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F4.1 Writing functions

Functions and structured code · GCSE · OCR J277 2.2.3, AQA 8525 3.2.10, Edexcel 1CP2 1.1.2 · about 12 min

What this lesson is about

def, calling, and why subprograms help.

• Defining a function
• Define first, then call
• What happens when a function is called
• A function can call other functions
• Why write functions?

Questions 6 marks in all

1. [1 mark]What happens when Python reaches def report(): and its indented body?

   1. AIt defines the function, but nothing in the body runs yet
   2. BIt runs the body once
   3. CIt runs the body forever
   4. DIt prints report
   Answer: A. Defining is like writing a recipe. The body runs when the function is called: report().

2. [1 mark]Which are good reasons to write a function?

   Tick every answer that is true.

   1. AWrite a piece of behaviour once and reuse it
   2. BA name like is_clear(40) explains what the code does
   3. CSmall pieces are easier to test
   4. DIt makes the robot faster
   Answer: A, B, C. Functions stop repetition, name ideas and split a program into testable pieces. Speed is not the reason.

3. [1 mark]What does this program print?

   def signal():
       print("  beep")
   
   print("before")
   signal()
   print("between")
   signal()

   Answer:

   before
     beep
   between
     beep

   At each call the program jumps into the function, runs its body, and comes back to the next line.

4. [1 mark]A program calls signal() on line 3 and defines def signal(): on line 5. What happens?

   1. ANameError: the function is not defined yet when line 3 runs
   2. BIt works, Python looks ahead
   3. CSyntaxError
   4. DNothing is printed and there is no error
   Answer: A. Python runs top to bottom, so a function must be defined before the line that calls it.

5. [1 mark]What is a subprogram?

   1. AA named block of code that does a task and can be called from elsewhere in the program
   2. BA program that runs on another computer
   3. CA comment above some code
   4. DThe first line of a program
   Answer: A. Procedures and functions are both subprograms.

6. [1 mark]In OCR's Exam Reference Language, which word ends a procedure? (one word)

   Answer: endprocedure. procedure name() ... endprocedure. A function ends with endfunction.

The task: signal at the corners

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

def signal():
    led("red")

forward(60, distance=25)
turn_right(30, angle=90)

from bugbot import *
connect()
def signal():
    led("red")
    tone(880, 0.2)
    led("off")

for i in range(4):
    forward(60, distance=25)
    signal()
    turn_right(30, angle=90)