1. Functions, Parameters and Scope
PowerShell
Functions, Parameters and Scope
Copyright © 2016 – Curt Hill

2. Introduction
Although we can define functions in PowerShell – there is little need for them
Each file is considered a procedure
No need for a header to define
We do have to consider the notion of parameters as well as scope
Copyright © 2016 – Curt Hill

3. Parameter Connection
In C++ there is only one way to connect a actual and formal parameter
This is the position in the parameter list void xyz(int a, float b){…} … xyz(4, z/2)
The first actual (4) connects to the first formal (a)
The second actual (result of z/2) connects with second formal (b)
Copyright © 2016 – Curt Hill

4. The Other Way
In several languages, such as Ada and JavaScript, named or keyword parameters are allowed
As well as positional parameters
In a keyword parameter the call may specify the parameter name and then the value
Often positional parameters are used for the required values and keyword parameters for the defaulted ones
Copyright © 2016 – Curt Hill

5. Parameters
Recall that in the batch language there was no declaration of parameters
Instead they were numbered: %1, %2 …
PowerShell is different, there is a declaration
This declaration is started with Param and followed with a parenthesized list of the parameters
Copyright © 2016 – Curt Hill

6. The Parameter Set
The form is: Param ( [Parameter(options)] [type] name, … [Parameter(options)] [type] name )
There may one or more parameters
Several of these pieces are optional
Copyright © 2016 – Curt Hill

7. Using Parameters
Positional parameters are given following the name of the script
In the same order as the parameter block
They follow the script name
Keyword parameters have the form: -name value
Where –name is the name given in parameter block
We now look at examples
Copyright © 2016 – Curt Hill

8. Example 1
Consider the beginning of a script named scr: Param( [string]$spec, [string]$fn ) $files = get-childitem $spec
This can be called by: scr *.ppt ppt_size.txt
Or by: scr -fn pptsz.txt -spec *.ppt
Copyright © 2016 – Curt Hill

9. Example 2 – Optional Type
The [String] that precedes the parameter is optional
Thus the parameter block could be: Param( $spec, $fn )
The line orientation is free as well: Param($spec,$fn)
Multiple lines are often more readable
Copyright © 2016 – Curt Hill

10. Example 3 - Defaults
Parameters may have default values by appending an = and the value
For example: Param($spec,$fn=“sizes.txt”)
Now it may be called with one parameter: scr *.zip scr –spec *.zip
Copyright © 2016 – Curt Hill

11. Example 4 - Mandatory A parameter left out becomes void
Empty string
We may use the options to require the parameter: Param( [Parameter(Mandatory=$True)] $spec, $fn="sizes.txt“ )
Now if called with no parameters it asks for the value of Spec
Copyright © 2016 – Curt Hill

12. Switch Besides the string type there is a switch type for parameters
By default a switch type is false
Only set to true if the parameter is present
Copyright © 2016 – Curt Hill

13. Example 5
Suppose the Script parameter: Param( [Parameter(Mandatory=$True)] $spec, $fn="sizes.txt", [switch]$announce ) … if($announce){ …
If –announce is in call then $announce is true otherwise false
Copyright © 2016 – Curt Hill

14. Example 6 – Position
Normally the position of the parameters must follow the order in the parameter set
This may be changed by the position option Param( $fn="sizes.txt", [Parameter( position = 1)] $spec, [switch]$announce)
$spec must still be first
Copyright © 2016 – Curt Hill

15. Functions
A function may be defined using the following form: function fname { … }
Where fname is the name of the function
If you wish to use parameters then a param block is included
Just like a script
Calling a function has the same form as calling a script file
Once a function is defined the name is retained for the duration of the session
Copyright © 2016 – Curt Hill

16. Variables Again Variable is an object in regards to cmdlets
The following cmdlets are available
Clear-variable
Get-variable
New-variable
Remove-variable
Set-variable
You should understand what each of these do
Copyright © 2016 – Curt Hill

17. Scope You will recall scope in C++ This is not the way of PowerShell
Every {} opens a new block
Search current block
If not found exit to enclosing block
Keep going until global scope or found it
This is not the way of PowerShell
Copyright © 2016 – Curt Hill

18. Scope Levels PowerShell believes in several different scopes
Global – the parent of all others
Local – the current scope
May be global, part of session or part of a script
Script – Each script file has its own scope
Copyright © 2016 – Curt Hill

19. Accessing A session which starts a script has the parent scope
It is part of the global scope
Each script that starts another script has the parent scope
It is the child of a session or another script
Copyright © 2016 – Curt Hill

20. Get-variable Several parameters of interest
The first parameter can be the name of a variable
Without the $
The –scope parameter shows all those variable in that scope
Values include global, local
Many of the variable cmdlets also allow you to specify scope
Copyright © 2016 – Curt Hill

21. Finally This should do it for us Now how about an exercise?
Copyright © 2016 – Curt Hill