《Python Data Structures》 Week4 List 课堂笔记

Coursera课程《Python Data Structures》 密歇根大学 Charles Severance

Week4 List

8.2 Manipulating Lists

8.2.1 Concatenating Lists Using +

使用“+”可以把存在的两个list加在一起。如：

>>> a = [1, 2, 3]
>>> b = [4, 5, 6]
>>> c = a + b
>>> print(c)
[1, 2, 3, 4, 5, 6]
>>> print(a)
[1, 2, 3]

8.2.2 Lists Can Be Sliced Using

list切片和string一样，第二个数字只是到这个数，但却不包括它。

>>> t = [9, 41, 12, 3, 74, 15]
>>> t[1:3]
[41, 12]
>>> t[:4]
[9, 41, 12, 3]
>>> t[3:]
[3, 74, 15]
>>> t[:]
[9, 41, 12, 3, 74, 15]

8.2.3 List Methods

list有很多方法，如append, count, extend, index, insert, pop, remove, reverse, sort等。

8.2.4 Building a List from Scratch

我们可以创建一个空list，然后再向里面添加元素，使用"append"方法。使用这个方法的话，新增的元素是放在list的末尾的。

>>> stuff = list()
>>> stuff.append('book')
>>> stuff.append(99)
>>> print(stuff)
['book', 99]

8.2.5 Is Something in a List?

判断一个元素是否在一个list里，可以使用"in"或"not in"，而python则会返回一个逻辑布尔值。而这项操作并不会改变list本身。

>>> some = [1, 9, 21, 10, 16]
>>> 9 in some
True
>>> 15 in some
False
>>> 20 not in some
True

8.2.6 Lists are in Order

使用"sort"可以对list进行排序。

>>> friends = ['Joseph', ' Glenn', 'Sally']
>>> friends.sort()
>>> print(friends)
['Glenn', 'Joseph', 'Sally']

8.2.7 Built-in Functions and Lists

>>> nums = [3, 41, 12, 9, 74, 15]
>>> print(len(nums))
6
>>> print(max(nums))
74
>>> print(min(nums))
3
>>> print(sum(nums))
154
>>> print(sum(nums)/len(nums))
25.6

8.3 Lists and Strings

8.3.1 Best Friends: Strings and Lists

split可以将一个字符串分割成几部分，形成一个字符串的list。这样我们就能取到这个字符串的每一个单词，或者使用一个循环把它都过一遍。

>>> abc = "With three words"
>>> stuff = abc.split()
>>> print(stuff)
['With', 'three', 'words']
>>> print(len(stuff))
3
>>> print(stuff[0])
With
>>> for w in stuff:
...     print(w)
...
With
Three
Words

同时split会把很多个空格当成一个来处理，默认把它当成定界符。当然，也可以自己决定用其他符号当作定界符。

>>> line = 'A lot           of spaces'
>>> etc = line.split()
>>> print(etc)
['A', 'lot', 'of', 'spaces']
>>> line = 'first;seconed;third'
>>> thing = line.split()
>>> print(thing)
['first;second;third']
>>> print(len(thing))
1
>>> thing = line.split(';')
>>> print(thing)
['first', 'second', 'third']
>>> print(len(thing))
3

8.3.2 The Double Split Pattern

如果我们想得到下面这一行中的加粗部分

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

那么我们可以这样做

words = line.split()
email = words[1]
pieces = email.split('@')
print(pieces[1])

8.4 Assignment

作业的代码如下

fname = input("Enter file name: ")
fh = open(fname)
lst = list()
for line in fh:
    temp = line.split()
    for word in temp:
        if word not in lst:
            lst.append(word)
lst.sort()
print(lst)

8.5 Assignment

作业的代码如下

fname = input("Enter file name: ")
if len(fname) < 1 : fname = "mbox-short.txt"

fh = open(fname)
count = 0
for line in fh:
    temp = line.split()
    if len(temp) < 1 or temp[0] != 'From':
        continue
    print(temp[1])
    count+=1

print("There were", count, "lines in the file with From as the first word")