Collections and iteration

Understand the idea

Choose a collection for the operations you need. Lists preserve order and allow indexed access. Sets answer membership questions. Dictionaries associate keys with values. Iteration visits elements without requiring an index.

Work through an example

for value in [4,1,7,2] visits values; enumerate also gives positions. Iterating a dictionary yields keys; .items() yields key/value pairs.

Time and space complexity

List traversal is O(n); expected set/dictionary membership is O(1), with possible worse cases.

Why it works

The iterator supplies each current element once unless the collection is modified during iteration.

Common mistakes

• Do not remove elements from a list while traversing it normally.
• An empty set is set(), while {} is a dictionary.

Chapter lab · Python implementation

Our running task is to find the largest value in a sequence. A program turns a precise input/output contract into small state changes. Read the example from the top, predict each update, and run its assertions. Python examples use ordinary built-in types; statements inside an indented block belong to that block.

This shared lab connects the chapter's topics. Follow the current lesson's worked example first, then trace the complete implementation. Additional algorithms described in topic exercises are variations you can implement independently.

def largest(values):
    if not values:
        return None
    best = values[0]
    for value in values[1:]:
        if value > best:
            best = value
    return best

def largest_without_slice(values):
    if not values:
        return None
    best = values[0]
    for i in range(1, len(values)):
        best = max(best, values[i])
    return best

Run and test the lab

Copy the implementation and these assertions into a Python 3 file and run it. An assertion failure means the actual result differs from the expectation. Add the current topic's exercise and boundary tests to deepen your understanding.

assert largest([]) is None
assert largest([-8, -3, -5]) == -3
assert largest_without_slice([4, 1, 7, 2]) == 7
assert largest_without_slice([0]) == 0