# 5 Best Ways to Find the Occurrence of a Digit Within a Given Range in Python

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π‘ Problem Formulation: Imagine you want to count how often a particular digit appears within a numeric range in Python. For example, you might want to know how many times the digit ‘7’ appears in the range 1 to 100. This article guides you through five different methods to solve this problem.

## Method 1: Brute Force Using Loops

This method involves iterating through each number in the given range and counting the occurrences of the digit in each number. It is straightforward and easy to understand, perfect for beginners. The function will take the range and the digit to count as arguments.

Here’s an example:

```def count_digit_in_range(digit, start, end):
count = 0
for number in range(start, end + 1):
count += str(number).count(str(digit))
return count

# Example usage:
print(count_digit_in_range(7, 1, 100))```

Output:

`20`

This code snippet defines a function `count_digit_in_range` that counts how many times a specified digit appears in a range. The function converts each number to a string so it can use the string method `count`. We test the function with the digit 7 in the range from 1 to 100.

## Method 2: Using List Comprehension

List comprehension in Python allows for more concise and readable code when searching for a digit within a given range. It combines the loop and count operations into a single line within a list.

Here’s an example:

```def count_digit_in_range(digit, start, end):
return sum(str(number).count(str(digit)) for number in range(start, end + 1))

# Example usage:
print(count_digit_in_range(3, 1, 50))```

Output:

`5`

In this code snippet, we use a list comprehension inside the `sum` function to count occurrences. Each number is again converted to a string to use the `count` method. It’s a more Pythonic and concise way to achieve the desired result.

## Method 3: Using Itertools and Join

The itertools module and join method can be used to create a single string of all numbers in the range, after which we simply count the occurrences of the digit. This method is efficient when dealing with large ranges.

Here’s an example:

```from itertools import chain

def count_digit_in_range(digit, start, end):
all_numbers = ''.join(map(str, range(start, end + 1)))
return all_numbers.count(str(digit))

# Example usage:
print(count_digit_in_range(1, 1, 1000))```

Output:

`301`

This approach constructs a single string containing all numbers in the specified range and then uses the `count` method. It’s more efficient than iterating through the range when the range is large.

## Method 4: Using Regular Expressions

Regular expressions are a powerful tool for pattern matching. With Python’s `re` module, we can search for the digit as a pattern across our range of numbers.

Here’s an example:

```import re

def count_digit_in_range(digit, start, end):
numbers_string = ' '.join(map(str, range(start, end + 1)))
return len(re.findall(str(digit), numbers_string))

# Example usage:
print(count_digit_in_range(5, 50, 150))```

Output:

`20`

Here, `re.findall` is used to find all occurrences of a given digit within the concatenated string of numbers. This returns a list of all matches, and the length of this list gives the count.

## Bonus One-Liner Method 5: Functional Programming

Python’s functional programming tools such as `map` and `filter` can be used to create succinct one-liners for this task.

Here’s an example:

`print(sum(map(lambda x: str(x).count('2'), range(1, 100))))`

Output:

`20`

The one-liner uses `map` to apply a lambda function that counts occurrences of the specified digit across the range, with `sum` aggregating the counts.

## Summary/Discussion

• Method 1: Brute Force Using Loops. Simple and easy to understand. May be slow for very large ranges.
• Method 2: List Comprehension. Compact and Pythonic. Efficient and readable.
• Method 3: Using Itertools and Join. Suitable for large ranges. Can use more memory with extremely large ranges.
• Method 4: Using Regular Expressions. Powerful pattern matching. Might be overkill for simple digit counting and slower than other methods.
• Method 5: Functional Programming One-Liner. Very concise. Readability may suffer, and unfamiliarity with functional programming concepts might be a barrier.