← VCE Software Development U3O1

Constants

A constant: a named value set once and never changed while the program runs. Written in UPPER_CASE by convention, it replaces magic numbers and is easy to update in one place.

Part of: VCE Software Development - U3O1 - Programming — Module 32, VCE Software Development - U4O1 - development and evaluation — Module 9, QCE Digital Solutions Unit 1 — Algorithms and Programming Techniques — Module 45, WA Computer Science ATAR Year 11 — Control Structures and Data Types — Module 7, WA Computer Science ATAR Year 11 — Modular Programming — Module 28

Understanding Constants in VCE Software Development U3O1

What it does: The 0.1 is a magic number because nobody can tell what it means. Replacing it with GST_RATE would fix this. The score variable is reassigned as the program runs. A constant such as GST_RATE is never reassigned after it is defined. By convention, constants are written in UPPER_CASE with underscores between words, so GST_RATE stands out as a value that should not change. Defining constants at the top makes them easy to find and change in one place. With a constant, one edit at the top of the program updates every line that uses it. Magic numbers must be hunted down one by one.

Common mistakes: This adds the rate (0.1) to the price instead of the GST amount. It should be total = price + gst. The constant was defined as MAX_ITEMS in upper case. Python is case-sensitive, so max_items causes a NameError. It should be MAX_ITEMS.

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This module's key terms are also in a printable crossword (Modules 10–47) and word search (Modules 10–71).

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Learning

Duo

Which line follows the Python convention for naming a constant?

GST_RATE = 0.1gstRate = 0.1

Matches

gst_rate = 0.1

↓

The constant is written in lower case. By convention constants use UPPER_CASE, such as GST_RATE, so readers know it must not change.

Applying

Meanings

Term

GST_RATE = 0.1

Meaning

A constant: a named value set once and never changed while the program runs. Written in UPPER_CASE by convention, it replaces magic numbers and is easy to update in one place.

Practice

GST_RATE = 0.1

Testing

Gaps

= 0.1

Check Answer

Spots

3gst = price * GST_RATE

4total = price + GST_RATE

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Quick Questions

1. Which line follows the Python convention for naming a constant?

A. GST_RATE = 0.1 ✓

B. gstRate = 0.1

By convention, constants are written in UPPER_CASE with underscores between words, so GST_RATE stands out as a value that should not change.

2. Where should constants normally be defined in a program?

A. At the top of the program, before they are used ✓

B. Buried in the middle of a loop

Defining constants at the top makes them easy to find and change in one place.

3. Which statement about a constant is correct?

A. Its value is set once and not changed while the program runs ✓

B. Its value changes every time the user enters data

A variable can change as the program runs. A constant is set once and then left alone.

4. Which one is best stored as a constant?

A. score = 0

B. MAX_ATTEMPTS = 3 ✓

The maximum number of attempts is fixed for the whole program. A score changes as the program runs, so it is a variable.

5. Which line avoids a 'magic number'?

A. total = price * 0.1

B. total = price * GST_RATE ✓

A magic number is an unexplained value written directly in code. A named constant such as GST_RATE tells the reader what 0.1 means.

Match the Pairs

GST_RATE = 0.1 → Defines a constant named GST_RATE with the value 0.1, set once and not changed while the program runs

total = price * GST_RATE → Uses the constant's name instead of a magic number, so the 0.1 is not hard-coded in the calculation

A magic number is an unexplained value typed directly into code, which a named constant avoids → tax = price * 0.1

A variable can change as the program runs, but a constant should stay the same → score = score + 1

gst_rate = 0.1 → The constant is written in lower case. By convention constants use UPPER_CASE, such as GST_RATE, so readers know it must not change.

GST_RATE = 0.1 GST_RATE = 0.15 → The constant is reassigned partway through the program. A constant must be set once and never changed while the program runs.

Key Term

GST_RATE = 0.1

A constant: a named value set once and never changed while the program runs. Written in UPPER_CASE by convention, it replaces magic numbers and is easy to update in one place.

Fill in the Blanks

1. GST_RATE = 0.1

Hint: Type the constant's name, written in UPPER_CASE with an underscore between the words

2. GST = 0.1

Hint: Type the operator that assigns a value to the constant's name

3. total = price * (1 + GST_RATE)

Hint: Type the constant name that replaces the magic number for the tax rate

4. if score >= PASS_MARK:

Hint: Type the UPPER_CASE constant that holds the pass mark instead of a magic number

5. seconds = minutes * SECONDS_PER_MINUTE

Hint: Type the arithmetic operator that converts minutes to seconds using the constant

Spot the Error

Question 1

A GST_RATE = 0.1

B price = 50

C gst = price * GST_RATE

D total = price + GST_RATE

E print(total)

Line D — This adds the rate (0.1) to the price instead of the GST amount. It should be total = price + gst.

Question 2

A MAX_ITEMS = 5

B items = 0

C items = items + 1

D if items > max_items:

E print("Full")

Line D — The constant was defined as MAX_ITEMS in upper case. Python is case-sensitive, so max_items causes a NameError. It should be MAX_ITEMS.

Question 3

A PI = 3.14159

B radius = 4

C area = 3.14159 * radius ** 2

D print(area)

Line C — This types the number directly (a magic number) instead of using the constant PI. It should be area = PI * radius ** 2.

Question 4

A TAX_RATE = 0.1

B price = "200"

C tax = price * TAX_RATE

D print(tax)

Line B — The quotes make price a string, so multiplying it by the float TAX_RATE causes a TypeError. It should be price = 200.

Question 5

A MAX SPEED = 100

B speed = 80

C if speed > MAX_SPEED:

D print("Too fast")

Line A — Names cannot contain spaces, so this is a syntax error. Use an underscore: MAX_SPEED = 100.

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