3.1.5 Scope

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1 The Scope of variables, constants, functions and procedures

Learn It

  • The scope of a variable is the range of code that it applies to.
  • You can think of scope as applying to the visibility of a variable.
  • A variable is in scope if it can be seen by a particular line of code.

Learn It - Block Scope

  • A variable with block scope is only valid within a particular block of code such as a for loop.
  • Look at the example below.
OUTPUT "Here are the even numbers up to 100"
FOR i <-- 1 TO 100
IF i MOD 2 = 0
OUTPUT "Finished"
  • The variable i has scope between lines 2 and 6.
  • If a programmer was to try and use the variable before line 2 or after line 6, there would be a run-time error.

Learn It - Function/Procedure scope

  • A variable with function scope is only valid within a particular function.
  • Look at the example below.
FUNCTION square(n)
RETURN n * n

FUNCTION sumSquares(n)
total <- 0 
i <- 0
WHILE i <= n:
total <- total + square(i)
i <- i + 1
RETURN total
  • Here we have two functions, both using a variable called n.
  • Because the two variables both have function scope, they are completely unrelated.
  • There is no risk of name collision when using the identifier n in either of the two functions.
  • Similarly, the second function contains variable identifiers i and total. As these both have scope only within the sumSquares function, the identifiers can be used elsewhere in the program, with no risk of altering the expected return of the function.

Learn It - Module Scope

  • Variables have module scope if they can be accessed by any line in a file or once a module has been imported.
  • This is tricky to show in Pseudocode, so we'll switch to Python 3.
  • Image we have a file called foo.py
bar = 'Hello'

def outputModuleVar:
  • The variable bar can be viewed (but not necessarily altered) by any part of the the file foo.py
  • We could also create a second file called baz.py
import foo
  • Because the variable bar has module scope, it can be accessed by any file that imports foo.py

Learn It - Global Scope

  • A variable has global scope if it can be accessed from anywhere in a project.
  • Variables with global scope are usually considered bad practice in most languages, due to the increased risk of name collisions.

Document It

  • In your chosen programming language, write a script that uses block, function and module scope for it's variables.
  • Take a screenshot of your code, and then annotate it in a graphics editor, identifying which variables have which scope.

Try It

  • In pseudocode, write two small scripts, that use block, function and module scope.
  • Give the code to a peer, and ask them to annotate and identify the scope of the variables.