The worksheetDownload the PDF
Answers

F4.3 Local and global variables

Functions and structured code · GCSE · OCR J277 2.2.3, AQA 8525 3.2.10, Edexcel 1CP2 6.6.3 · about 15 min

BugBotLab

What this lesson is about

Scope: where a variable exists, and why passing values beats global.

Questions 6 marks in all

  1. [1 mark]A function sets gap = 10 inside its body. After calling it, the program runs print(gap). What happens?

    1. ANameError: gap only exists while the function runs
    2. BIt prints 10
    3. CIt prints gap
    4. DIt prints nothing
    Answer: A. A variable made inside a function belongs to it. If the outside needs the value, return it.

  2. [1 mark]What does this program print?

    speed = 80

def gentle():
    speed = 30
    print(speed)

gentle()
print(speed)
    Answer:
    30
80

    The speed inside the function is a new local variable, so the global speed is unchanged.

  3. [1 mark]beeps = 0 outside, and inside a function beeps = beeps + 1 with no global line. What happens when the function runs?

    1. AUnboundLocalError: assigning makes beeps local, and it has no value yet
    2. Bbeeps becomes 1
    3. CThe global beeps becomes 1
    4. DNameError
    Answer: A. Assigning to a name anywhere in a function makes it local throughout that function.

  4. [1 mark]Which are reasons local variables are good practice?

    Tick every answer that is true.

    1. AA subprogram can be understood and tested on its own
    2. BThe same names can be used safely in different subprograms
    3. CThe memory is freed when the subprogram ends
    4. DThey make the program run without errors
    Answer: A, B, C. Local variables keep subprograms self-contained. They do not stop errors on their own.

  5. [1 mark]What does this program print?

    count = 0

def add():
    global count
    count = count + 1

add()
add()
print(count)
    Answer:
    2

    global count means the function changes the global variable, so two calls make it 2.

  6. [1 mark]What is the scope of a variable?

    1. AThe part of the program where it can be used
    2. BThe largest value it can hold
    3. CIts data type
    4. DHow long its name is
    Answer: A. A local variable's scope is its subprogram; a global variable's is the whole program.

The task: count the beeps

This program should beep three times and print beeps: 3, but it crashes. Fix it, with parameters and return or with global.

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

beeps = 0

def beep():
    tone(880, 0.1)
    wait(0.1)
    beeps = beeps + 1

beep()
beep()
beep()
print("beeps:", beeps)

The hint students can ask for: A function cannot change a variable made outside it unless you say so. Either pass the count in and hand the new one back, or declare the outside variable global at the top of the function. Pick one and keep it consistent.

A solution

from bugbot import *
connect()
def beep(count):
    tone(880, 0.1)
    wait(0.1)
    return count + 1

beeps = 0
beeps = beep(beeps)
beeps = beep(beeps)
beeps = beep(beeps)
print("beeps:", beeps)

Any program that meets the task's checks is marked correct in the simulator; this is one way, not the only way.