Why use constants in a program

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Quick Questions

Shade in the A or B circle for each question.

1. Why is CONSTANT VatRate ← 0.2 clearer than writing 0.2 in every calculation?

A The name VatRate tells the reader what the value means

B The name VatRate makes the program run faster

2. The VAT rate changes from 0.2 to 0.25. The program uses the constant VatRate. How many places must you edit?

A One: the CONSTANT line

B Every line that uses VatRate

3. Which line is better for a price calculation?

A Total ← Price + Price * VatRate

B Total ← Price + Price * 0.2

4. A program has the number 0.2 typed in 15 different lines. The rate changes. What is the risk?

A The program will refuse to start

B You might miss one line, so some results use the old rate

5. What is a magic number?

A A number that always equals zero

B A bare value in code with no name to explain its meaning

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1CONSTANT VatRate ← 0.2		AGives the number 0.2 a meaningful name, so later lines can use VatRate instead of an unexplained magic number
2Total ← Price * VatRate		BCONSTANT VatRate ← 0.25
3A magic number in a calculation is hard to understand, because the reader cannot tell what 0.2 stands for		CThe rate 0.2 is typed as a magic number on every line. When the rate changes, every line must be edited, which is slow and easy to get wrong.
4Changing a rate once: edit only the CONSTANT line and every calculation using the name gets the new value		DUses the named constant in a calculation, so if the rate changes only the CONSTANT line needs editing
5Tax1 ← Price1 * 0.2 Tax2 ← Price2 * 0.2 Tax3 ← Price3 * 0.2		EOne line was missed when the rate was updated, so the program uses two different rates and gives wrong results.
6Tax1 ← Price1 * 0.25 Tax2 ← Price2 * 0.2 Tax3 ← Price3 * 0.25		FTax ← Price * 0.2

Definitions

CONSTANT VatRate ← 0.2

Fill in the Blanks

Complete each line of code by filling in the missing parts.

________ VatRate ← 0.2 Tax ← Price * VatRate

CONSTANT VatRate _ 0.2 Tax ← Price * VatRate

CONSTANT _______ ← 0.2 Tax ← Price * VatRate OUTPUT Tax

CONSTANT VatRate ← 0.2 Tax ← Price _ VatRate

CONSTANT VatRate ← 0.25 Tax1 ← Price1 * _______ Tax2 ← Price2 * VatRate

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A DECLARE Price : REAL

B CONSTANT VatRate = 0.2

C INPUT Price

D OUTPUT Price * VatRate

Question 2

A CONSTANT PassMark ← 40

B IF Score >= 40

C THEN

D OUTPUT "Pass"

E ENDIF

Question 3

A CONSTANT VatRate ← 0.2

B INPUT Price

C Total ← Price * (1 + 0.2)

D OUTPUT Total

Question 4

A DECLARE MaxScore ← 50

B INPUT Mark

C Pct ← Mark / MaxScore * 100

D OUTPUT Pct

Question 5

A CONSTANT Rate ← 0.05

B INPUT Savings

C Interest ← Savings + Rate

D OUTPUT Interest

Answer Key

Why use constants in a program

Section A: Quick Questions

A — The name VatRate tells the reader what the value means

A — One: the CONSTANT line

A — Total ← Price + Price * VatRate

B — You might miss one line, so some results use the old rate

B — A bare value in code with no name to explain its meaning

Section B: Match the Pairs

CONSTANT VatRate ← 0.2 → Gives the number 0.2 a meaningful name, so later lines can use VatRate instead of an unexplained magic number

Total ← Price * VatRate → Uses the named constant in a calculation, so if the rate changes only the CONSTANT line needs editing

A magic number in a calculation is hard to understand, because the reader cannot tell what 0.2 stands for → Tax ← Price * 0.2

Changing a rate once: edit only the CONSTANT line and every calculation using the name gets the new value → CONSTANT VatRate ← 0.25

Tax1 ← Price1 * 0.2 Tax2 ← Price2 * 0.2 Tax3 ← Price3 * 0.2 → The rate 0.2 is typed as a magic number on every line. When the rate changes, every line must be edited, which is slow and easy to get wrong.

Tax1 ← Price1 * 0.25 Tax2 ← Price2 * 0.2 Tax3 ← Price3 * 0.25 → One line was missed when the rate was updated, so the program uses two different rates and gives wrong results.

Section C: Definitions

CONSTANT VatRate ← 0.2 — A named value used instead of a magic number. It makes code easier to read, and when the value changes you edit it in one place only, so no missed edits cause mistakes.

Section D: Fill in the Blanks

CONSTANT VatRate ← 0.2
Tax ← Price * VatRate

CONSTANT VatRate ← 0.2
Tax ← Price * VatRate

CONSTANT VatRate ← 0.2
Tax ← Price * VatRate
OUTPUT Tax

CONSTANT VatRate ← 0.2
Tax ← Price * VatRate

CONSTANT VatRate ← 0.25
Tax1 ← Price1 * VatRate
Tax2 ← Price2 * VatRate

Section E: Spot the Error

B — A constant is given its value with the ← operator, not =. It should be CONSTANT VatRate ← 0.2, so the named value is set in one place.

B — 40 is a magic number here. The check should use the constant PassMark, so if the pass mark changes it only needs editing on line 1.

C — The rate 0.2 is typed directly instead of using VatRate. If the VAT rate changes, this line could be missed. It should use VatRate.

A — MaxScore never changes, so it should be declared with CONSTANT, not DECLARE. A constant keeps the fixed value in one named place.

C — Interest is found by multiplying the savings by the rate, not adding. It should be Savings * Rate, using the constant Rate.