GCSE · Computer Science · AQA · Spec 8525

Addition (arithmetic operator)

A line of code can work out exactly the right answer and still leave your total unchanged. By the end of this page, you'll spot that line in seconds.

Programming · Addition

Follow the score, one line at a time

The arrow ← means 'store the value on the right in the variable on the left'. A new value appears in the table only when a line stores one.

Line 5 works out 35. Before you step past it, predict: what does score hold straight afterwards?

 1  score ← 0
 2  points ← 10
 3  score ← score + points
 4  points ← 25
 5  OUTPUT score + points
 6  score ← score + points
 7  score ← score + 5
 8  OUTPUT score

score	points
0

Output

Step 1: The game starts. score is set to 0 — nothing has been added yet.

1 / 8

Press Next to run one line. Say what each variable holds BEFORE you look.

Step 1 of 8: The game starts. score is set to 0 — nothing has been added yet..

Exam line: When you trace code, ask of every line: which variable, if any, is on the left of the arrow? That is the only one that can change.

Watch out: On line 5, watch the table as well as the screen. Something appears on screen — but does anything get stored?

What do you think?

The line that looks impossible

A program has just stored 7 in score. The very next line is score ← score + 5.

What does that line do? Pick the idea closest to what you think right now.

Nothing sensible. score can't equal score + 5, so the line is impossible.It doubles score, because score appears on both sides.It only works if score is 0 before the line runs.It works out 7 + 5 = 12, then stores 12 in score, replacing the 7.

How sure are you?

I'm sureI think soI'm guessing

Nothing sensible. score can't equal score + 5, so the line is impossible.

This is a common misconception

It would be impossible as an equation — but it isn't one. It's an instruction: work out the right-hand side, then store the answer in the variable on the left. So the computer works out score + 5 = 7 + 5 = 12, then stores 12 in score. In Python the same line is written score = score + 5, which is exactly why it looks like maths. Read that = as 'becomes', not 'equals'.

It doubles score, because score appears on both sides.

This is a common misconception

score does appear twice, but it's only read once — on the right-hand side, where its current value (7) is used. The score on the left isn't added in; it just names where the answer goes. 7 + 5 = 12, so score becomes 12 — not 14, and not 19.

It only works if score is 0 before the line runs.

This is a common misconception

The line works from whatever score holds right now. With 7 it gives 12; if score held 0 it would give 5; if it held 100 it would give 105. Starting a running total at 0 is a good habit before you add up a list — but this line runs perfectly well from any value.

It works out 7 + 5 = 12, then stores 12 in score, replacing the 7.

This matches the evidence

Exactly. Right-hand side first, using the value score holds at that moment. Then the result replaces the old value. The 7 is gone; score is now 12.

Right side first, using the current value. Then store the answer on the left, replacing what was there.

Exam line: Read x ← x + 5 as 'x becomes its current value plus 5'.

Spot the bug

Where did the 3 go?

This program should add up three prices — 4, 3 and 2 — and output the total, 9. It outputs 6. Which line is wrong?

A student's program — which line goes wrong?

total ← 0
price ← 4
total ← total + price
price ← 3
subtotal ← total + price
price ← 2
total ← total + price
OUTPUT total

Exam line: Every value you add into a total must be stored back into that total.

Your turn

Write the code for a cinema booking

A cinema program already stores the price of a ticket in ticketPrice and the price of popcorn in popcornPrice. Write pseudo-code or program code that adds a booking fee of 2 to ticketPrice and stores the result in a variable called cost, then adds popcornPrice to cost, and then outputs cost. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Adds 2 to ticketPrice using + (e.g. ticketPrice + 2) (1 mark)
Stores that result in cost with an assignment (e.g. cost ← ticketPrice + 2) (1 mark)
Adds popcornPrice to cost and stores the result back in cost (e.g. cost ← cost + popcornPrice) (1 mark)
Outputs cost after both additions have been stored (1 mark)

Model answer

cost ← ticketPrice + 2 cost ← cost + popcornPrice OUTPUT cost (In Python: cost = ticketPrice + 2, then cost = cost + popcornPrice, then print(cost).)

Writing cost + popcornPrice on its own gets the sum right but loses it. The arrow into cost is what keeps it.

Exam line: One idea per line: add, store, output. Each one can earn credit on its own.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Working out a sum is only half the job. The other half is putting the answer somewhere.

What you need to know

The + operator adds two values to give a result. The values can be literals (numbers typed straight into the code, like 5), variables (like price), or one of each.
Working out a sum does not keep it. To use the result later, store it in a variable with an assignment, e.g. total ← price + 2. (In Python, assignment is written with =.)
In an assignment, the right-hand side is worked out first, using each variable's current value. The result is then stored in the variable on the left, replacing whatever it held.
A running total starts at 0 and grows one value at a time with total ← total + value.

The big picture

The + operator adds values in a program — two literals, two variables, or one of each — to give a result. That result is only kept if an assignment stores it in a variable. A running total starts at 0 and grows with lines like total ← total + price, where the right-hand side is worked out first and then stored.

Key points

1+ adds literals, variables, or a mix of both.

2No assignment, no stored result — outputting a sum does not change any variable.

3total ← total + 5 is an instruction, not an equation: right side first, then store.

4Only the variable on the left of the arrow changes; variables on the right are just read.

5Set a running total to 0 before you start adding into it.

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Worked example

Problem

An app tracks a swimmer's laps. Trace this program and give the values of laps and bonus after each line. laps ← 3 bonus ← laps + 4 laps ← laps + bonus bonus ← bonus + 1

⚠ Watch out

Writing a sum without storing it — such as OUTPUT total + price — and then expecting total to have changed. Only a line that assigns into total changes total.

🧠

Memory hook

Work it out on the right, put it away on the left. A sum that's never put away is lost.

✓

Check yourself

x holds 8. What does x hold after x ← x + x + 1 runs? (Answer: 17 — work out 8 + 8 + 1 first, then store it in x.)

Flashcards

(7)

What does the + operator do in a program?

It adds two values together to give a result.

What can go either side of + ?

A literal (like 3), a variable (like price), or one of each: 3 + 4, a + b, price + 2.

What is an assignment?

An instruction that stores a value in a variable, e.g. total ← 5. Python writes it total = 5.

After x ← y + 2 runs, has y changed?

No. y is only read on the right-hand side. Only the variable on the left changes.

Why isn't total ← total + 1 an impossible equation?

It's an instruction, not a claim that two things are equal. It means 'total becomes its current value plus 1'.

What is a running total?

A variable that starts at 0 and has each new value added into it in turn, with total ← total + value.

What happens to a sum that is output but never stored?

It appears on screen and is then gone. No variable changes, so the program can't use it later.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Follow the score, one line at a time

The line that looks impossible

Where did the 3 go?

Write the code for a cinema booking

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