve in microseconds.

Extras

  • rich colorized output, pyfiglet ASCII banner, Pillow screenshots
  • Library API: count_digit() returns a per-place DigitCount
  • 74 tests, ruff, mypy, pre-commit, CI docs + container publishing
  • MIT-licensed, Python 3.8+

Source: https://github.com/rkriad585/h9a
Docs: https://rkriad585.github.io/h9a/
PyPI: https://pypi.org/project/h9a/

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