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Welcome back to this lecture on loop's We'll talk about for loops and while loops.

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Let's start by talking about four loops.

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First many objects in Python are what's known as iterable.

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That means we can use for loops to execute a block of code for every iteration through these objects.

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So that term iterable just means that you can iterate over the object.

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For example you can iterate over every character in a string iterate over every item in a list or iterate

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over every key in a dictionary.

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So let's talk about the actual Python syntax for a for loop.

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Let's imagine that my iterable object is a list.

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So I created a variable called my iterable and I said equal to this list.

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One two three.

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So what happens here is I say for item name in my iterable print the item name.

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So let's talk about the actual steps that's going on here.

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First I'd define an iterable object.

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In this case it's a list but this could be a string or a dictionary or a tuple or many other object

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types.

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So later on you've built an intuitive understanding of what's iterable and what's not.

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But right now we'll focus on lists since they're basically the most straightforward.

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So once you have your iterable defined then you're going to choose some sort of temporary variable name.

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So you're going to say 4 and this item underscore name you can choose whatever you want to plaister.

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So in this case I chose underscore name and then we say the in keyword and then we say my iterable basically

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that variable.

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So for every item in my iterable colon and then we're going to execute a block of code.

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In this case I can print out every single item inside of the iterable.

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So the output here would just be those numbers 1 2 and 3.

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Basically for every item in that list go ahead and print out that item.

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Now let's talk about while loops while loops will continue to execute a block of code while some condition

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remains true.

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For example while my pool is not fool keep filling my pool with water or while my dogs are still hungry

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keep feeding my dogs.

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So let's talk about the syntax of an actual wild.

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Here we're going to say while some boolean condition and then we're going to execute a block of code

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continually until that boolean condition is no longer true.

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So often we're going to have some sort of way to effect that boolean condition inside the while loop

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itself.

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Now you can also combine a while and else if you want to show you could say while some boolean condition

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do something else do something different although often you just see while loops by themselves ok let's

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explore all these concepts by heading over to the Adam text editor and working through some examples.

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Ok let's begin by showing some examples of a for loop.

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I'm going to start by using a list so I created s e q as my variable name.

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You can really choose any variable name you want and I'm going to create a list the numbers 1 2 3 4.

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So let's imagine I wanted to perform some sort of action with each element.

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I could say for item in CQ or sequence.

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Go ahead and print that item and again notice how there's indentation here and this should be automatically

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done for you in any ID.

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If it's not then you should probably save this as a profile and check again.

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So now I'm going to run this code and if I scroll up I get to see one two three four five.

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So it went ahead and printed every single number in that list.

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What I could do is say maybe print every number squared.

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So I'll take the item square it and then print out the result.

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So let's run those results and I see 1 4 9 16 25.

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Now often beginners get confused they say well we're that this item come from I didn't see the find

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anywhere.

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Technically it's just a temporary variable name for the iteration through the sequence so you could

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have this be anything and I like to make something weird here just to make that very clear so I could

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say for jellyfish in the queue.

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Go ahead and take jellyfish.

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And maybe times it by 2.

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So again it's totally up to you.

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What term is going to be chosen for your sequence throughout that iteration.

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Now often you're probably not going to want to use something like jellyfish.

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Instead you should choose something pretty irrelevant.

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Maybe nom would be a good one in this case because there numbers so this is how I would do this.

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I would say for them sequence print name but keep in mind any variable name there is OK.

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So something else you should know is when you're saying for some item in a sequence you don't necessarily

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need to actually use that item in the sequence.

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Let's say I just wanted to print Hello five times.

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This case for every number in that sequence 1 2 3 4 5.

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I'm going to perform the action.

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Notice I'm not actually using numb instead I'm using almost this as some sort of counting device.

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So now when I run this I get back.

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Hello hello hello hello.

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Just one hello for each item in this list.

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OK so that's how a for loop works of a list.

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Let's see how it works now with a string.

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So I'll say hello is a string and I want to save for now I'm going to make this actually say C H A R

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because it's more reflective what it's going to do with a string.

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So I'm going to say my string is equal to hello and for C H R for character in my string.

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Go ahead and print out that character.

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So when you actually for loop through a string you basically loop through every single character in

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that string.

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So for a list every item or element in that list for strings.

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It's every character in that string.

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Then for dictionaries.

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Let's go ahead to make a dictionary.

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We'll say salary's again and we'll say John 10.

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Let's have Sally B 20 and let's say Lisa is 30 I'm going to start this again for.

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And when you loop through a dictionary what's going to happen is you get a loop through the keys of

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the dictionary so you can say for key in salary's print the key.

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So when I read this and I loop through a dictionary I see all the keys John Salley and Lisa.

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So something to know here is you could technically call this whatever you want.

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So maybe it makes more sense to call this four employee in salary's again to really taking advantage

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of Python's clean syntax to make code very readable.

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So here it's very clear what's happening.

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I'm saying for employee in salary's Prince employ and there we go.

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John Salley and Lisa.

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Now let's imagine actually wanted the values associated with those keys.

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Well all we need to do is take my salary's dictionary and then index it for that particular employee.

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So I'm going to grab that employee and paste it in there.

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And let's print out salary's for that employee.

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So now when I run this piece of code I see 10 20 30 and if I want all the information I can just use

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multiple print statements.

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So Nel's prints employ here and let's do some more prints so I can say the let's say employ

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has a salary of.

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And then we'll print out that salary and I'm going to print out a blank line there.

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So save that and zoom a little bit so we can see the whole picture here and collapse this little bit.

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So have a salary's dictionary.

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I'm saying for every employee in salaries.

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Remember when you loop through a dictionary it's going to default through the Keys that dictionary.

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So then in print that the employee has a salary of and they're going to grab that associate a key value

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pair and then prints a blank line.

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So when I run this I get back.

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Lisa has a salary of 30.

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Salary a salary of 20.

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And John has a salary of ten.

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So again just keep in mind when you loop through a dictionary it's going to loop through the Keys by

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default.

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Now the last special thing I want to mention is something called tuple unpacking.

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As you begin to work a lot with Python you'll notice that often functions return a list of tuple pairs.

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So you'll see something like this a one.

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And then maybe C-3.

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So notice what's happening here.

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I have a list of tuples.

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So if I were to print out the length of my pairs I would expect a length of three because there's three

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tuples inside of this my pairs list.

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So here you have these tuples.

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There's three of them.

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So what happens when actually loop through this I'll say for item in my pairs print the item will save

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that and run it and I get back each particular tuple which is fine but because this sort of structure

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is so common in Python having a list of tuples where you can do something called tuple and packing so

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you could do is instead of saying for item and each item being a tuple you can actually have a little

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tuple here and then say something like.

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Item one item two.

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And then you can print out item 1 and print out item 2.

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So this is actually going to deconstruct or unpack the tuple as you are going through your for loop.

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So now look what you see.

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You see one you see a one BE2 and then see three because you're printing out first these letters and

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then printing out these numbers.

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So you're unpacking the tuple inside of your for loop.

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Now you actually don't need these princes here.

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Instead you could say Item one.

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Item two and my pairs and this by itself will also work just fine.

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To get that exact same result.

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I just wanted to show you with the princes to make it clear.

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The idea of what's happening.

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And again we can choose whatever variable names we want for these particular items inside of this list.

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So it might make more sense to do something like four letter numb in my pairs.

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Prince

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and again you don't have to use both of these.

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Maybe I only want to print the letters inside of those pairs.

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So after you save it you can run it and just get back the letters.

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Or maybe you only want the numbers.

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In that case just print out them save that.

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Run it again and easy one two three.

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Again this is called tuple unpacking and it's a pretty important topic because you see this sort of

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structure a lot and some more advanced Python functions.

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OK so that's the basics of for loops.

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Now let's quickly discuss how we can use while loops and we'll also talk about a few other topics such

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as the in keyword and the range function so let's say that so remember that while loops will continually

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perform action until that condition becomes true.

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So I will set a variable.

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I equal to 1 and I will say while I is less than 5 go ahead and Prince I is currently and then I'll

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go ahead and use an F string literal to Prince.

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I so what was happening here him say is equal one.

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And then while I was less than five I'll say I is currently in the current value of i.

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Now if I were to just run this by itself right now it would go on forever because I'm not changing the

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value of I one will always be less than five.

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So this would constantly Forever print out eyes.

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Currently one is currently one is currently one.

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So in order to make sure that we don't get a forever while loop peer analysts a prince actually not

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going to print anything I'll just say I is not going to be equal to I plus one.

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So what does that mean.

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It means I started off as one.

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I'm saying while I less than 5 print out the current value of i and then take the current value of i

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add 1 to it and reset that to be I.

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So then that's going to become two then three.

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Then it's four and eventually it'll be five.

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And this while loop will no longer execute so it should stop by printing.

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I's currently 4 and by the time I get added 1 and it's 5.

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This while loop ceases to continue.

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So let's run this and see what we get.

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So we say eyes currently 1 2 3 4 and on that very last iteration of the while loop we said 4 plus 1

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is 5 and 5 is no longer less than 5.

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So this while loop terminates.

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OK that's the very basic idea of a while loop.

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You want to be really careful with this condition because if you do something like.

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While one is equal to 1 then it doesn't matter what you put here.

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This is always going to be true so it will go on forever and you want to use Control-C to stop permanent

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while loops OK we just discussed for loops and while loops.

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The last thing I want to do is discuss a few more useful operators.

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One is the range function which can quickly generate a list of integers for you.

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So if you use Range what you can do is signify a start a stop and a step size so you can say range from

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0 to 5 and you can actually loop through this or iterate through it so I can say for x in range 0 to

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5 print X and what range does it automatically generates integers for you.

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So now I want to run this I get back.

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0 1 2 3 4.

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So it starts at 0 and then goes up to but not including 5.

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So again you can kind of play around with this maybe do 11 so that will go all the way up to but not

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including 11.

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So we'll see.

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0 1 2 3 4 5 6 7 8 9 10 and something else to know is that you can add a third parameter.

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That's a step size.

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So hopefully this feels really really reminiscent of slicing a list or slicing a string.

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So if you want to take jumps and two cause by default it just jumps one integer at a time.

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When you run this you get back 0 2 4 6 8 10.

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So again that's the step size.

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So you have a start in exclusive stop.

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So it goes up to and not including a stopping point and then a step size.

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Now if you just want the list itself where you need to do is cast this into a list and then say something

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like result is equal to that list and then if you aren't you can then print that result.

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So if you don't intend on actually iterating through this range instead you actually want the raw result

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as a list.

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You can just cast it as a list and then get back to that list.

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OK the last keyword I want to dive in a little more on is the in keyword.

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We already saw as a keyword for 4 loops but I just want to clarify.

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You can actually use it to test if something is in an iterable.

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So let's say I want to check.

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Let's say I have a really long string so I'm just going to type a bunch of code there and I want to

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check is the letter S inside of that string.

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All I can say s in this string and then I can actually print out the result of this.

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So I just print s in right here and this should return back a boolean value.

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And in this case it's true because S isn't here.

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Now let's put Z.

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I can tell that zees not in there.

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So I run this they'll give me back false.

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So this works both for strings and for a list.

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So let's move this and I'll say one two three.

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So I'm going to ask is the string Z an element inside of this list.

229
00:15:48,910 --> 00:15:50,230
You run that you get false.

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00:15:50,260 --> 00:15:52,320
So let's change it to 1.

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So now is 1 inside of this list.

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Run that and that's true.

233
00:15:56,650 --> 00:15:59,140
So that's the in keyword operator by itself.

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Kind of outside of a for loop.

235
00:16:00,640 --> 00:16:03,330
You can just check if an element is inside.

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In other iterable element.

237
00:16:05,200 --> 00:16:11,500
OK so we talked about range in the keyword and then while loops and for loops as well as tuple and packing.

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Kind of a special condition for for loops.

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If you have any questions feel free to post the Kewney forums.

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We'll see you at the next lecture.