Decisions and Repetition

Programming FoundationsLesson 3 of 860 minutesLab included

Learning objectives

By the end of this lesson

• Predict which branch an if statement will take.
• Trace changing state through for and while loops.
• Recognize boundary mistakes and non-terminating loops.
• Add temporary observations without changing the question being tested.

Conditions choose a path

score = 17

if score >= 20:
    result = "high"
elif score >= 10:
    result = "medium"
else:
    result = "low"

print(result)

Python tests from top to bottom and executes the first matching branch. Indentation defines which instructions belong to each branch.

Predict results for 9, 10, 19, and 20. Values at the edges deserve attention because < and <= disagree exactly there.

Loops repeat while state changes

total = 0

for value in [3, 5, 7]:
    total = total + value
    print(value, total)

Trace a table with iteration, value, and total. The final answer matters less than explaining every transition.

A while loop repeats while its condition remains true:

remaining = 3
while remaining > 0:
    print(remaining)
    remaining -= 1

Removing the final line creates a loop whose controlling state never approaches termination. Stop a runaway terminal program with Ctrl+C and read the resulting KeyboardInterrupt.

Knowledge check

Question 1 of 2

Why should boundary values be tested?

Comparison operators differ at their exact edgesThey make Python compile fasterOnly boundary values have types

Check answer

What this gives us as researchers

You can turn control flow into testable paths and trace state across repetitions. The next lesson packages behavior into functions and moves data through files.