The 16-year-old who paid the child fare

Imagine a bus app charges a 16-year-old the child fare. The company loses money every day. Nobody sees it for a month.

The mistake is one small symbol in one line of code. Finding a mistake like this is a skill. You will practise it now.

Key words

Word	What it means	Example
Bug	A mistake in a program.
Debug	To find a bug and fix it.
Syntax error	The code breaks the rules of Python. The program does not start.	a missing colon
Runtime error	The program starts but crashes while it runs.	dividing by zero
Logic error	The program runs but gives a wrong answer.	<= instead of <
Trace table	A table where you write the value of each variable after each step.
Variable	A name that holds a value.
Loop	A part of a program that repeats.
Iteration	One run through a loop.

The cells students fill in are left blank in this preview.

Three kinds of error

Kind	What happens	How you find it
Syntax error	Python stops at once and shows a message	Read the line number in the message
Runtime error	The program crashes part of the way	Read the message. Ask what data caused it
Logic error	No message, but the answer is wrong	Trace the code with a table. Test small parts

The cells students fill in are left blank in this preview.

Fix it: a syntax error

Code: python

age = 15
if age < 16
    print("Child fare")
else:
    print("Adult fare")

# Run it and read the message. Which line does it point to?
# Fix the line and run it again.

Logic errors are the hard ones

A syntax error shows a message. A logic error shows no message. The program runs and gives a wrong answer.

The best tool is a trace table. Go through the code one line at a time. Write down each value. Compare what you wrote with what you expected.

Fix it: the wrong edge

Code: python

age = 16
if age <= 16:
    print("Child fare")
else:
    print("Adult fare")

# A 16-year-old should pay the adult fare. Fix the condition.
# Then test the ages 15, 16 and 17.

Fix it: wrong by one

Code: python

# This should print the numbers 1 to 5.
for i in range(5):
    print(i)

# It prints 0 to 4. Change the range so it prints 1 to 5.

Fix it: the wrong order

Code: python

total = 0
print("Total:", total)
total = total + 5
total = total + 3

# It prints 0. It should print 8.
# Move one line so the steps run in the right order.

Trace table: a loop that never stops

Step	count < 3 ?	What happens
Start	True	The code: count = 0, then while count < 3: print(count)
After iteration 1	True	The line that adds 1 is missing
After iteration 2	True	The loop runs again
After iteration 3	True	It will never become False

The cells students fill in are left blank in this preview.

Read this code

Code: python

# i != 2 means: i is not equal to 2.
total = 0
for i in range(1, 4):
    if i != 2:
        total = total + i
print(total)