Global & Local Variables (OCR A Level Computer Science): Revision Note

Global Variables

What is a global variable?

• A global variable is a variable declared at the outermost level of a program

• This means that they are declared outside any modules such as functions or procedures.  

• Global variables have a global scope, which means they can be accessed and modified from any part of the program.

Python example

In this python code, you can see that the globalVariable (with the value 10) is declared outside of the function printValue. This means that this function and any other modules can access and change the value in the global variable. 

globalVariable = 10 # Defines a global variable

def printValue():
    global globalVariable # Access the global variable inside a function
    print("The value into the variable is:", globalVariable)

printValue() # Call the function

Usage & need for global variables

• Data Sharing: Global variables facilitate data sharing between different parts of the program, allowing data to be passed between different modules easily without the use of parameter passing. 

• Persistent Storage: Global variables retain their values throughout the program's execution, making them suitable for storing data that needs to persist across function calls

• Global Configuration: Global variables can be used to store configuration settings or constants that are relevant across the entire program

Benefits and drawbacks

Local Variables

What is a local variable?

• A local variable is a variable declared within a specific scope, such as a function or a code block

• Local variables are accessible only within the block in which they are defined, and their lifetime is limited to that particular block

• Once the execution of the block ends, the local variable is destroyed, and its memory is released 

Python example

In this python code, you can see that the localVariable (with the value 10) is declared inside of the function printValue. This means that only this function can access and change the value in the local variable. It cannot be accessed by other modules in the program. 

def printValue():
    localVariable = 10  # Defines a local variable inside the function
    print("The value of the local variable is:", localVariable)

printValue()  # Call the function

Benefits and drawbacks

Converting Between Global and Local Variables

• In this Python code below, a global variable named global_string is declared and initialised with the string "Hello"

• Two functions are defined, add_world and add_name

• The add_world function appends the string ", world!" to global_string

• The add_name function takes a string name as an argument and appends it to global_string

• add_world is called and prints global_string, which results in "Hello, world!"

• After that, add_name is called with the argument "Alice" and prints global_string again, which results in "Hello, world! Alice"

# Global variable
global_string = "Hello"

def add_world():
    global global_string
    global_string += ", world!"

def add_name(name):
    global global_string
    global_string += " " + name

# Call the functions
add_world()
print(global_string)  # Outputs: Hello, world!

add_name("Alice")
print(global_string)  # Outputs: Hello, world! Alice

• In this revised Python code, a local variable named local_string is declared and initialised with the string "Hello"

• Two functions are defined, add_world and add_name

• The add_world function takes a string s as an argument, appends the string ", world!" to it and returns the modified string

• The add_name function takes two arguments: a string s and a string name. It appends name to s and returns the modified string

• add_world is called with local_string as an argument and assigns the result back to local_string. local_string is printed, which resulted in "Hello, world!"

• add_name is called with local_string and "Alice" as arguments and assigns the result back to local_string. local_string is printed again, which results in "Hello, world! Alice"

def add_world(s):
    s += ", world!"
    return s

def add_name(s, name):
    s += " " + name
    return s

# Initialize a local variable
local_string = "Hello"

# Call the functions
local_string = add_world(local_string)
print(local_string)  # Outputs: Hello, world!

local_string = add_name(local_string, "Alice")
print(local_string)  # Outputs: Hello, world! Alice