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Now,

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one of the main reasons for working with PowerShell is the ability for you

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to create functions that contain all of the things that we've talked about

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and allow you to build reusable components.

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So the first thing to understand is functions. What is a function?

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Well functions are literally the building block of PowerShell scripts,

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that's how you construct your code into logical reusable components.

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They are reusable throughout the entire script,

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and they can contain variables, parameters,

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statements, and even calls to other functions as needed.

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Now what we can do is we can create functions that

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use arguments and also parameters.

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Now you may wonder what the difference is so let's look at what the

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difference is between an argument and a parameter. Now arguments

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are not specified within the function,

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so there's no property that needs to be filled in.

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Arguments are populated simply by passing values at the point of execution.

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So if my PowerShell script is called testps1,

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I would call it as testps1 space and then whatever I typed as spaced values,

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they would be the arguments and the values for those arguments are

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retrieved using the offset or the ID identifier. A parameter,

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on the other hand,

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is defined as a variable in the function. Parameter values

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are populated when you call the function.

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So we would load the testps1 and then I would call a

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function that would say new message,

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and then I would have a property that I would populate with a value.

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Now, parameters can have properties and parameters can be mandatory or optional.

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So how do we create a basic PowerShell function?

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Well, here is one that we created,

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so function Get‑Answer, it's going to return me an

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answer, it's going to ask me a question,

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and then I'm going to give the answer. So you can see

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we're using a few components here.

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The first one is a variable inside the function called question,

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and then we're using something called Read‑Host.

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So I'm calling a function,

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and it's going to ask the user for a piece of information.

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It will say what is the capital city in Australia,

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which in all fairness as a side note,

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most people get wrong, it's not Sydney or anything

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else like that, it's something else.

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So the answer is clearly written in the function,

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but I'm using an if statement to render that.

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So if the question, so the returned value from the question equals Canberra,

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then that's correct. Write the message in green to the home page

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there on the front screen, or if it's incorrect,

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write the message again.

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So a basic PowerShell function is using the components

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that we already know and learned.

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So what about if we change that to use arguments because the original

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one is requesting something from the user. So what about if we allow

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them to just put a value in as part of it?

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So what we can do here is we've got this function, Get‑Answer, and

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you'll notice something different has happened.

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Now it means that somebody can type get space answer space, and then in quotes,

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they can type what they think the answer would be,

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or maybe they could type their name, or maybe they could do something else.

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So in this instance,

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I'm asking for a name. So they would type Get‑Answer

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Liam, and when the question popped up, it would say Hi,

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read the value from the offset ID from what's called args,

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$args is a default way of retrieving arguments at the end of a

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function or a call, and it would say, Hi,

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Liam, what is the capital city in Australia and then

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the same piece would then go ahead.

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Now what about if we change the PowerShell function

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to utilize variables this time?

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So once again, same piece of PowerShell, but this time we're not

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using args, we've now got Get‑Answer, and then in the parentheses

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right as you call the function is $name.

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That means this function cannot be called without passing a value.

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So to utilize this, I would have to say,

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Get‑Answer space Liam and then it would prompt me again, Hi,

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Liam, what is the capital city, and then the same process would work.

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So three different ways so far of achieving the same thing.

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So what about if we wanted to use parameters instead?

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So we're creating a function and want parameters to be

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available either mandatory or optional.

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So in this instance, I've changed the function.

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The function is called Test‑WhatIsCapitalCityofAustralia

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because I'm also not prompting with a question.

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I want to make this really easy,

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so I want someone to be able to type in

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Test‑WhatIsCapitalCityofAustralia, and then I have a parameter called

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city,

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which they can type a value, so they could type

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Test‑WhatIsCapitalCityofAustralia and then in quotes they

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could type whatever the place would be, Melbourne,

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Perth, etc, but I don't want them to type that.

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I want them to be able to tab through specific values

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when they execute this function.

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So we do what's called a ValidateSet. You can see

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I've got four options available,

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and then as they pick the right value from the variable because the

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if evaluation now changes to be check the parameter value is equal to

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what I'm needing, then it goes ahead.

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So what are some of the common function enhancements because you saw we

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did the function multiple times, so what are the enhancements? Well the

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first one is comments. We put comments in to enhance the functions so

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anybody goes in to modify or look at them,

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they can see what this function is all about. Then it could

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be error handling where we capture in or try catching etc

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errors that could take place.

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Then, of course, console messages.

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You can see that I'm using the write host and then putting a

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message out into the console in different colors.

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It could be that we use loop statements to loop around sets of

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data, or we could be using flow logic to be able to say if,

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else, catch, etc. all the way around, even switch statements.

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And then, of course,

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we can output the results into something that we wish to use.

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So let's have a look at a basic script that would iterate functions.

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So I created a new function called Get‑Files.

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The Get‑Files has a parameter called file type,

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which is going to expect me to type the file type I wish to get, so is it a

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Docx, is it an Excel, Is it a PDF, and then it calls a standard out‑of‑the‑box

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cmdlet called Get‑ChildItem, and then I have a path,

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and then it has recurse and force.

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So what we can see here is when I simply type Get‑Files space txt, it will

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then go and retrieve all the txt files, or the text files, and iterate a

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list for me in one go, so really straightforward.

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Another option could be well what about if you wanted

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to create picker values instead?

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So remember we were doing our check in the population of specific cities.

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I could also create what's referred to as a new class.

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So, for example,

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if we're looking at the cities again and saying what's the population of city A,

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B, and C, and D, etc, then I can create what's called an autocomplete

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value, a custom set of values so that when the end user goes to call that

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function and it requires a user interaction,

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they don't have to type anything, they can select from here.

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Now,

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this is what's called an IValidateSetValuesGenerator, so kind of like what

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we did with the ValidateSet where we give you picker options, this means we

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have a series of values that we can utilize.

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So what does this look like in the question function?

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Well, if we start our function out,

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test the population of Hawaii, the first thing I have is a ValidateSet,

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which is Cities, which comes from the previous function that we just looked at,

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which is generating values and then it's a variable called Answer.

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We then have the if statement that's going to look

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for the answer of 1.4 million,

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which is 1 of those items. Instead of me having to statically write

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the values in the ValidateSet, I'm just calling the object that I

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created previously, and then of course, when the end user calls it,

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they'll call Test‑PopulationOfHawaii space, then they can

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press the Tab key, it will say Answer,

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and then they can tab again and it will iterate through the

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values that are in that city's object.

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So it's all about creating something that's easy for the

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end user or even yourself to use again.

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Now, if we talk about math functions in a basic one,

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you can see that I'm defining some math operators here because I want to

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create a function that basically does my math for me, so add,

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subtract, multiply,

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divide. So you can see I've created a ValidateSet and the

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ValidateSet here, I can check the operator, so I can say if

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the mathoperator equals Add,

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then I've got another method called Invoke‑AddNumbers

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and it will add the two numbers.

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I've got one that says well elseif is equal to Subtract,

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then use the another function called Invoke‑SubtractNumbers.

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If it's a multiply,

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do the multiply, if it's a divide, do the divide. So you can see how we

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start to build these scripts that we take all the things that we've done

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so far operators equal to if, else, elseif, else, etc. and add them all together to create these scripts.