After this lesson, you will be able to:

• Build a new list while iterating instead of editing the source list.
• Filter and normalize values using conditions and .append().
• Count repeated values in a dictionary using .get().
• Group values into dictionary entries whose values are lists.

Loops can construct a collection, not just print its values. Start with an empty destination, then add values as you process the source. For a list, use .append() to add each selected result while preserving its encounter order.

Filtering uses a condition inside the loop: some source items are appended and others are skipped. Keep the input collection unchanged when it represents data you may need later; editing a list while iterating over that same list can skip elements unexpectedly.

A dictionary is useful for counts and groups. counts.get(key, 0) retrieves the current count or starts at zero. For grouping, initialize a list the first time a category appears, then append its items. You are combining methods from earlier lessons with functions, conditions, and loops from Unit 1.

1. Normalize values into a new list

raw_regions = [" EU ", "Us", " APAC "]
regions = []
for region in raw_regions:
    regions.append(region.strip().lower())
print(regions)

Output:

['eu', 'us', 'apac']

Each cleaned value is appended to the new list.

2. Filter with a condition

scores = [31, 0, 18, -3]
valid_scores = []
for score in scores:
    if score >= 0:
        valid_scores.append(score)
print(valid_scores)

Output:

[31, 0, 18]

The source stays intact; only valid values enter the result.

3. Count repeated values with a dictionary

states = ["open", "closed", "open"]
counts = {}
for state in states:
    counts[state] = counts.get(state, 0) + 1
print(counts["open"])

Output:

2

The fallback count starts an unseen key at zero.

4. Group values by category

requests = [("api", "urgent"), ("worker", "normal"), ("mail", "urgent")]
groups = {}
for service, category in requests:
    if category not in groups:
        groups[category] = []
    groups[category].append(service)
print(groups["urgent"])

Output:

['api', 'mail']

A grouped dictionary combines a key with an ordered list of matching values.

1. Modifying the list being iterated can skip elements.
2. Forgetting to create a group's list before appending causes KeyError.
3. Reinitializing the accumulator inside the loop discards earlier progress.
4. Printing the constructed list is not the same as returning it from a function.