第七章 数据结构

数据结构

[]返回list,（）返回生成器

列表推导式

​ 列表推导式提供了从序列创建列表的简单途径。

>>> vec = [2, 4, 6]
>>> [3*x for x in vec]
[6, 12, 18]
#输出每个值 并将每个值2次方
[[x, x**2] for x in vec]
[[2, 4], [4, 16], [6, 36]]

用 if 子句作为过滤器：

>>> [3*x for x in vec if x > 3]
[12, 18]
>>> [3*x for x in vec if x < 2]
[]

(1) 一般形式

mylist=[x+1  for x in range(1,100,2)]

print(mylist)

(2) 一般形式+判断

mylist=[x+1  for x in range(1,100,2)  if x<50]

(3) 生成列表嵌套列表

mylist=[[x,x+1,x*x] for  x in range(10)  if x>5]

(4) 嵌套循环

mylist=[x+y  for  x  in range(10)  for y in range(10)]
mylist=[[x,y]  for  x  in range(10)  for y in range(10)]

拆分
for  x  in range(3):
    for y in range(10):
        print([x,y])

嵌套列表解析

Python的列表还可以嵌套。

以下实例展示了3X4的矩阵列表：

>>> matrix = [
...     [1, 2, 3, 4],
...     [5, 6, 7, 8],
...     [9, 10, 11, 12],
... ]

以下实例将3X4的矩阵列表转换为4X3列表：

>>> [[row[i] for row in matrix] for i in range(4)]
[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]

另外一种实现方法：

>>> transposed = []
>>> for i in range(4):
...     # the following 3 lines implement the nested listcomp
...     transposed_row = []
...     for row in matrix:
...         transposed_row.append(row[i])
...     transposed.append(transposed_row)
...
>>> transposed

字典推导式

可能你见过列表推导时,却没有见过字典推导式,在2.7中才加入的:

1	d = {key: value for (key, value) in iterable}
	d = {key: value for key, value in [('a','a'),('b','b')]}