for Loops and range()

Unit ID: M04-U03 Estimated active time: 25-35 minutes

Process each item

module_titles = ["Values", "Collections", "Control flow"]

for title in module_titles:
    print(title)

On each iteration, title refers to the next list item. The indented block runs once per item.

Use a singular loop name for items from a plural collection. for record in records is clearer than for x in data.

Transform into a new list

clean_titles = []

for title in module_titles:
    clean_title = title.strip().title()
    clean_titles.append(clean_title)

The source list remains unchanged. The result list records each transformed value.

Use range for a numeric sequence

for number in range(1, 4):
    print(number)

Output:

1
2
3

The stop value 4 is excluded. range(start, stop, step) follows the same stop-excluded idea as slicing.

Use enumerate when position matters

for position, title in enumerate(module_titles, start=1):
    print(position, title)

This produces human-friendly positions starting at 1 without manually changing a counter.

Do not change list length while iterating over it

Removing items from the same list being traversed can skip values or create confusing results. Build a new result list or iterate over a copy.

scores = [8, -1, 6, 11]
valid_scores = []

for score in scores:
    if type(score) is int and 0 <= score <= 10:
        valid_scores.append(score)

Practice

Given hours = [8, 12, 0, 45, "6"], build separate valid_hours and invalid_hours lists using a for loop. Valid hours must have exact type int and be from 0 to 40.

Expected valid values: [8, 12, 0].

Takeaway

A for loop makes repeated processing explicit. Build new result collections instead of mutating the source while traversing it. Next, we will use while for repetition controlled by a changing condition.