Introduction to Computer Science and Programming Using Python(3)

 
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Week three: Structured Types

1. TUPLES 元组

  • an ordered sequence of elements, can mix element types.

  • immutable, cannot change element values

  • represented with parentheses ()

  • >>> te = ()
>>> t = (2, "one", 3)
>>> t
(2, 'one', 3)
>>> t[0]
2
>>> t + (5,6)
(2, 'one', 3, 5, 6)
>>> t[1:2] #不包括后括号里面的参数
('one',)
>>> t[1:3] #必须增加一位来包括前面的参数
('one', 3)
>>> ('one',) #，代表这是一个元组
('one',)
>>> ('one') #无，代表这是个字符串
'one'

2. List 列表

  • ordered sequence

  • square brackets: []

  • mutable!!!

3. Operations on Lists

  • ADD—.append/.extend（object_name.do_something()）

    >>> l = [2,1,3]
>>> print(l)
[2, 1, 3]
>>> l.append(5) #添加单个对象至列表最后
>>> print(l)
[2, 1, 3, 5]
>>> L1 = [2,1,3]
>>> L2 =[4,5,6]
>>> L3 = L1 + L2 #列表合并
>>> print(L3)
[2, 1, 3, 4, 5, 6]
>>> L1.extend([0,10]) #添加列表
>>> print(L1)
[2, 1, 3, 0, 10]
  • Remove—del(List[])/L.pop()/L.remove
    >>> L
[1, 3, 6, 3, 7, 0]
>>> L.remove(3)
>>> L
[1, 6, 3, 7, 0]
>>> del(L[1])
>>> L
[1, 3, 7, 0]
>>> L.pop()
0
>>> L
[1, 3, 7]
>>> L.pop(0)
1
>>> L
[3, 7]
  • Convert Lists to Strings and back—list()/’‘.join()
    >>> s = 'I < 3 cs'
>>> list(s) #字符串变列表
['I', ' ', '<', ' ', '3', ' ', 'c', 's']
>>> s.split('<') #分割字符串
['I ', ' 3 cs']
  
>>> L = ['a','b','c']
>>> ''.join(L) #列表变字符串
'abc'
>>> L
['a', 'b', 'c']
>>> '_'.join(L)
'a_b_c'
  • Other List operations—sorted()/.sort()/.reverse()
    >>> L = [9, 6, 0, 3]
>>> sorted(L)
[0, 3, 6, 9]
>>> L
[9, 6, 0, 3]
>>> L.sort
<built-in method sort of list object at 0x000001C63CA297C8>
>>> L.sort()
>>> L
[0, 3, 6, 9]
>>> L.reverse()
>>> L
[9, 6, 3, 0]

    MORE: https://docs.python.org/3/tutorial/datastructures.html

3. Mutation

  • If two lists print the same thing, does not mean they are the same structure.

    >>> cool
['blue', 'green', 'grey']
>>> chill
['blue', 'green', 'grey'] #列表中对象一样，不一定列表相同
>>> chill[2] = 'blue'
>>> chill
['blue', 'green', 'blue']
>>> cool
['blue', 'green', 'grey']

  • Cloning a list: create a new list and copy every element using chill = cool[:]

    >>> cool = ['blue','green','grey']
>>> chill = cool[:] #克隆列表，生成新的列表，不指向原列表。
>>> chill.append('black')
  
  
>>> chill
['blue', 'green', 'grey', 'black']
>>> cool
['blue', 'green', 'grey']

  • calling.sort()mutates the list,returns nothing

  • calling sorted() does not mutate list, must assign result to a variable.

  • Lists of Lists of Lists

    • can have nested lists

    • side effects still possible after muation

      >>> warm = ['yellow', 'orange']
>>> hot = ['red']
>>> brigtcolor = [warm]
>>> brigtcolor
[['yellow', 'orange']]
>>> brigtcolor.append(hot) #添加另一个列表
>>> brigtcolor
[['yellow', 'orange'], ['red']]
>>> hot.append('pink') #原列表变化
>>> hot
['red', 'pink']
>>> bright.color
>>> brigtcolor #所有指向列表也跟着变化
[['yellow', 'orange'], ['red', 'pink']]

  • AVIOD mutating a list as you are iterating over it!!!

    def remove_dups (L1, L2):
        for e in L1:
                if e in L2:
                        L1.remove(e)                       
                          
>>> L1 = [1,2,3,4]
>>> L2 = [1,2,5,6]
>>> remove_dups(L1,L2)
>>> L1 #重复项[2]并未被剔除，因为代码中列表被改变，程序终止。
[2, 3, 4]
>>> L2
[1, 2, 5, 6]
  
#改进方法，给列表重新赋值建立新的列表
def remove_dups_new(L1,L2):
    L1_copy = L1[:] #对新列表进行运算，不涉及原列表改变
    for e in L1_copy:
        if e in L2:
            L1.remove(e)
  
>>> L1 = [1,2,3,4]
>>> L2 = [1,2,5,6]
>>> remove_dups_new(L1,L2)
>>> L1
[3, 4]
>>> L2
[1, 2, 5, 6]

  • Map Function Return an iterator that applies function to every item of iterable, yielding the results.

    >>> list(map(abs, [1,-2,3,-4,3.5, 6.8]))
[1, 2, 3, 4, 3.5, 6.8]
  
>>> list(map(int, [1,-2,3,-4,3.5, 6.8]))
[1, -2, 3, -4, 3, 6]

4. Quick Review

  • STRINGS, TUPLES, RANGES, LISTS

    • Common operations
    • seq[i]—i th element of sequence
    • len(seq)—length of sequence
    • seq1 + seq2—concatenation of sequences (not range)
    • n*seq—sequence that repeats seq n times (not range)
    • seq[start:end] —slice of sequence
    • e in seq—True if e contained in sequence
    • e not in seq—True if e contained in sequence
    • for e in seq—iterates over elements of sequence
  • Properties

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5. Dictionary 字典

  • my_dict = {}

    >>> grades = {'Ann':'B', 'John':'A+','Denise':'A','Katy':'A'}
>>> grades
{'Ann': 'B', 'John': 'A+', 'Denise': 'A', 'Katy': 'A'}
>>> grades['John']
'A+'
>>> grades['A']
Traceback (most recent call last):
  File "<pyshell#77>", line 1, in <module>
    grades['A']
KeyError: 'A'

  • values

    • any type(immutable and mutable)
    • can be duplicates
    • dictionary values can be lists, even other dictionaries
  • keys
    • must be unique
    • immutable type: int,float,string,tuple,bool
    • careful with float type as akey

  • no order to keys or values!!!

    >>> d = {4:{1:0},(1,3):'twelve', 'const':[3.14, 2.7, 8.44]}
>>> d
{4: {1: 0}, (1, 3): 'twelve', 'const': [3.14, 2.7, 8.44]}
  • LIST VS. DICTIONARIES

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