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Welcome back everyone to this lecture on strings strings are sequences of characters and the syntax

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of either single quotes or double quotes.

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So here we have a couple of string examples the first one uses single quotes of Hello.

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The second one uses double quotes of Hello.

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And the third one.

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Notice what's happening here.

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We have a single quote already inside the string.

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I don't do that.

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So in order to make sure your string doesn't prematurely stop with the single quote inside of don't

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what we do is we add two double quotes and block it off.

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That way we can understand with Python that we don't want the strings and at don't.

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Instead we can wrap it in the other quote tight.

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So that's why we have the use of both single and double quotes.

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If one of your double quotes or some quotes is planning to be inside of your string you just wrap it

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again and the other quote type Now more importantly because strings are ordered sequences.

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It means we can use indexing and slicing to grab subsections of the string indexing notation.

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Use a square bracket notation after the string or after the variable assigned the string indexing allows

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you to grab a single character from the string.

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So what does that actually mean.

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Let's see some examples of that.

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So again those indexing actions and the slicing actions are going to use square brackets notation and

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we do as you pass in a number index instead of the square brackets to indicate the positions of what

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you want to grab.

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So here I have the string hello and now I have a corresponding index for each of these characters.

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So zero goes of H one goes if e to the first L three at the fourth L or second L and then for the Oh.

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So there we have hello and if I want to grab back the letter H.

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All I need to do is pass an 0 into the square brackets.

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Keep in mind Python also has the ability to do a reverse indexing so we can go starting at age 0 and

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then go all the way to the end of the string and that's negative one.

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Then we have it too.

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And so on.

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So reverse indexing is really useful when you may not know the string length and you want to grab maybe

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the last character string.

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For example if someone's registering for a web site and they provide a user name and the user name is

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not going to be known beforehand.

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And for whatever reason you wanted to grab the last letter of the username instead of trying to guess

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the index position of the very last letter where you could do is just pass a negative one and that will

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automatically grab the last letter.

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Regardless of how long the string is.

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So that's indexing which allows you to grab a single character element from the string.

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Slicing allows you to grab a subsection of multiple characters otherwise known as a slice of the string.

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So slicing has the following syntax.

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You start with square brackets and then you have a start colon a stop colon and then this step.

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So start as a numerical index for the slice start stop is the index you're going to go up to but not

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include and then step is the size of the jump you're going to take between start and stop.

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OK.

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So that's going to be more clear when we actually work through those examples.

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For now let's go to Adam and explore these concepts overcoached strings indexing strings slicing strings.

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And then we'll also talk about string properties methods and how we can't print format variables inside

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of her own strings.

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What to cover so let's get started.

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OK here I am back at home and I have my exampled up my file open.

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Let's go and just type out a string.

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Hello.

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And again notice what's happening if I run this python my example that pi actually doesn't get any output

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in order to see the output.

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I'm going to need to print this out.

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So say a print and then pass in the string you want to print hello with double quotes or single quotes

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and then we see Hello as the output.

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So keep that in mind as you're working if not postscripts.

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It's easy to forget.

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They need to print things out in order to see them as output at your command line.

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So that's the basics of printing a string.

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I also want to show you a really useful function called Alien which checks the length of a string.

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And we'll also see how it's able to check the length of many object types.

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But if you want to know the length of the string you just passing that string inside of the end function.

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So I pass in Hello.

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Inside the length of this function and then print out the result.

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Now tell me how many characters are inside of this string.

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In my case it's 5.

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So 0 1 2 3 4 5.

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You should know it's going to count spaces as well.

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So if I add in three spaces there and then run this it's now going to report 8.

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Because these three spaces also count as part of that sequence of characters.

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OK.

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So that's printing out a string and that's checking the length of a string.

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Let's talk about indexing now.

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I'm going to start for variable assignment.

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I'll see my string is equal to hello.

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And then I want to get it to do is let's imagine I wanted to print out the first letter of the string.

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What I would do is say my string and then I have this square bracket notation for indexing you pass

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in the index number of whatever character you want.

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So if I want h.

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Remember that's going to start at zero.

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So then I pass an 0 here and then I can print out that result.

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So let's do that then a print out my string 0 run this.

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And now I get back in h over here.

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Let's imagine.

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I want it.

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E Well that's going to be 1 over so 0 1.

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So I just pass in one save that and when they run this I get back.

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Let's imagine I want it.

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Oh there's two ways I can do that.

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I could go 0 1 2 3 4.

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So capacity a four here save it run it and I would get back in 0.

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Or alternatively I could use negative indexing to say hey go to negative 1 essentially grab the last

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letter in that string and then that also gives it back.

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So that's indexing describing a single element or character from the string.

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The next thing I want to talk about is slicing.

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So we just talked about here was indexing.

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Now let's talk slicing a string.

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So in order to slice a string we're going to use a little bit of different syntax and I'm actually going

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to type in something like a b c d e f g h i j.

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There you are slicing and to be more obvious.

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So right now we know how to grab a single letter from the string but would have actually wanted to grab

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a slice of letters such as ABC.

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Oh what I could do is the following.

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I'll say my string and the syntax is going to be your start colon and then your stop.

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So let's imagine I want to grab ABC.

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That means I want to start at zero colon and then you're going to go stop up to the index you want to

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grab two.

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So I want to get a b c so that's 0 1 2.

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And the way Python works is you're going to go up to not including index position 3 to go 0 1 2 and

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then up to but not including three so that's important to know that the start position is inclusive

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and the end position or stop position is Xclusive.

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So that sometimes confuses a beginner so just keep that in mind.

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I'm going to print out the results of this and it's go all the way to the end.

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Close off that parentheses run this.

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And now I get back a b c Let's explore for another example let's imagine I want to grab the F G what

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I'm going to do is figure out where I'm going to start.

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So start is inclusive starting to go 0 1 2 3 4.

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So there's that e.

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So say for colon.

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And then I want to go grab F.G. up to and including index h 0 1 2 3 4 5 6 7.

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There we go for seven.

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And let's see if that grass is f g.

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We run it and there we go F G again.

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Careful note here on the stop index.

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It's again going up to but not including that top index is included.

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Stop goes up to and not including it.

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That's a really common point for beginners to kind of get confused by.

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Now I also want to show you that it can add in the step size here so for.

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Let's go from 0 up to let's say G.

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So we go from 0 to 7.

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So if I run that that gives me back ABC D E F G.

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But let's add in a third parameter here and I say colon again and then pasand to remember goes.

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Start Colan stop colon step.

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So now that I have that what it's going to do is by default it takes a step size of one goes a then

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one over B than one over see if I pass and two as it's third parameter it's going to go a go to over

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to see go to over to E and so on.

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So then when I print out this result I get back a C E G again it's going in step size of two and you

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can play around with the step size to keep making larger and larger jumps.

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So they want to make a jumps or steps or three because a.

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One two three.

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The one two three to G and so on.

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OK.

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So that's the start the stop and the step size now because it's so common to start at the very beginning

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of a string let's go back here to this example 0 7 which gives us everything up to this sort of command

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is really common.

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Starting at the beginning of a string and then going all the way to some index that you can actually

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if you remove 0 it will default to zero.

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So if say colon and some number that's going to say starting at the beginning of the string go all the

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way up to but not including that index.

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So running that is going to get you ABC the BFG.

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Now just like starting from the very beginning.

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You can also say go all the way to the end.

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So if you put a number down and then type in a colon that's going to say starting in Expedition 7.

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Give me the string all the way to the end so that you get IGY.

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So this one is pretty nice that you don't need a count over what the last index position of the string

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is.

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So you say start here colon go all the way to the end and the opposite is colon start at the very beginning

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and go all the way to this particular index position.

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OK.

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Now the last thing I want to know which is kind of interesting thing is if we say colon colon that means

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start from the very beginning and go all the way to the end.

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Now that by itself isn't really useful because it's basically just asking for the whole string back.

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But often what people like to do is they like to then add in the step parameter so that says starting

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all the way from the beginning going all the way to the end going to step size of two.

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So when I run that we should see a step size of two to see step size to eat and so on.

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All the way until we get to the end the string which in this case is going to be I given our step size.

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OK.

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So again colon colon start from the very beginning all the way to the end.

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And give me a step size or two.

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What's really interesting is that you can actually then use a reverse step size and we want to do this

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too often.

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Basically still in time we're going to show it but you can reverse the strain quite easily using something

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like start from the very beginning to the end.

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But give me is the size of negative one.

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So it's going to basically start here and then go backwards to J I h.

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And so on.

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So when you save it and run it you actually get the reverse of the string.

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So this is a really quick way to reverse a string in Python.

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Again won't be doing it too much in this course but just keep in mind that it's available to you.

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Finally I want to talk about some string properties and string methods as well as how we can use what's

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known as string interpellation to print out strings.

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So we talking about indexing and slicing.

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Very important things.

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I also want to talk about the fact that strings are immutable.

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Meaning if I wanted to change the character here a to z what it cannot do is I cannot do the following.

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I can't say hey my stream index position zero.

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Go ahead and assign that to Z.

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In fact if you just try to do that it's going to say hey type error an SDR object a string object does

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not support item assignment.

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So that means we see a string like this.

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You can no longer mutate it or change it.

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So this is known as E mutability the unability to mutate the actual object.

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Later on we'll learn about different data types and different data structures that we can mutate that.

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But keep in mind strings are immutable.

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You can't however concatenate strings.

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So that's kind of cool.

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So I can say my string plus my string and that's going to concatenate it together so that you'll see

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your string once and then it's going to concatenated again off itself.

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So then that means you can do something like this.

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Hello space concatenated with which is just using the plus sign and then someone's name for example

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Hello Jose.

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So that's a really quick way to do some print formatting of strings just concatenate them together like

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that.

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Just keep in mind.

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You'll have to be aware spacing because if I just said hello Plus Jose there's technically no space

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there.

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So when you run that you just see Hello.

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Jose is one giant string.

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So keep that in mind.

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You can concatenate strings in that fashion.

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Now I want to talk about some string methods.

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So there's lots of methods available on strings so I'm going to change just say capital H.

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Hello world.

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So if I ever want to capitalize every single letter in the string basically make everything uppercase

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I could take my string and then call up or on it.

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And if I just call Upper like this with no print C's it's actually going to give a weird response which

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will take on just a second.

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But in order to uppercase everything you call the variable name dot and then this method upper open

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and close princes.

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So these are known as methods on an object and we'll talk about methods and a lot more detail when we

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discuss object oriented programming.

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But right now just kind of keep these things in mind that they are available to you.

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So notice now.

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Hello world.

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Everything's uppercase when I called the upper there now.

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Likewise there is a lower so I can say lower save that.

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And when I run this back.

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Hello world.

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Everything's in lowercase now so H and W now become lowercase.

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Now if I forget those print C's and I just print out my string the lower what's going to happen is Python's

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going to report back hey you have this built in method called lower of the string object at this location

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inside your memory to actually execute that method.

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Essentially make it run.

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You have to have these open and close print C's and that method of open or close print sees versus no

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parentheses.

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It will make a lot more sense when we actually learn how to create our own functions which is happening

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at the end of this section.

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OK so those are two methods.

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Upper and Lower that are really useful.

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Last one I want to mention which is going to kind of be a precursor to our next lecture is the split

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method and what happens is the split method will split the string on whitespace to create a list.

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We don't technically know what the list is yet so it's kind of when to show you this hello world.

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And notice making a list now based off the two strings here separated by the whitespace and then you

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can technically expand on that by providing the actual letters you want to split on.

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So if you wanted to split every time there was a w such as its capital W I would just say capital W

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inside a split.

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Run that and then I get to see it split on W..

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OK again this is more of a preview than something you need to really learn right now.

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Main thing you should be aware of is that you can index and slice from strings and then finally I'm

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going to talk about print formatting.

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So there's two main ways to do print formatting.

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There's actually lots of different ways but the two main ones were going to be using the first one is

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let's say we have a user name and we'll just set the sequel to Sammy as the username and then let's

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say they have a favorite color so will create a color variable and their favorite color is blue.

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Let's imagine I wanted to print something out that said this user names.

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Favorite color is blah blah blue.

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So that's a little tricky right now because right now I have a user name and color as variables.

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What I can't do is say this.

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The username save favor it is color.

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If I just run this then that's what I'm going to get back to the stream that username favorite is color.

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So that's not really helpful because Python doesn't realize I'm asking for the variable username not

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the string user name.

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So in order to actually make that work I have two options.

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One is using the dot format method so I can replace what I want to insert with curly braces so I'll

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say the curly braces favorite is curly braces and then right after that double quote I can type that

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format and then pasan what I want to be filled in to those curly braces such as username and color.

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So what's happening now and you can really see this in the syntax highlighting user name and color here.

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Now these are variables in Python using the format method is going to insert them in order to wherever

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the curly braces are.

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So we'll save that.

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Run this and then I get to see the Samie favorite is blue.

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So takes the user name Sammy format's it sticks it in there takes the color variable blue formats and

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then sticks it in there.

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OK the last thing I want to mention is the ability to use f string literals for printing.

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And this is something we will see in the course.

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However I want to make it very clear this is only available on Python 3.6 and above.

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So if you're running Python 3.5 maybe you downloaded Python in early 2018 and 3.6 wasn't available free

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yet you may get an error when running this line.

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So double check that your version of Python is 3.6 or above.

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So this is going to be F string literals.

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Is called F string.

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Literals and this makes it actually really easy to print format stuff.

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So what you do is you type in an F and then a set of either single quotes or double quotes and then

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you just pass in your variable names instead of curly braces directly.

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So we can say something like the user name

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chose color.

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So that's going to do is because these are now curly braces and decided to have an F in there.

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When I run this I now get back to see the Samie chose blue.

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If you forget that f and try running this you're going to get the actual string itself the username

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chose color.

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So you let Python know hey I'm referring to variables by starting that string with an F outside of the

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quotes.

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So again it's called F string literals and it's only available in 3.6 and above and you'll be seeing

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us using the course because this is essentially the most compact way.

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It's more compact than using the dot format method.

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But if you don't have 3.6 yet you can always use that format.

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OK.

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That's really the basics of strings.

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We just covered and actually index strings slice strings a few properties of strings and some methods.

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And then as well as print formatting if you have any questions feel free to post the Cunio forms.

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We'll see at the next lecture.