Loops — PythonLessonLab

THE IDEA

Understand the concept

A for loop visits items in a sequence. range(1, 4) produces 1, 2, and 3; the stop value is excluded. A running total is a common reason to loop.

A while loop repeats while its condition remains true. Its body must eventually change something that makes the condition false. break can end a loop early when needed.

for iterates over values
range stop is excluded
while needs a stopping condition

WORKED EXAMPLES

See it step by step

Read the code, predict the output, then compare it with the result.

01. Visit a range

PYTHON

for number in range(1, 4):
    print(number * 2)

EXPECTED OUTPUT

2
4
6

range(1, 4) provides 1, 2, and 3; the stop value 4 is excluded.

02. Accumulate a total

PYTHON

total = 0
for value in [2, 3, 4]:
    total += value
print(total)

EXPECTED OUTPUT

Each iteration adds the next list item to total. The final print runs after the loop.

HANDS-ON

Try it in Python

Edit the example and run it. Python starts in your browser the first time you click Run.

Python console

Ready to run

total = 0
for number in range(1, 4):
    total += number
    print(f"Added {number}; total is {total}")
print(f"Final total: {total}")

Need input()? Add one value per line

Input lines

Ctrl / ⌘ + Enter to run

OUTPUT

Your output appears here.

Your turnChange the loop to add the numbers 1 through 5. Print the final total and check that it is 15.

Need a hint?

range(1, 6) produces 1 through 5. Keep total += number inside the loop, then print the final total after the loop.

Complete the task and select Check task to verify your code.

KNOWLEDGE CHECK

Quick quiz

Three questions. You can change your answers and try again.

DEBUG BETTER

Typical mistakes

Everyone meets these errors. See what causes them and how to fix them.

A while loop never progresses

COMMON MISTAKE

count = 0
while count < 3:
    print(count)

FIX

count = 0
while count < 3:
    print(count)
    count += 1

What happens: The condition stays true, so the loop keeps printing 0.
Update the value tested by the condition.

Expecting the stop value