← IGCSE Computer Science Paper 2

Choosing a variable or a constant

Use CONSTANT for a value fixed while the program runs, e.g. VAT rate or pi. Use a variable (DECLARE) for a value that changes or is input, e.g. score or answer.

Part of: IGCSE Computer Science 0478 — 8.1a Data Types and Variables — Module 13

Understanding Choosing a variable or a constant in IGCSE Computer Science Paper 2

What it does: A value that stays the same throughout the program is declared with CONSTANT, as Pi is here. The VAT rate is fixed for the whole run of the program, so it should be a constant. Being a REAL number does not decide it. Pi never changes, so use a constant. A value entered by the user is not known in advance and can differ each time, so use a variable. Constants hold fixed values. Values that are input belong in variables. Lives left changes during play. The days in a week is fixed, so it suits a constant. DECLARE creates a variable, which is the right choice when the value changes during the program.

Common mistakes: Price is entered by the user, so its value changes. It must be a variable, DECLARE Price : REAL, not a CONSTANT. The VAT rate is fixed while the program runs, so it should be a constant: CONSTANT VATRate ← 0.2.

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Learning

Duo

Which should store the VAT rate of 0.2 that never changes while the program runs?

A variable: DECLARE VATRate : REALA constant: CONSTANT VATRate ← 0.2

Matches

DECLARE VATRate : REAL VATRate ← 0.2

↓

The VAT rate is fixed for the whole program, so it should be a constant (CONSTANT VATRate ← 0.2), not a variable that could be changed.

Applying

Meanings

Term

CONSTANT

Meaning

Use CONSTANT for a value fixed while the program runs, e.g. VAT rate or pi. Use a variable (DECLARE) for a value that changes or is input, e.g. score or answer.

Practice

CONSTANT VatRate ← 0.2

Testing

Gaps

VATRate ← 0.2

Check Answer

Spots

1CONSTANT VATRate ← 0.2

2CONSTANT Price ← 0.0

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Quick Questions

1. Which should store the VAT rate of 0.2 that never changes while the program runs?

A. A variable: DECLARE VATRate : REAL

B. A constant: CONSTANT VATRate ← 0.2 ✓

The VAT rate is fixed for the whole run of the program, so it should be a constant.

2. Which should store pi (3.14159) used to calculate the area of circles?

A. A constant, because pi never changes ✓

B. A variable, because it is a REAL number

Being a REAL number does not decide it. Pi never changes, so use a constant.

3. A quiz program adds 1 to the player's score after each right answer. What should score be?

A. A constant, because it starts at 0

B. A variable, because its value changes ✓

Score changes while the program runs, so it must be a variable declared with DECLARE.

4. The user types their answer to a question using INPUT. What should store it?

A. A variable: DECLARE Answer : STRING ✓

B. A constant: CONSTANT Answer ← ""

A value entered by the user is not known in advance and can differ each time, so use a variable.

5. Which statement is a correct reason for using a constant?

A. The value is fixed while the program runs ✓

B. The value will be typed in by the user

Constants hold fixed values. Values that are input belong in variables.

Match the Pairs

CONSTANT VATRate ← 0.2 → A fixed value that is set once and never changes while the program runs

DECLARE Score : INTEGER → A variable for a value that can change as the program runs, such as a player's score

Pi never changes while the program runs, so store it as a constant → CONSTANT Pi ← 3.142

The user's answer is typed in and can be different each time, so store it in a variable → DECLARE Answer : STRING INPUT Answer

DECLARE VATRate : REAL VATRate ← 0.2 → The VAT rate is fixed for the whole program, so it should be a constant (CONSTANT VATRate ← 0.2), not a variable that could be changed.

CONSTANT Score ← 0 Score ← Score + 10 → A score changes as the program runs, so it must be a variable (DECLARE Score : INTEGER), because a constant cannot be updated.

Key Term

CONSTANT

Use CONSTANT for a value fixed while the program runs, e.g. VAT rate or pi. Use a variable (DECLARE) for a value that changes or is input, e.g. score or answer.

Fill in the Blanks

1. CONSTANT VATRate ← 0.2

Hint: Type the keyword used to declare a value that stays fixed while the program runs, such as a tax rate.

2. DECLARE Score : INTEGER

Hint: Type the keyword used to declare a variable whose value changes as the program runs.

3. CONSTANT Pi ← 3.14

Hint: Type a suitable identifier for the fixed circle ratio.

4. DECLARE Answer : STRING INPUT Answer

Hint: Type the variable that stores the value the user enters.

5. CONSTANT VATRate ← 0.2 Total ← Price * (1 + VATRate)

Hint: Type the name of the fixed value that is used in the calculation.

Spot the Error

Question 1

A CONSTANT VATRate ← 0.2

B CONSTANT Price ← 0.0

C INPUT Price

D OUTPUT Price * (1 + VATRate)

Line B — Price is entered by the user, so its value changes. It must be a variable, DECLARE Price : REAL, not a CONSTANT.

Question 2

A DECLARE Price : REAL

B DECLARE VATRate : REAL

C INPUT Price

D OUTPUT Price * (1 + VATRate)

Line B — The VAT rate is fixed while the program runs, so it should be a constant: CONSTANT VATRate ← 0.2.

Question 3

A CONSTANT MaxLives ← 3

B DECLARE Lives : INTEGER

C MaxLives ← Lives

D Lives ← Lives - 1

Line C — MaxLives is a constant and cannot be assigned a new value. The changing value must go into the variable: Lives ← MaxLives.

Question 4

A DECLARE Answer : STRING

B CONSTANT Answer ← "yes"

C OUTPUT "You typed ", Answer

Line B — The answer is typed in by the user and changes each time. It needs a variable and INPUT Answer, not a CONSTANT with a fixed value.

Question 5

A DECLARE Score : INTEGER

B Score ← 0

C CONSTANT Score ← Score + 1

D OUTPUT Score

Line C — A score changes as the program runs, so it is a variable. It is updated with an assignment: Score ← Score + 1. CONSTANT cannot be used.

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