How would you approach writing a method to count the elements of a sorted list with -1 as an out-of-range marker, without built-in methods?

Question Analysis

This question asks you to implement a method to count the elements in a sorted list where -1 is used as an out-of-range marker. The key points to consider are:

• Sorted List: The list is already sorted, which means that all valid elements come before any out-of-range markers (-1).
• Out-of-range Marker: The -1 signifies the end of the valid elements in the list.
• No Built-in Methods: You must manually count the elements without using built-in methods like len() in Python or similar functions in other programming languages.

The challenge is to effectively traverse the list until you encounter the first -1 and count the elements up to that point.

Answer

To solve this problem, you will need to iterate through the list and count each element until you encounter the first -1. Here is a step-by-step approach to implementing this method:

def count_elements(lst):
    count = 0
    for element in lst:
        if element == -1:
            break
        count += 1
    return count

# Example usage:
sorted_list = [3, 5, 7, 9, -1, -1, -1]
print(count_elements(sorted_list))  # Output: 4

Explanation:

• Initialize a Counter: Begin by initializing a variable count to 0. This will keep track of the number of valid elements in the list.
• Iterate through the List: Use a for loop to iterate over the elements in the list.
• Check for Out-of-range Marker: Inside the loop, check if the current element is -1. If it is, break out of the loop as it indicates the end of the valid elements.
• Increment Counter: If the current element is not -1, increment the count by 1.
• Return the Count: Once the loop is complete, return the count which represents the number of elements in the list before the first -1.

This approach ensures that you count only the valid elements and efficiently handle the sorted list with the -1 markers.