The till charges too much

A member pays 6 pounds for an hour. The sign says members pay 4.

Is the sign wrong? Is the till wrong? Is the code wrong? You do not know yet.

A defect is a mistake in a program. To find one, follow a method.

Key words

Word	What it means	Example
Defect	A mistake in a program. Also called a bug.	price is wrong
Debug	To find and fix a defect.	fix the price
Syntax error	Code that breaks the rules of the language. It will not run.	a missing colon
Runtime error	Code that starts, then stops with an error message.	a name that does not exist
Logic error	Code that runs but gives the wrong answer.	the wrong price
Trace table	A table you fill in by hand. It shows the values changing line by line.	total: 0, 1, 1
Test	A check that shows if code gives the right answer.	price_for(True, 2) is 8

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

Step 1: read the code against the brief

Question to ask	Example for price_for
What should this code do?	Members pay 4 an hour. Visitors pay 6 an hour.
What does it really do?	Run it with a member for 2 hours. It shows 12, not 8.
Where do they differ?	The two rates are in the wrong places. They are swapped.

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

Try it: a syntax error

Code: python

def is_child(age)
    return age < 16

print(is_child(12))

# Press Run. Read the message.
# Python says where it got stuck.
# Fix the line and run it again.

Try it: a runtime error

Code: python

def total_price(hours):
    rate = 6
    return rate * hour

print(total_price(2))

# This starts, then crashes.
# Which name does Python not know?
# Fix it and run it again.

Trace a logic error by hand

Line run	one["member"]	Is it Sam?	total
total = 0			0
first booking	Sam	yes	total = 1
second booking	Sam	yes	total = 1 again, not 2
third booking	Aisha	no	1
The answer			1, but Sam has 2 bookings

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

Change one thing at a time

data-callouts.warning.label

If you change three things and the code works, you do not know which change fixed it. If you change three things and it breaks, you do not know which one broke it. Change one thing. Run. Then decide.