python筆記_列表解析

相比於for迴圈，列表解析的語法是由底層c語言實現的，它和使用for迴圈遍歷pyobject物件相比，效能會有很大的提升。

無條件子句的列表解析式

In [2]: [2*i for i in range(4)]
Out[2]: [0, 2, 4, 6]

帶條件子句的列表解析式

In [13]: [i if i%2==0 else -1 for i in range(7)]
Out[13]: [0, -1, 2, -1, 4, -1, 6]

In [14]: [i for i in range(7) if i%2==0 else -1]
  File "<ipython-input-14-09b7845471ee>", line 1
    [i for i in range(7) if i%2==0 else -1]
                                      ^
SyntaxError: invalid syntax


In [15]: [i for i in range(7) if i%2==0]
Out[15]: [0, 2, 4, 6]

In [18]: [i if i%2==0 for i in range(7)]
  File "<ipython-input-18-49bed909dcb4>", line 1
    [i if i%2==0 for i in range(7)]
                   ^
SyntaxError: invalid syntax
 

注意：當條件子句在for前時必須帶上else，此時else表示不符合if條件時列表元素的取值；當條件子句在for後時不能帶上else。

多重迴圈的列表解析式

In [3]: [(i,j) for i in [1,3,5] for j in [2,4,6]]
Out[3]: [(1, 2), (1, 4), (1, 6), (3, 2), (3, 4), (3, 6), (5, 2), (5, 4), (5, 6)]

In [11]: matrix
Out[11]: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

In [12]: [i for row in matrix for i in row]
Out[12]: [1, 2, 3, 4, 5, 6, 7, 8, 9]
 

注意：多重迴圈是有順序的，按照從列表前到列表後的for出現的次序執行。

字典解析

In [19]: {key:value for key,value in [(1,11),(2,22),(3,33)]}
Out[19]: {1: 11, 2: 22, 3: 33}