1.5 For Loops - IP
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>> Hello everyone and welcome back to
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intro to Python here on Cybrary OnDemand.
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I'm your instructor Joe Perry.
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This is the unfortunately pun based lesson,
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Lesson 4, for loops.
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Those of you who just came from
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Lesson 3 will be aware that I had
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the depressing realization that I had unintentionally
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written a pun into my lessons and that's okay,
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but that's what happens and
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I guess we just have to live with that now.
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In lesson 4, we're going to
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understand the concept of looping,
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what it actually is, what it's for,
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what it does, how we use it?
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Then we're going to learn and we're going to implement
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a specific type of loop called a for loop.
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Loops are a fundamental part of flow control.
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They're one of the things you
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implement based on Boolean logic,
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just like you do with if statements.
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Now all of programming,
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as I've said before and all of computer science
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and a huge chunk of math is in some way
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derived or derivable from
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Boolean logic but loops are
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a very specific implementation of it.
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What they do is they take a piece of code,
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they take some sequence of actions,
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what we would often call an algorithm or a set of rules,
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and they repeat that sequence of actions
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until some condition is met.
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They may apply logic to every member of a set,
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if you have a list of items,
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they might apply logic to all of that.
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They might count to a specified index.
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They might just count up by one number each time
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or by multiple numbers depending
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on how you structure it,
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or they may cycle through some series of options.
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Loops can be used to implement all sorts of things.
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In the next lesson, we'll talk about while loops,
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you'll see how programs that are always
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on or programs that last
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forever are executed and
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a lot of those will make use of while
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loops to perform what we call an infinite loop,
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or basically a loop that doesn't terminate.
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You can see the comic right above me,
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"Bob is our infinite loop specialists."
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Generally speaking, when you're creating a loop,
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you're going to give some initial value
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to help set the condition.
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We'll talk about that a little bit
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more in the next lesson as well.
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For loops in Python,
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you actually can get away with not using
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initial values pretty often.
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Just because of the way Python is structured,
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it will often fill in your special variable for
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you or fill in your conditional for you. For loops.
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An example here for all of the items in this list,
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remember I said we could apply
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logic to every member of a set,
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for all of the items in this list, the Number 1,
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the letter b,
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and the Number 150, print that item.
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Below that you see pretty
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close to what you would see in Python.
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You wouldn't say for i in list,
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you would use whatever the lists variable name is.
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You couldn't use lists as the variable name
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for reasons that we'll discuss later.
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But this is roughly Pythonic.
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For i in list, print i.
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We're not going to talk deep about functions.
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All the print statement here is doing is
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taking whatever value is given to
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it and it's writing it to the screen.
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For i in list, print i.
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Well, let's have a look.
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Is there an item in the list?
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That is the conditional
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upon which this is actually being evaluated.
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The for i in list,
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this for loop is an implementation saying,
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is there an item in the list?
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If yes, retrieve that item.
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That's where if comes in,
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that's where the Boolean logic evaluation,
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the execution decision based
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upon internal logic comes from.
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Is there an item in the list? Of course there is.
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There are three items at present. Print that item.
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In this case, the first item is going to be the Number 1.
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Generally speaking, lists are
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evaluated from left to right as they are written.
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Largely, it's just a programming standard,
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it's the way it generally implements.
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However, lists are not inherently sorted so
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sometimes you'll want to
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control the way it interprets your list.
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But generally speaking, it's going to be
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left to right as it is written.
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Is there another item in the list?
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We've printed our first item,
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we've retrieved it, we've used it, we're done with it.
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Now we're going to go to the next item in the list,
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we're going to increment what we call the index.
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We started at item 0,
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now we're going to go to the item that is
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indexed at one,
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in the same way that you would use,
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for example, an index and a card catalog system.
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You'll see indexes referenced often.
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All an index is a number that
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identifies the location in the set,
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in the list in the array and whatever.
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Is there another item in the list?
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We increment our index and we see
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that yes, of course, there is.
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In this case, it is the letter b.
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Then for a third item, we say,
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is there another item in the list?
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Sure enough, there is, it's the number
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150, so we print that.
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Now we reach a point that's a little bit trickier
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because we've run out of items in this list.
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There's nothing left in the list to print,
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there's nothing left to retrieve.
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Is there another item in the list evaluates to
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false and your for loop will terminate execution.
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The condition here is actually an inverted condition.
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We talked about logical inversion
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in our earlier lesson about Boolean logic.
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What's happening here is a logical inversion.
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It's saying so long as there
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are items in the list, keep printing.
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When it becomes false,
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when there are no more items left to print,
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then you'll continue the program
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and end the execution of this loop.
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That's just important to understand, again,
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all that's happening here is a piece
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of Boolean logic being evaluated every time
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this loop is run until such time as it
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evaluates to false and then the loop terminates.
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Here's another example for i,
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where i is each number between zero and 100, print i.
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This is also similar to Python,
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though not exactly the same for reasons we'll
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discuss when we talk about range in a later video.
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But for i and the range 100,
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so basically the numbers between zero
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and 100, you will print i.
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We're going to start out Python.
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We don't have to set this value.
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Python is going to assign that value for us.
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So i is going to be set to
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zero at the beginning of this loop.
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The logic will be evaluated.
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This is our case of Boolean logic,
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is i less than or equal to 100?
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Of course, in this case, it is because zero
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is substantially smaller than 100.
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It evaluates to true and we
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perform the logic underneath it.
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We print i and then we increment i.
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Now the way this for loop is designed,
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the way it's constructed, it will do
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all of the incrementation for us.
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We don't have to pay any attention to that,
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the for-loop is covering.
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We see that i is one,
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which of course is less than 100,
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we continue our evaluation.
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This is going to happen for another 100 loops until
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finally we reach i equals 101.
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Now it's important to know that with
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the list that we were evaluating in the last slide,
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it didn't actually magically know,
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that was all of the items I've done.
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It had to first get to a condition where
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the evaluation turned out to be false.
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In this case, we see that 101 is still greater than 100.
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Therefore, it is going to evaluate to false,
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and therefore, we're going to
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end the execution of our for loop.
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Here's a knowledge check. I'm going to
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let you try and do this one at home.
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For i in range 10, again,
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evaluate that the same way we did with range 100.
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If you already know Python,
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you'll know why that's a little bit off.
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Don't worry about it.
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Also, if you already know Python,
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maybe starting the Python lessons.
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Anyway, [NOISE] for i in range 10,
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print i times 2.
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Go ahead and pause this video and take
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a couple of seconds to give it a shot.
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You're either back because you figured it
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out or your back because you gave up or
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you just watched me sit very still for a
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second as my dog snored in the background.
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Either way, we're now going to look at what happens.
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I is going to start out equaling 0,
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of course, and therefore we're going to
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perform our operation against the Number 0.
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Print i times 2,
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of course 0 times 2 is 0.
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Then we're going to move on to one.
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Now 1 times 2 obviously is 2,
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so we're going to print that.
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Here we're combining the two concepts we talked about
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earlier of performing a piece of logic to
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every item in a set and incrementing a number.
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As we increment this number,
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we're going to multiply it by two
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each time until finally,
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we get to the last number,
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which of course is going to be 10 in this case.
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We print 10 times 2,
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we say, "We're done."
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Then it's going to evaluate under the hood.
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This is all happening in Python.
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You're not seeing any of it.
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Under the hood is going to value it and say,
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the next Number 11 is not actually a valid case for this,
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the conditional is false and we're done executing.
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That's all there is to for loops.
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That's the lesson. In this lesson again,
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we talked about the concept of looping.
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We talked about what looping is used for,
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and we examined a few different implementations
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of a for loop.
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In our next lesson when we come back,
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we're going to talk about another type of loop
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called a while loop.
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I mentioned that one a few times in this lesson,
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but when you come back, we're going to spend
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a little bit of time really digging into it.
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I want to thank you all for being a part of this course.
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I want to thank you for watching,
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and of course I look forward to
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having you back for our next lesson.
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As always, I'm your instructor Joe Perry,
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and thank you for watching this on Cybrary OnDemand.
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