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Welcome back. Recall that types are like categories of

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data and that different types are good for storing different kinds of things.

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So for example, we've seen integers which are good for storing round numbers,

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and we've seen floats which are good for storing floating points or decimals.

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In this lesson, we're going to learn three new kinds of types,

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strings, lists and tuples.

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All three of these types are sequences.

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A sequence is an ordered collection,

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meaning that it's a collection of items and it has an order.

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Meaning that it has the first item,

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a second item, a third item and so on.

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It also has a length.

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So, let's start by talking about strings.

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The simplest way to create a string is with the string literal expression.

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So, for example here,

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we're creating a string and storing its value in the variable

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s. I create a String by starting with either double quotes,

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or as I'll talk about in a little bit, single quotes.

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Then inside of those quotation marks,

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then I put the content of the string.

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The content of the string is a sequence of characters or letters.

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So, in the case of this string,

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the first character is capital H,

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the second character is lowercase e, and so on.

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In this string, we have exactly one, two, three,

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four, five, six, yes the space counts,

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seven, eight, nine, 10,11 characters.

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So, I would say that the string s is 11 characters long.

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We can also create strings using single quotes.

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So, if I replace these double quotes with single quotes,

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then s is still a valid string.

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If we wanted to create a multi-line string,

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we can use triple quotes.

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So, now if I print out the value of s and I get Hello world.

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If I print out the value of m,

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then I get this multi-line string.

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For multi-line string, we can make these triple quotes either double quotes or

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single quotes, either is valid.

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So, we can use either double quotes or single quotes as

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long as we're consistent about which kind of quotation we're using.

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So, for example, I can't start out this string

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s with double quotes and end it with a single quote.

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If I tried to do that,

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then I get a syntax error on line one.

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Instead, I need to either use double quotes for both the start and end,

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so I could do this, or I'd need to use single quotes for both the start and end.

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The same is true for multi-lines strings.

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We can either use three double quotes or three single quotes.

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So, I'm going to modify this string s a little bit.

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So, first, here's another valid string,

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s equals capital M,

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and so if I print it out then I get M. So,

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what this is doing is, it's creating a string that's one character long,

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and that first character is capital M. Here's another slightly more confusing string.

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So here, s is a string that has zero characters in it.

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So, s has a length of zero,

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and that's not to be confused with a string that has one character,

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but that character is a space.

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Here the length of s is one,

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whereas here if I delete this space,

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the length of s is zero.

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One more note for strings,

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I can have a string that looks a lot like an integer.

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So, for example, I can say s equals five,

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and when I print out the value of s, then I get five.

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To Python, this is entirely different than saying something like i equals the integer i.

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So, even though when I print these two things out,

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they look almost exactly the same in their representation,

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there are things that I can do with s or things that I

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can do with i that I can't do with the other,

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because s is a string and i is an integer.

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Just to show that these are different types,

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I'm going to print out the type of s,

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and I'm going to print out the type of i.

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So, when I run this code,

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I see that i and s have different types,

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but this actually has consequences for what I can do with i and s. So, for example,

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let's suppose that I wanted to print out what 10 plus i was,

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I can say print out i plus 10.

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Now, when I print out s then I get five,

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and when I print out i plus 10,

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then I get 15.

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What if I tried to add 10 to s?

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So, I printed out s plus 10.

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Well, if I try to do this,

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then Python is actually going to give me a run time error,

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and it's going to say that it can't concatenate string and integer objects on line four.

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Just to break that down, what it's saying is that s is

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a string and I can't add an integer to a string,

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because even though s is a string whose first character happens to be five,

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to Python that's still a string.

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But, even though to us,

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s looks like a number, to Python,

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s is just a sequence of characters,

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and so we can't add a number to a sequence of characters.

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To Python that doesn't make any sense to do,

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and so we get a run time error.

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If we did want to add five to something like s,

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then we would need to cast it first.

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So, I could say print int(s),

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in this call to the function int is going to cast the string s to be an integer,

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so the value of this overall expression is the integer five.

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So, now when I run my code,

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then I can see that I can add 10 to the integer value of s. Alternatively,

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I could cast it to be a floating point number,

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so I could say float s plus 10,

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and then I would get 15.0,

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because the value of this expression is the string s or five casted to a float,

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and so the value of this overall expression is 5.0.

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When we add 5.0 plus 10,

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then the value of this overall expression is 15.0.

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So, one thing to note when we cast strings like

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s into numbers by calling something like float s,

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is that when we call float or int to cast the string,

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then the string needs to be a valid number.

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What that means is that if I have something other than numbers in the string,

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so for instance let's suppose that I say 500,

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when I try to cast s into a float,

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then I get a run time error,

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and what this is saying is that the string 500 is not a number.

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In other words, Python doesn't know how to convert this string into a float,

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and the same goes for integers.