How to Count the Occurrences of a Specific Digit in Python

Learn how to create a function in Python that counts the occurrences of a specific digit in integers from 0 to n with a simple example.
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This video is based on the question https://stackoverflow.com/q/67087608/ asked by the user 'Spiros' ( https://stackoverflow.com/u/9048691/ ) and on the answer https://stackoverflow.com/a/67087764/ provided by the user 'Johnny John Boy' ( https://stackoverflow.com/u/7553746/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Count the Occurrences of a Specific Digit in Python: A Step-by-Step Guide

When it comes to programming in Python, you might find yourself facing various challenges, especially when you are trying to manipulate numbers. One such challenge is counting the occurrence of a specific digit across a range of integers. In this guide, we will not only define this problem but also walk you through how to write a function that effectively counts how many times a certain digit appears in integers from 0 to a given number, n.

The Problem

Consider that you want to count how many times the digit 9 appears from 0 to an integer n. You might have written an initial function, but perhaps you noticed that it didn’t give you the expected results. Here’s an example of what you might have tried:

[[See Video to Reveal this Text or Code Snippet]]

Expected Outputs:

For n = 8, the output should be 0 (0 nines in 8).

For n = 100, the output should be 20 (20 nines in 100).

For n = 909, the output should be 191 (191 nines in 909).

However, in many cases, your function might return 0 unexpectedly. Let’s explore why this happens and how to fix it.

The Solution

Identifying the Issue

Upon closer inspection, you may find that there are a couple of minor mistakes in your function:

Loop Limit: You are iterating only up to n, which means n is being excluded in your count. To include n, your loop should run until n + 1.

Parsing the Digits: Instead of converting n to a string to check for occurrences, you should convert i to a string (the current number in the iteration) to count its digits.

Corrected Code

With these points in mind, here’s how you should rewrite your function:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Changes

Loop Adjustment (range(0, n + 1)): This change ensures that the loop includes the number n in its count.

Digit Parsing (str(i)): By checking each digit of i, you capture all occurrences of the digit 9 across the entire range from 0 to n.

Conclusion

By applying these adjustments, you should now have a fully functional Python function capable of counting the occurrences of the digit 9 from 0 up to any integer n. This is a great example of how small tweaks in your code can lead to significant improvements in functionality. Happy coding!

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