GCSE Computer Science Theory

Scope

Basics

• Scope refers to the visibility of variables - in other words, in which parts of your program can see or use it. Normally, every variable has a global scope. Once defined, every part of your program can access that variable. However, it is very useful to be able to limit a variable’s scope to a single function. In other words, the variable will have a limited local scope. This way, changes inside the function can’t affect the main program in unexpected ways
• The scope of variables can be local or global.
  
  • Local variables only work in the loop, procedure or class in which they are created whilst
    global variables can be accessed from any point in a program.
• For example:

OUTPUT "Here are the even numbers up to 100"  
FOR i <-- 1 TO 100 
    IF i MOD 2 = 0 
        OUTPUT i
    ENDIF 
ENDFOR 
OUTPUT "Finished"

The variable i has scope between lines 2 and 6. This is called block scope because i can only be used within the block of the for loop.
If a programmer were to try and use the variable before line 2 or after line 6, there would be a run-time error.

• A variable has global scope if it can be accessed from anywhere in a project.

X <-- "test value"

FUNCTION message()
    OUTPUT X
ENDFUNCTION

FUNCTION changeMessage()
    OUTPUT "The message is:" + X
    X <-- INPUT "What new message would you like?"
ENDFUNCTION

Variables with global scope are usually considered bad practice in most languages, due to the increased risk of name collisions.
- Function scope is similar:

FUNCTION square(n)
    RETURN n * n
ENDFUNCTION

FUNCTION sumSquares(n)
    total <-- 0
    i <-- 0
    WHILE i<= n:
        total <-- total + square(i)
        i <-- i + 1
    ENDWHILE
    RETURN total
ENDFUNCTION

Task

Write a program for a user to input to input two numbers, then write the result of them being added together. The result from this should then be multiplied by three.

Do this in three ways, using function, module and global scopes.

Python One Liner:
print(sum([int(input('Input {} number'.format(i))) for i in ['first', 'second']]) * 3)


Practice Questions

June 2014

Q1>

The variable i in Figure 1 only has scope between lines 3 and 7. Explain with reference to the variable i what scope means.
[1 mark]

Q2>

Fig 2.

    VAT = 0.20
    def calculations(amount):
        tax = (VAT/amount)
        print (tax)    
        totalCost = amount + tax
        return totalCost
    total = float(input("How much does the item cost? ")
    print (calculations(total))

2a

What does scope mean?

2b

What is the scope of VAT?

2c

What is the scope of tax?

2d

What is the scope of totalCost?

2e

What is the scope of total?

2f

Why would it necessary for a variable to have limited scope?