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  • Welcome back for Unit 3.

  • This unit introduces the next big idea we need for a web crawler,

  • which is structured data.

  • And by the end of this unit you will have finished building a working web crawler.

  • The closest thing we've seen so far to structured data

  • is the string data type introduced in Unit 1 and used

  • in many of the procedures in Unit 2.

  • A string is a kind of structured data, and that's because

  • we can break it down into its characters.

  • The string has a sequence of characters,

  • and we can operate on sub sequences of the string.

  • What we could do with strings was somewhat limited, though,

  • because the only thing we can put in a string is a character.

  • Today, we're going to introduce the list data type,

  • and lists are much more powerful than strings,

  • so whereas for a string, all of the elements had to be characters,

  • in a list, the elements can be anything we want.

  • They could be characters. They could be strings.

  • They could be numbers. They could also be other lists.

  • Let's look at an example.

  • When we created a string, we just put a sequence of characters

  • surrounded by either single or double quotes.

  • Here's an example of a string,

  • and we could store that string in a variable by using an assignment.

  • With a list, instead of using quotes to identify the list

  • we use square brackets, and the elements are separated by commas.

  • And just like with a string, we can assign the list that we created

  • to a variable, so we'll store that list in the variable "p."

  • With a string, we could use the square brackets

  • to select elements, and when we index element 0,

  • we'll get the first element of the string, a sequence of that character,

  • which is the character "y."

  • With lists, we can also use square brackets to access elements,

  • so if we do p[0],

  • that will evaluate to the first element of p,

  • which is the string containing the single letter y.

  • With strings, we saw that we could use the colon inside the square brackets

  • to select a sub string of more than 1 character.

  • Here we're selecting from position 2 through position 4.

  • That will give us the third and fourth characters of the string,

  • which is the sub sequence, the string "bb."

  • We can do the same thing with lists.

  • We can select from position 2 to position 4,

  • but instead of returning a string, it will return a list

  • containing those elements.

  • It will give us a list of the third and fourth element

  • of the variable p, which is the list that we have here.

  • The general grammar for constructing a list

  • is to have a square bracket followed by a list

  • of any number of expressions where the expressions

  • are separated by commas.

  • We could create a list using just 2 brackets,

  • a left bracket and a right bracket, and this would create a list

  • containing 0 elements, also known as the empty list.

  • We could create a list containing 1 element.

  • That would be the square brackets with 1 element between them.

  • Here we've created a list containing just 1 element,

  • which is the number 3.

  • Or we could create a list with many elements, as we did in the first example,

  • where we have all of the strings separated by commas.

Welcome back for Unit 3.

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B1 string list element square unit variable

Introduction - Intro to Computer Science

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    羅紹桀 posted on 2015/06/10
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