markdown 4. Python - 列表

Posted 2021-05-05

## Table of Contents


<div class="alert alert-block alert-info" style="margin-top: 20px">
<li><a href="#ref0">About the Dataset</a></li>
<li><a href="#ref1">Lists</a></li>
<li><a href="#ref2">Quiz</a></li>
<br>
<p></p>
Estimated Time Needed: <strong>15 min</strong>
</div>

<hr>

<a id="ref0"></a>
<center><h2>About the Dataset</h2></center>

Imagine you received many music recommendations from your friends and compiled all of the recommendations into a table, with specific information about each movie.

The table has one row for each album and several columns:

- **artist** - Name of the artist
- **album** - Name of the album
- **released_year** - Year the album was released
- **length_min_sec** - Length of the album (hours,minutes,seconds)
- **genre** - Genre of the album
- **music_recording_sales_millions** - Music recording sales (millions in USD) on [SONG://DATABASE](http://www.song-database.com/)
- **claimed_sales_millions** - Album's claimed sales (millions in USD) on [SONG://DATABASE](http://www.song-database.com/)
- **date_released** - Date on which the album was released
- **soundtrack** - Indicates if the album is the movie soundtrack (Y) or (N)
- **rating_of_friends** - Indicates the rating from your friends from 1 to 10
<br>
<br>

The dataset can be seen below:

<font size="1">
<table font-size:xx-small style="width:25%">
  <tr>
    <th>Artist</th>
    <th>Album</th> 
    <th>Released</th>
    <th>Length</th>
    <th>Genre</th> 
    <th>Music recording sales (millions)</th>
    <th>Claimed sales (millions)</th>
    <th>Released</th>
    <th>Soundtrack</th>
    <th>Rating (friends)</th>
  </tr>
  <tr>
    <td>Michael Jackson</td>
    <td>Thriller</td> 
    <td>1982</td>
    <td>00:42:19</td>
    <td>Pop, rock, R&B</td>
    <td>46</td>
    <td>65</td>
    <td>30-Nov-82</td>
    <td></td>
    <td>10.0</td>
  </tr>
  <tr>
    <td>AC/DC</td>
    <td>Back in Black</td> 
    <td>1980</td>
    <td>00:42:11</td>
    <td>Hard rock</td>
    <td>26.1</td>
    <td>50</td>
    <td>25-Jul-80</td>
    <td></td>
    <td>8.5</td>
  </tr>
    <tr>
    <td>Pink Floyd</td>
    <td>The Dark Side of the Moon</td> 
    <td>1973</td>
    <td>00:42:49</td>
    <td>Progressive rock</td>
    <td>24.2</td>
    <td>45</td>
    <td>01-Mar-73</td>
    <td></td>
    <td>9.5</td>
  </tr>
    <tr>
    <td>Whitney Houston</td>
    <td>The Bodyguard</td> 
    <td>1992</td>
    <td>00:57:44</td>
    <td>Soundtrack/R&B, soul, pop</td>
    <td>26.1</td>
    <td>50</td>
    <td>25-Jul-80</td>
    <td>Y</td>
    <td>7.0</td>
  </tr>
    <tr>
    <td>Meat Loaf</td>
    <td>Bat Out of Hell</td> 
    <td>1977</td>
    <td>00:46:33</td>
    <td>Hard rock, progressive rock</td>
    <td>20.6</td>
    <td>43</td>
    <td>21-Oct-77</td>
    <td></td>
    <td>7.0</td>
  </tr>
    <tr>
    <td>Eagles</td>
    <td>Their Greatest Hits (1971-1975)</td> 
    <td>1976</td>
    <td>00:43:08</td>
    <td>Rock, soft rock, folk rock</td>
    <td>32.2</td>
    <td>42</td>
    <td>17-Feb-76</td>
    <td></td>
    <td>9.5</td>
  </tr>
    <tr>
    <td>Bee Gees</td>
    <td>Saturday Night Fever</td> 
    <td>1977</td>
    <td>1:15:54</td>
    <td>Disco</td>
    <td>20.6</td>
    <td>40</td>
    <td>15-Nov-77</td>
    <td>Y</td>
    <td>9.0</td>
  </tr>
    <tr>
    <td>Fleetwood Mac</td>
    <td>Rumours</td> 
    <td>1977</td>
    <td>00:40:01</td>
    <td>Soft rock</td>
    <td>27.9</td>
    <td>40</td>
    <td>04-Feb-77</td>
    <td></td>
    <td>9.5</td>
  </tr>
</table></font>

<hr>

<a id="ref1"></a>
<center><h2>Lists</h2></center>

We are going to take a look at lists in Python. A list is a sequenced collection of different objects such as integers, strings, and other lists as well. The address of each element within a list is called an 'index'. An index is used to access and refer to items within a list.


<a ><img src = "https://ibm.box.com/shared/static/eln445fv5nzv3wlm4u8dnfhbrcrv0hff.png" width = 1000, align = "center"></a>
  <h4 align=center> Representation of a list  
  </h4> 





 To create a list, type the list within square brackets **[ ]**, with your content inside the parenthesis and separated by commas. Let’s try it!


```python
L = ["Michael Jackson" , 10.1,1982]
L
```




    ['Michael Jackson', 10.1, 1982]



We can use negative and regular indexing with a list :

 <a ><img src = "https://ibm.box.com/shared/static/a7ac9lnvmcaz29n86ffez4as27fl3n9m.png" width = 1000, align = "center"></a>
  <h4 align=center> Representation of a list  
  </h4> 



```python
print('the same element using negative and positive indexing:\n Postive:',L[0],
'\n Negative:' , L[-3]  )
print('the same element using negative and positive indexing:\n Postive:',L[1],
'\n Negative:' , L[-2]  )
print('the same element using negative and positive indexing:\n Postive:',L[2],
'\n Negative:' , L[-1]  )
```

    the same element using negative and positive indexing:
     Postive: Michael Jackson 
     Negative: Michael Jackson
    the same element using negative and positive indexing:
     Postive: 10.1 
     Negative: 10.1
    the same element using negative and positive indexing:
     Postive: 1982 
     Negative: 1982


Lists can contain strings, floats, and integers. We can nest other lists, and we can also nest tuples and other data structures. The same indexing conventions apply for nesting:    



```python
[ "Michael Jackson", 10.1,1982,[1,2],("A",1) ]

```




    ['Michael Jackson', 10.1, 1982, [1, 2], ('A', 1)]



 We can also perform slicing in lists. For example, if we want the last two elements, we use the following command:


```python
L = [ "Michael Jackson", 10.1,1982,"MJ",1]
L
```




    ['Michael Jackson', 10.1, 1982, 'MJ', 1]




<a ><img src = "https://ibm.box.com/shared/static/pt3pfp1sg5okwuwwpy0dnj8e94fl2mwy.png" width = 1000, align = "center"></a>
  <h4 align=center> Representation of a list  
  </h4> 


```python
L[3:5]
```




    ['MJ', 1]



We can use the method "extend" to add new elements to the list:


```python
L = [ "Michael Jackson", 10.2]
L.extend(['pop',10])
L
```




    ['Michael Jackson', 10.2, 'pop', 10]



Another similar method is 'appended'. If we apply 'appended' instead of 'extended', we add one element to the list:


```python
L = [ "Michael Jackson", 10.2]
L.append(['pop',10])
L
```




    ['Michael Jackson', 10.2, ['pop', 10]]



 Each time we apply a method, the list changes. If we apply "extend" we add two new elements to the list. The list **L** is then modified by adding two new elements:


```python
L = [ "Michael Jackson", 10.2]
L.extend(['pop',10])
L
```




    ['Michael Jackson', 10.2, 'pop', 10]



If we append the list  **['a','b']** we have one new element consisting of a nested list:


```python
L.append(['a','b'])
L
```




    ['Michael Jackson', 10.2, 'pop', 10, ['a', 'b']]



As lists are mutable, we can change them. For example, we can change the first element as follows:


```python
A = ["disco",10,1.2]
print('Before change:', A)
A[0] = 'hard rock'
print('After change:', A)
```

    Before change: ['disco', 10, 1.2]
    After change: ['hard rock', 10, 1.2]


 We can also delete an element of a list using the **del** command:


```python
print('Before change:', A)
del(A[0])
print('After change:', A)
```

    Before change: ['hard rock', 10, 1.2]
    After change: [10, 1.2]


 We can convert a string to a list using 'split'.  For example, the method **split** translates every group of characters separated by a space into an element in a list:



```python
'hard rock'.split()

```




    ['hard', 'rock']



We can use the split function to separate strings on a specific character. We pass the character we would like to split on into the argument, which in this case is a comma.  The result is a list, and each element corresponds to a set of characters that have been separated by a comma: 


```python
'A,B,C,D'.split(',')

```




    ['A', 'B', 'C', 'D']



 When we set one variable  **B** equal to **A**;  both **A** and **B** are referencing the same list in memory :


```python
A = ["hard rock",10,1.2]
B = A
print('A:', A)
print('B:', B)
```

    A: ['hard rock', 10, 1.2]
    B: ['hard rock', 10, 1.2]


 <a ><img src = 'https://ibm.box.com/shared/static/7g2u8hqqb4birdwn7m9uir4s9wfj8mko.png' width = 1000, align = "center"></a>
  

 Initially, the value of the first element in ** B** is set as hard rock. If we change the first element in **A**  to 'banana', we get an unexpected side effect.  As **A** and **B ** are referencing the same list, if we change list **A**, then list **B** also changes.  If we check the first element of **B** we get banana instead of hard rock:


```python
print('Before changing A[0], B[0] is ',B[0])
A[0] = "banana"
print('After changing A[0], A[0] is ',A[0])
print('After changing A[0], B[0] is ',B[0])
```

    Before changing A[0], B[0] is  hard rock
    After changing A[0], A[0] is  banana
    After changing A[0], B[0] is  banana


This is demonstrated in the following figure: 

 <a ><img src = https://ibm.box.com/shared/static/thdu6y5pzh99qpun4tu2fjvj86st0hbu.gif width = 1000, align = "center"></a>




You can clone list  **A** by using  the following syntax:


```python
B = A[:]
B
```




    ['banana', 10, 1.2]



 Variable **B** references a new copy or clone of the original list; this is demonstrated in the following figure:


 <a ><img src = https://ibm.box.com/shared/static/gwx86gaoeizqjvx7xj96cb8i9hn684ei.gif width = 1000, align = "center"></a>




Now if you change **A**, **B** will not change: 



```python
print('Before changing A[0], B[0] is ',B[0])
A[0] = "apple"
print('After changing A[0], A[0] is ',A[0])
print('After changing A[0], B[0] is ',B[0])
```

    Before changing A[0], B[0] is  banana
    After changing A[0], A[0] is  apple
    After changing A[0], B[0] is  banana



```python

```

 <a id="ref2"></a>
<center><h2>Quiz</h2></center>

#### Create a list 'a_list' , with the following elements 1, “hello”, [1,2,3 ] and True. 


```python

```

 <div align="right">
<a href="#q1" class="btn btn-default" data-toggle="collapse">Click here for the solution</a>
</div>
<div id="q1" class="collapse">
```
a_list=[1, 'hello', [1,2,3 ] , True]
a_list

```
</div>

####  Find the value stored at index 1 of 'a_list'.


```python

```

 <div align="right">
<a href="#q2" class="btn btn-default" data-toggle="collapse">Click here for the solution</a>
</div>
<div id="q2" class="collapse">
```
a_list[1]


```
</div>

####  Retrieve the elements stored at index 1 and 2 of 'a_list'.


```python

```

 <div align="right">
<a href="#q3" class="btn btn-default" data-toggle="collapse">Click here for the solution</a>
</div>
<div id="q3" class="collapse">
```
a_list[1:3]

```

#### 4) Concatenate the following lists A=[1,'a'] abd B=[2,1,'d']:


```python

```

 <div align="right">
<a href="#q4" class="btn btn-default" data-toggle="collapse">Click here for the solution</a>
</div>
<div id="q4" class="collapse">
```
A=[1,'a'] 
B=[2,1,'d']
A+B
```

 <a href="http://cocl.us/bottemNotebooksPython101Coursera"><img src = "https://ibm.box.com/shared/static/irypdxea2q4th88zu1o1tsd06dya10go.png" width = 750, align = "center"></a>

# About the Authors:  

 [Joseph Santarcangelo]( https://www.linkedin.com/in/joseph-s-50398b136/) has a PhD in Electrical Engineering, his research focused on using machine learning, signal processing, and computer vision to determine how videos impact 
 ]human cognition. Joseph has been working for IBM since he completed his PhD.


 <hr>
Copyright &copy; 2017 [cognitiveclass.ai](cognitiveclass.ai?utm_source=bducopyrightlink&utm_medium=dswb&utm_campaign=bdu). This notebook and its source code are released under the terms of the [MIT License](https://bigdatauniversity.com/mit-license/).​


```python

```