The Secret Life of Python: Manual List Iteration with While and Reverse Loops

#python#coding#programming#softwaredevelopment

Why your 'while' loop skips items—and two ways to fix it without copying.

Timothy was feeling confident. He had spent the morning cleaning up his lists using Margaret’s "Snapshot" method. But as he sipped his coffee, a thought bothered him.

"Margaret," he called out, "the snapshot copy works great for my small task list. But what if I had a list with ten million items? Creating a full copy just to delete a few things feels… expensive."

Margaret nodded, impressed. "You’re thinking about memory efficiency, Timothy. That’s the mark of a growing engineer. If each item in that list was a pointer, a snapshot would cost you about 80 megabytes of extra RAM. The manual way? It costs zero."

"I wonder if we could use a while loop," Timothy continued, showing her his screen. "It’s not as elegant as a list comprehension, but it should work without a copy. I tried to write one, but I think I broke Python."

He pointed to his console, where the cursor was blinking frantically in a sea of never-ending output.

The Infinite Loop of Doom

"I tried to use a while loop to manually walk through the list," Timothy explained. "But it just keeps printing the same thing forever."

He showed her his attempt:

# Timothy's Infinite Loop
tasks = ["done", "todo", "done", "done", "todo"]
i = 0

while i < len(tasks):
    if tasks[i] == "done":
        tasks.remove("done")

    # Timothy's logic: Just keep moving!
    i += 1 

print(f"End: {tasks}")

"Look at the logic," Margaret said gently. "You are moving your index forward (i += 1) every single time. But when you remove an item, the list shifts toward you. You’re stepping forward while the sidewalk is moving backward."

The "Stay-in-Place" Strategy

"If I remove the item at index 0," Timothy realized, "the item that was at index 1 is now the new index 0. If I move to index 1, I’ve jumped over it!"

"Exactly," Margaret said. "In a for loop, the 'driver' is automatic—it always steps forward. In a while loop, you are the driver. You only shift gears when it’s safe."

She helped him rewrite the logic, replacing .remove() with .pop().

tasks = ["done", "todo", "done", "done", "todo"]
i = 0

while i < len(tasks):
    if tasks[i] == "done":
        # We use .pop(i) because we already know the exact index.
        # It's faster than searching the whole list again with .remove()
        tasks.pop(i)  

        # CRITICAL: We do NOT increment 'i' here.
        # The next item just slid into our current position!
    else:
        i += 1  # Only move forward if we didn't delete anything

"Now," Margaret explained, "if you find a 'done' task, you delete it and stay exactly where you are. You look at the same spot again to see what slid into it. You only move the pointer i when you're sure the current item is one you want to keep."

The Professional Backstep

"Is there a way to use a for loop without a copy?" Timothy asked.

"There is one trick," Margaret smiled. "The Reverse Commute. If you start at the end of the list and walk toward the beginning, the shifting doesn't matter."

She wrote out a range that looked like a secret code: range(len(tasks) - 1, -1, -1).

"That's range(start, stop, step)," she explained. "We start at the last index, stop just before -1, and step backward by 1."

# Walking backwards
tasks = ["done", "todo", "done", "done", "todo"]

for i in range(len(tasks) - 1, -1, -1):
    if tasks[i] == "done":
        tasks.pop(i)

"Think about it," Margaret said. "When you remove an item at the end, the items before it—the ones you haven't visited yet—stay exactly where they are. You're removing the rug from behind you, not from under your feet."

Margaret’s Cheat Sheet: Manual Iteration

Margaret flipped to a new page in her notebook.

• Small list, readability matters: Use a Snapshot Copy (tasks[:]).
• Huge list, memory is tight: Use a Reverse Loop. It uses zero extra memory.
• Complex logic needed: Use a While Loop with manual index control.
• The Golden Rule: If you delete, stay still. If you keep, move forward.

Timothy closed his laptop. "I used to think loops were just about repeating things. Now I realize they’re about navigating a changing world."

"That," Margaret said, "is the difference between a coder and an engineer."

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In the next episode, Margaret and Timothy will face "The Lying Truth"—where Timothy learns that in Python, even a 'Zero' can be a lie, and 'Nothing' can be quite something.

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Aaron Rose is a software engineer and technology writer at tech-reader.blog and the author of Think Like a Genius.