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Instructor: Let's talk about loops.

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We've talked about conditional operators,

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logical operators,

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but loops gives us a whole new power

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when it comes to our machines.

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The concept of loops or looping in programming

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is really, really powerful.

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We saw that with logical operators

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and conditional logic.

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We're able to skip lines in our program.

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So that we don't always go one, two, three,

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four, five, six, so on

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and so forth.

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But loops do a really interesting thing.

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It allows us to run

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lines of code,

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over and over and over

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and that's really powerful

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because that means we can run things thousands of times,

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millions of times.

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And this is where machines excel.

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Machines excel

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at doing small tasks, over

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and over really, really fast.

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Way better than humans.

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So loops are one of the most powerful features

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of programming languages.

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So how do we create one?

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Well, it's as simple as using, what we call 'for',

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or in Python, we call them 'for' loops.

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With this keyword 'for',

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we're able to say 'for',

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let's say item,

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in zero to mastery,

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do something.

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So they're kind of like conditional operators

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instead of 'if' we have 'for',

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but then we have this

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thing, right, where we have item, which,

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what is that we, we don't really know.

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Well, this is a variable that we create.

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We can name it whatever we want.

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We can say I,

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We can say

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teddybears,

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whatever you want.

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This is a variable.

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And a variable is created here,

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for each item

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after the 'in'.

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So it's saying, hey for every item in zero to mastery,

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do something.

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And we've seen this 'in' keyword before,

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and we'll dig deep into this later,

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but we call this an iterable.

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An iterable is something that is able to get looped over.

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So an iterable allows us to use this notation

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of a 'for' something 'in' an iterable,

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to iterate over each item.

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In our case, if I print item here

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and I click run,

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look at that.

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It prints each item in the iterable,

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which is the string zero to mastery.

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So every letter, it goes into every

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bookshelf in our machine's memory

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and prints each item one at a time.

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Now, this works with strings,

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but we can also use lists,

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like one, two, three, four, five,

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that we've seen before.

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If I click run,

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again this is an iterable,

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we're able to iterate over it

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and it grabs each item

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in the list.

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Let's do a

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set.

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Could we do this with a set?

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Well, if I click run,

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that works as well.

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What about a topple?

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Let's do it with a topple.

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If I click run, that works as well.

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That's amazing.

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What about a dictionary?

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Well, we'll get to dictionaries in a bit.

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So for loops, allow us to iterate

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over anything that has a collection of items.

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So that in this case,

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we're looping one, two, three, four, five times.

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And you can see over here that we have the colon,

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and then the indentation to tell Python,

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hey, whatever comes here,

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I can print item again.

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I can print item again.

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I can do it as many times as possible,

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as long as I have indentation,

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it's going to keep printing

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our

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numbers,

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but as soon as I open up

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here

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and print something else,

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well, I only get that once because it's not in the loop.

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So our program first runs this,

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for one, so it's going to print one, three times,

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then two, then three, then four, then five,

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and then only finally then it goes to print.

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You see how we're looping over

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and over and over.

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Now what happens if I try

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and print item here?

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And I click run.

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Hmm.

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Did you see that?

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Item gets printed at the very end here?

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I don't know if you can notice it,

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but you see that there's four fives.

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We've printed one, two, three, four,

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each, three times, but five gets printed,

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four times because this last print

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at the very end is,

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well, five.

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Because by the time the loop ends, item,

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the value of item is actually five.

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Let me ask you this.

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Can I do something like for,

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let's say X, in another list

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that contains strings, a, b,

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and c.

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Could I do something like this?

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Absolutely!

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I can nest things in Python.

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And as a matter of fact,

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when we do conditionals in Python,

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like if statements,

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we can always nest those as well.

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Because in Python it's always the indentation, right?

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So I can say print here, item,

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and then also

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print, and you know what?

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Let's print them one next to the other.

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So I'm going to say print item and print X.

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If I click run,

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you see that we're printing,

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one and a,

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one and b,

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one and c,

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because item we

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run line one.

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So item is currently one.

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So we're going to go to the next line, line two,

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and we're going to say,

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we're going to loop over this iterable

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and we're going to say, this is X.

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So this is going to be one, this is going to be a.

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And then Python comes back to line two,

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because we're still in this loop,

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and we're going to say, hey, what's X now?

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Well, we're done with a, so let's go to b.

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So it's going to run b

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and then c.

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And only once this is done,

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does it go back out,

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and now starts with two,

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which is right here, two a, two b, two c.

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So you can have nested loops

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over and over and over as well.

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Right, so these are loops.

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And right now,

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maybe it's not completely obvious why these may be useful,

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but before we get into that I want to talk about,

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this idea of iterable in the next video.