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So let's talk about using Do‑While and Do‑Until as

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part of a loop within PowerShell.

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So the first thing to understand is what is a loop?

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Well,

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a loop is a programming and a scripting language construct that allows

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you to process a sequence of instructions inside code.

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It also allows iteration of several items or several times,

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depending on how you're iterating, as long as the condition is met.

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So, for example,

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let's say you want to see the number of files in a

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folder that can execute and say, go and get me the folder,

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iterate through the folder, identify the items,

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and then once you've identified all of them,

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then stop.

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Or it could be, count a certain amount and then write those values out.

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Now there are specific loop types that are available to us.

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The first one is what's referred to as a For loop.

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So a For loop is known also as a For statement,

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and it's typically used to iterate through a set of

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commands a specified number of times.

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You then also have what's called a While.

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This repeats a command or a group of commands where the given condition is true.

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Then, of course, we have the Do‑While.

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So just like the While loop,

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it will execute the block of statements while the condition evaluates to a true,

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so do while this is true.

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But there's a small difference.

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That is the body of the Do‑While is executed first,

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then the condition gets tested at the end,

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whereas the Do‑Until have a similar syntax to the Do‑While

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except the condition is evaluated first.

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So think of that logic.

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Iterate through these items, compare the value at the bottom,

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or compare the value at the top that you've got,

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and then continue.

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We also then have what's referred to as a Foreach‑Object.

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So this is a cmdlet, not a loop, but just like the Foreach statement,

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which is another one,

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it iterates through each object in a collection and performs operations.

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Then we have what's called the Foreach statement, or the Foreach loop.

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This is used to iterate through values in a collection of items,

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for example, in an array, and then perform actions against each item.

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And then, of course, we have an odd one, which is the Foreach() method.

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So you can specify a collection or an array.

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So let's say we create a variable called at variable

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and populate it with numbers 1 to 10.

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We can then use the Foreach() method,

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which effectively is accessed by going to the end of the variable,

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type in a period, and then using Foreach(),

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and that can then go through and iterate as well.

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Now,

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we need to understand the difference between an entry and an exit

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loop before we can choose which type to use.

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So an entry loop tests the condition is checked before it enters into the loop.

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So that's your first one.

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The entry loop is, does this value do something?

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If so, then continue the loop.

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Whereas an exit loop is keep looping and then check the

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value at the end and drop out that way.

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So what does that look like?

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Well, let's look at the difference between the two.

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So I have a variable, which is number 1,

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and then I have what's referred to as an entry loop,

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so this is using a While loop.

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So what you can see here, it says, While $number,

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so 1, is less than 10, then perform your specific actions.

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So in this instance,

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it's going to just increment the number by one and keep

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going until or while it's less than 10.

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So if my number in the right‑hand side, which is $number++,

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increments up to 10,

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it'll then stop working at that point because it's no longer less than 10.

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If we do the Do option, you can see it flips the syntax the other way around.

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So it now says, here's my number, increment the number,

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and while that implemented number is less than 10,

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continue to happen.

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Now you may look at that and think, well,

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that's okay, it looks very similar,

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but it can have completely different effects on the

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PowerShell output that we're using.

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Now the Do‑While loop is almost the same as a regular While loop.

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It executes the block of statements while the condition returns true.

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The Do‑While executes first, then the condition test is at the end of the loop.

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Now for the Do‑While, the syntax can be very similar.

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So you can see it'll say do something while this condition is met.

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So if we have a variable again with a number 1,

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you can see that it looks very similar to the Do‑While.

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So Do $number, which would say number 1,

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increment number 2 while that number that you've incremented is less than 10.

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So that would then continue to run until that point.

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So what are the differences between a While and a Do‑While loop?

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Well, if we compare them, the While loop is an entry controlled loop,

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so it comes in at the point here and then tests the condition

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before the execution of the first loop or iteration.

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The loop doesn't execute when the condition evaluates to false.

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So, remember the less than 10?

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Once it reaches 10, it's no longer less than, so the loop continues to execute.

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And it doesn't require any other syntax to be able to make that work.

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The Do‑While loop is an exit controlled loop.

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It tests the condition after the execution of the first iteration.

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So it will keep going down, and until it reaches that specific number,

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it won't stop.

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The loop executes at least once every single time.

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So whereas the While loop can actually evaluate to false

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immediately and then never execute, the Do‑While will always execute once.

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It uses the Do keyword at the starting of the loop and then a

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While keyword with a condition at the end.

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Now the Do‑Until loop is very similar to a Do‑While loop.

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The loop will repeat until the condition returns true,

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and these are the opposite of a Do‑While loop.

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Now that syntax is very similar.

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Instead of it saying Do‑While, it's Do‑Until,

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so do this command until this condition is met.

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So if we use our standard variable, $number = 1,

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and you can see we have a Do‑Until.

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So, think how this works.

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Do 1.

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Increase the number by one until it's less than or

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greater than or something else.

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Now notice it would be different, so Do‑Until the number is less than 10.

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Sounds like a strange way of doing it, but that's what I said.

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Depending on the looping mechanism that you use will have different

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effects on the output and what you want to achieve.

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Now the Do‑While loop is an exit controlled loop.

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It uses the While keyword for the condition and then

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executes when the condition returns true.

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The Do‑Until loop is an exit controlled loop.

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It uses the until keyword and then continues execution until the condition returns true.