Choosing a variable or a constant

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Quick Questions

Shade in the A or B circle for each question.

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

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

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

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 ← ""

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

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		ACONSTANT Pi ← 3.142
2DECLARE Score : INTEGER		BA fixed value that is set once and never changes while the program runs
3Pi never changes while the program runs, so store it as a constant		CThe 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.
4The user's answer is typed in and can be different each time, so store it in a variable		DDECLARE Answer : STRING INPUT Answer
5DECLARE VATRate : REAL VATRate ← 0.2		EA variable for a value that can change as the program runs, such as a player's score
6CONSTANT Score ← 0 Score ← Score + 10		FA score changes as the program runs, so it must be a variable (DECLARE Score : INTEGER), because a constant cannot be updated.

Definitions

CONSTANT

Fill in the Blanks

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

________ VATRate ← 0.2

_______ Score : INTEGER

CONSTANT __ ← 3.14

DECLARE Answer : STRING INPUT ______

CONSTANT VATRate ← 0.2 Total ← Price * (1 + _______)

Spot the Error

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

Question 1

A CONSTANT VATRate ← 0.2

B CONSTANT Price ← 0.0

C INPUT Price

D OUTPUT Price * (1 + VATRate)

Question 2

A DECLARE Price : REAL

B DECLARE VATRate : REAL

C INPUT Price

D OUTPUT Price * (1 + VATRate)

Question 3

A CONSTANT MaxLives ← 3

B DECLARE Lives : INTEGER

C MaxLives ← Lives

D Lives ← Lives - 1

Question 4

A DECLARE Answer : STRING

B CONSTANT Answer ← "yes"

C OUTPUT "You typed ", Answer

Question 5

A DECLARE Score : INTEGER

B Score ← 0

C CONSTANT Score ← Score + 1

D OUTPUT Score

Answer Key

Choosing a variable or a constant

Section A: Quick Questions

B — A constant: CONSTANT VATRate ← 0.2

A — A constant, because pi never changes

B — A variable, because its value changes

A — A variable: DECLARE Answer : STRING

A — The value is fixed while the program runs

Section B: 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.

Section C: Definitions

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.

Section D: Fill in the Blanks

CONSTANT VATRate ← 0.2

DECLARE Score : INTEGER

CONSTANT Pi ← 3.14

DECLARE Answer : STRING
INPUT Answer

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

Section E: Spot the Error

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

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

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

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.

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.