Introduction to Python Programming

Chapter 5: Loops

Chapter outline

• While loop
• For loop
• Nested loops
• Break and continue
• Loop else

Figure 5.1

• credit: modification of work “Quantum Computing”, by Kevin Dooley/Flickr, CC BY 2.0

5.1 While loop

• Learning Objectives
• Explain the loop construct in Python.
• Use a while loop to implement repeating tasks.

While loop

• Repeats a block while its condition remains True.
• Condition is checked before each iteration.
• Remember to update state to avoid infinite loops.

While loop (example video)

Counting with while loop

count = 1
while count <= 5:
    print(count)
    count += 1

Counting with while (video)

While loop — key takeaways

• Use for unknown iteration counts; guard against infinite loops.
• Update loop variables inside the body.

5.2 For loop

• Learning Objectives
• Explain the for loop construct.
• Iterate over containers and sequences with for.
• Use range() to generate sequences.

For loop overview

• Iterates directly over items in an iterable (e.g., list, string, range).
• Concise and avoids manual index bookkeeping.

for item in container:
    # process item
          

Iterating over a container

roster = ["Alice", "Bob", "Carol"]
for student in roster:
    print(student)

For loop with containers (video)

Using range()

for i in range(3):        # 0,1,2
    print(i)

for j in range(2, 5):     # 2,3,4
    print(j)

for k in range(1, 10, 3): # 1,4,7
    print(k)
          

For loop — key takeaways

• Use for known iteration counts or container iteration.
• range() supports start, stop, step.

5.3 Nested loops

• Learning Objectives
• Implement nested while loops.
• Implement nested for loops.

Nested while loops

x = 1
while x <= 2:
    y = 1
    while y <= 3:
        print(x, y)
        y += 1
    x += 1
          

Nested while loops (video)

Nested for loops

courses = [["Alice", "Bob"], ["Carol", "Dan"]]
for course in courses:
    for student in course:
        print(student)
          

Nested for loops (video)

Nested loops — notes

• Inner loops fully run for each outer-loop iteration.
• Time complexity grows with product of loop sizes.

5.4 Break and continue

• Learning Objectives
• Analyze a loop’s control flow.
• Use break and continue to control iteration.

break — exit loop early

while True:
    s = input("Enter 'q' to quit: ")
    if s == "q":
        break
    print("You entered:", s)
          

break (video)

continue — skip to next turn

for i in range(5):
    if i == 2:
        continue
    print(i)
          

continue (video)

Break & Continue — key points

• break ends the loop immediately.
• continue skips to the next iteration.

5.5 Loop else

• Learning Objectives
• Use a loop else clause to handle “no break” cases.
• Apply with either for or while loops.

Loop else — behavior

• else runs only if the loop **did not** encounter a break.
• Useful for search patterns (“found” vs “not found”).

nums = [1, 2, 3, 4]
for n in nums:
    if n == 10:
        print("Found 10!")
        break
else:
    print("10 not found")
          

Loop else (video)

Finding the number 10 in a list

• Demonstrates loop+break vs loop else “not found”.

Chapter 5 reference

• Table 5.4 — Chapter 5 reference.

License

• This ancillary resource is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 (CC BY NC-SA) license; it may be distributed, remixed, built upon for noncommercial purposes only, and must be attributed to OpenStax and redistributed under the same license.