How to Count, Double, and Find Max in a Python List

Three beginner-friendly Python functions that count even numbers, double each value, and find the maximum in a list using simple loops.

Easy Python 3.9+ Aug 9, 2026 Lists & loops 14 views 0 copies

lists loops beginner

Python code

34 lines
Python 3.9+
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def count_even_numbers(numbers):
    """Return the count of even numbers in a list."""
    count = 0
    for num in numbers:
        if num % 2 == 0:
            count += 1
    return count


def double_values(numbers):
    """Return a new list with each value doubled."""
    doubled = []
    for num in numbers:
        doubled.append(num * 2)
    return doubled


def find_max(numbers):
    """Return the largest value in a list."""
    if not numbers:
        return None
    max_value = numbers[0]
    for num in numbers:
        if num > max_value:
            max_value = num
    return max_value


if __name__ == "__main__":
    sample = [3, 7, 2, 9, 4, 6, 1]
    print(f"Original list: {sample}")
    print(f"Even count: {count_even_numbers(sample)}")
    print(f"Doubled values: {double_values(sample)}")
    print(f"Max value: {find_max(sample)}")

Output

stdout 
Original list: [3, 7, 2, 9, 4, 6, 1]
Even count: 3
Doubled values: [6, 14, 4, 18, 8, 12, 2]
Max value: 9

How it works

Each function uses a for loop to iterate over the list. count_even_numbers checks num % 2 == 0 to identify even numbers and increments a counter. double_values builds a new list by appending num * 2 for every element. find_max initializes the maximum to the first element and updates it whenever a larger value is found; it returns None for an empty list. These patterns are fundamental for processing collections in Python.

Common mistakes

  • assuming the input list is never empty in `find_max` — always handle that case
  • modifying the original list instead of creating a new one in `double_values`
  • comparing to `max_value` with `>=` instead of `>` in `find_max`, which is fine but changes behavior for duplicates

Variations

  1. Use list comprehension: `[num * 2 for num in numbers]` for doubling.
  2. Use built-in functions: `sum(1 for n in numbers if n % 2 == 0)` and `max(numbers)`.

Real-world use cases

  • Counting the number of failed requests in a list of HTTP status codes to trigger alerts.
  • Scaling product prices by a factor in an e‑commerce inventory import pipeline.
  • Determining the highest salary in a list of employee records for analytics reports.

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