Scope

scope:

• where does a given declaration apply?
• what declarations apply at a given point?

scope of a declaration:

• from where it is made, to the end of the subprogram that contains it

"cannot see in; can see out"

Which names are in scope?

can refer to:

• names imported via WITH (imports)
• names declared in procedures that contain the current procedure (globals)
• locally declared names, parameters (locals)

local declaration masks global declaration of same name

AVOID global variables !

An example shows the visibility of each name defined in a program, at different points throughout the program.

Another example also shows the visibility of each name defined in a program, at different points throughout the program. It also shows the the output produced when the program is run. You need to understand the scope rules in order to understand the output.

Scope - good practice

localise scope as far as possible

procedures

• task/subtask: local
• common operation: global

constants

• used in only 1 part of the program: local
• used in >= 2 parts of the program: global

variables

• never use globals for external values
• always use parameters for external values
• local variables for internal use