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Sep 29: Tracing Loops by Hand

Learning Objectives

After today's class, you should be able to:

  • Trace the execution of loops that change variables.
  • Predict the output of code segments having a loop.
  • Use a loop to build a container of selected results.

Reminders

Containers Review

Note: The following table is not exhaustive. But these details can be useful to memorize.

Name Create empty Operators Important methods
List [] or list() [i], in, +, * append, count, index, insert, pop, remove
Tuple () or tuple() [i], in, +, * count, index
Set set() only in add, pop, remove
Dict {} or dict() [k], in get, items, pop

Tracing while loops

Exercise 1  (10 min)

For each while loop:

  • Predict the output/behavior by hand on paper.
  • Check your answer using Thonny's debugger.

W-1 

count = 1
while count <= 3:
    count += 1
    print(count)

W-2 

count = 7
while count > 3:
    print(count)
    count -= 2

W-3 

count = 1
while count <= 6:
    if count % 2 == 0:
        count += 3
    else:
        count += 1
    print(count)

W-4 

count = 1
while count < 10 and count % 7 != 0:
    print(count)
    count += 1

W-5 

count = 10
while count > 0:
    count += 1
    print(count)

W-6 

grades = [93, 82, 67, 99]
while len(grades) > 1:
    min_grade = min(grades)
    print(min_grade)
    grades.remove(min_grade)

Infinite Loops

Classic while True loop

Straightforward, but uses break:

while True:
    num = int(input("Positive integer: "))
    if num > 0:
        break
    print("Invalid input, try again.")

Using the walrus operator

More concise, but harder to read:

# The previous example had the opposite condition
while num := int(input("Positive integer: ")) <= 0:
    print("Invalid input, try again.")

More Practice

  • Complete the Sep29 assignment on Runestone
    • If you are absent today, do this on your own