← VCE Software Development U3O1

Program structure with a main() function

Defines a function called main that holds the main steps of your program. You call main() once at the end of the file, after the other functions are defined, to start the program.

Part of: VCE Software Development - U3O1 - Programming — Module 86, VCE Software Development - U4O1 - development and evaluation — Module 66, 7-8 Digital Technologies: Programming in Python — Module 32, VCE Applied Computing U1O2 Programming: Procedures and functions — Module 7, ACARA Digital Technologies Years 7–8 — Programming with Control Structures and Functions (Python) — Module 54, ACARA Digital Technologies Years 9–10 — Modular and Object-Oriented Programming (Python) — Module 14, QCE Digital Solutions Unit 1 — Algorithms and Programming Techniques — Module 67, QCE Digital Solutions Unit 1 — Programmed Solutions — Module 46, QCE Digital Solutions Unit 2 — Prototype Data Solutions — Module 73, QCE Digital Solutions Unit 3 — Innovative Digital Solutions — Module 59, QCE Digital Solutions Unit 4 — Complex Digital Data Exchange Problems and Solution Requirements — Module 100, QCE Digital Solutions Unit 4 — Prototype Digital Data Exchanges — Module 32, WA Computer Science ATAR Year 11 — Modular Programming — Module 14

Understanding Program structure with a main() function in VCE Software Development U3O1

What it does: greet is defined before main, so main can call it. Functions above main are ready to use. The call main() sits at the bottom with no indent. It runs the function once and starts the program. A def line must end with a colon. Without the colon you get a syntax error. The main function holds the main steps, such as asking for input, calling other functions and printing results. Defining main() only saves the steps. You must call main() to run them. The second option never calls it, so nothing prints. Indentation shows which lines are inside the function. Lines that are not indented are outside main.

Common mistakes: The first line of a function definition must end with a colon. It should be def main():. Python is case-sensitive, so Print is not the same as print. The step inside main() should be print('Start').

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This module's key terms are also in a printable crossword (Modules 72–96) and word search (Modules 72–113).

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Learning

Duo

Where should you call main() in a simple Python program?

At the end of the file, after the functions are definedAt the very top, before any functions are defined

Matches

def main(): print("Start")

↓

main() is never called, so the program does nothing. Add main() at the end of the file with no indent.

Applying

Meanings

Term

def main():

Meaning

Defines a function called main that holds the main steps of your program. You call main() once at the end of the file, after the other functions are defined, to start the program.

Practice

def greet(name):

Testing

Gaps

main():

Check Answer

Spots

1def main()

2 name = input('Name? ')

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Quick Questions

1. Where should you call main() in a simple Python program?

A. At the very top, before any functions are defined

B. At the end of the file, after the functions are defined ✓

Python reads from top to bottom. If main() is called before the functions it uses are defined, you get a NameError. Call main() last.

2. Which line correctly defines a main function?

A. def main()

B. def main(): ✓

A def line must end with a colon. Without the colon you get a syntax error.

3. What does main() hold in a well-structured program?

A. The main steps of the program ✓

B. Only the comments for the program

The main function holds the main steps, such as asking for input, calling other functions and printing results.

4. Which code actually starts the program?

A. def main(): print("Hi") main() ✓

B. def main(): print("Hi")

Defining main() only saves the steps. You must call main() to run them. The second option never calls it, so nothing prints.

5. True or false: you should call main() once at the end of the file.

A. True ✓

B. False

You call main() once to start the program. Calling it again would run all the steps again.

Match the Pairs

def main(): → Defines the main function that holds the main steps of the program

main() → Calls the main function once at the end of the file to start the program

Helper functions are defined above main, so they already exist when main runs → def greet(): print("Hi!") def main(): greet()

The program starts by calling main() once at the very end of the file, with no indent → def main(): print("Start") main()

def main(): print("Start") → main() is never called, so the program does nothing. Add main() at the end of the file with no indent.

main() def greet(): print("Hi!") def main(): greet() → main() is called before it is defined, which causes a NameError. Move main() to the end of the file, after all the functions.

Key Term

def main():

Defines a function called main that holds the main steps of your program. You call main() once at the end of the file, after the other functions are defined, to start the program.

Fill in the Blanks

1. def main(): print("Let's go!") main()

Hint: Type the keyword that starts a function definition

2. def main(): print("Start") main()

Hint: Type the name of the function that the last line calls to start the program

3. def main(): print("Game on") main()

Hint: Type the symbol that ends a function definition line

4. def main(): print("Hello") main()

Hint: Type the built-in function that shows text on the screen from inside main

5. def main(): print("Welcome") main()

Hint: Type the name of the function that holds the main steps of the program

Spot the Error

Question 1

A def main()

B name = input('Name? ')

C print('Hi', name)

D main()

Line A — The first line of a function definition must end with a colon. It should be def main():

Question 2

A def main():

B Print('Start')

C print('End')

D main()

Line B — Python is case-sensitive, so Print is not the same as print. The step inside main() should be print('Start')

Question 3

A import random

B define main():

C n = random.randint(1, 6)

D print(n)

E main()

Line B — Python uses the keyword def to create a function, not define. It should be def main():

Question 4

A def main():

B x = 5

C print(y)

D main()

Line C — The variable y was never created inside main(). The program made x, so it should be print(x)

Question 5

A def hello():

B print('Hello')

C def main():

D hello

E main()

Line D — Without brackets, hello only names the function and does not run it. A function call needs brackets: hello()

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