Master Basic List Operations in Python Without Using Built-in Methods in Python (Part 2)

In this part of the series, we continue exploring how to work with lists in Python without relying on built-in functions. These kinds of exercises are great for building strong problem-solving skills and a deeper understanding of data structures and algorithms.

Problem One: Count the Frequency of the Smallest Element

Task: You are given an array of integers. Write a function that returns the number of times the smallest element appears in the array.

Note: Do not use min() or count(). Try to solve this with a single list traversal.

Breakdown:

• If the list is empty, return 0.
• Initialize the smallest value with float('inf') and set a counter to 0.
• Traverse the list:
  • If the current number is smaller than the smallest, update the smallest and reset the counter to 1.
  • If it's equal to the smallest, increment the counter.

def count_min(numbers):
    if not numbers:
        return 0

    smallest = float('inf')
    count = 0

    for num in numbers:
        if num < smallest:
            smallest = num
            count = 1
        elif num == smallest:
            count += 1

    return count

Problem Two: Find the Second Largest Number

Task: You are given a list of integers. Write a function that returns the second-largest number.

If there are fewer than two unique numbers, return None.

Breakdown:

• First, check if the list has at least two numbers. If not, return None.
• Initialize largest and second_max to float('-inf').
• Loop through the numbers:
  • If the number is greater than largest, update second_max to largest, then update largest.
  • If the number is greater than second_max and not equal to largest, update second_max.
• If second_max is still -inf, return None.

def second_max(nums):
    if len(nums) < 2:
        return None

    largest = float('-inf')
    second_max = float('-inf')

    for num in nums:
        if num > largest:
            second_max = largest
            largest = num
        elif num > second_max and num != largest:
            second_max = num

    if second_max == float('-inf'):
        return None

    return second_max

Final Thoughts

Mastering these kinds of operations without relying on Python’s built-ins forces you to think like a computer and truly understand how things work under the hood. This knowledge becomes invaluable when working on more complex applications or preparing for coding interviews.