How to Create a List with a Fixed Size in Python

1. Introduction

In Python, lists are dynamic data structures, which means they can grow or shrink in size as needed. However, there might be scenarios where you need a list with a fixed size, where the number of elements does not change after its creation. This is often required in applications where the data size is known beforehand, and maintaining a constant size can prevent errors from adding or removing items unintentionally.

Definition

A fixed-size list is a list that has a predefined number of elements and does not change in size. In Python, while lists are inherently dynamic, we can simulate fixed-size lists by pre-allocating a list with a specific size and then controlling how elements are added or replaced.

2. Program Steps

1. Decide the size of the list.

2. Create a list with the desired size, initializing all elements with a default value, such as None.

3. Define functions or control structures to prevent adding or removing elements beyond the fixed size.

4. Implement any additional logic for your specific application, like updating elements without changing the list's size.

3. Code Program

# Step 1: Decide the size of the list
fixed_size = 5  # This is the desired fixed size of the list

# Step 2: Create a list with the fixed size, initialized with a default value
fixed_list = [None] * fixed_size  # Initialize all elements with None

# Step 3: Functions to handle element assignments without changing list size
def set_element(lst, index, value):
    """
    Sets the value of an element at a specific index if the index is within the bounds of the list size.
    """
    if 0 <= index < len(lst):
        lst[index] = value
    else:
        raise IndexError("Attempted to set an element outside of the fixed list size.")

# Step 4: Example of setting elements in the list
try:
    set_element(fixed_list, 0, 'Python')  # Set the first element
    set_element(fixed_list, 2, 'Fixed')   # Set the third element
except IndexError as e:
    print(e)

# Optional Step: Preventing the list from growing or shrinking
# (Not including this part in the code as it requires overriding list methods or creating a new class)

Output:

['Python', None, 'Fixed', None, None]

Explanation:

1. fixed_size is set to 5, determining the size of the fixed list.

2. fixed_list is created with 5 None elements, using list multiplication.

3. The set_element function is defined to control the update of list elements.

4. Within the set_element function, there is a check to ensure the index is within the correct range.

5. If an invalid index is provided, the function raises an IndexError.

6. The set_element function is used to set values at index 0 and 2.

7. If there is an attempt to set a value at an index outside the fixed size, it would trigger an IndexError.

8. The final output shows the list with the updated values at the specified indexes, while the list size remains unchanged.