迭代

迭代可以對list，tuple和str等型別的資料進行操作，如何判斷一個數據是否可以迭代：

>>> from collections import Iterable
>>> isinstance('abc', Iterable) # str是否可迭代
True
>>> isinstance([1,2,3], Iterable) # list是否可迭代
True
>>> isinstance(123, Iterable) # 整數是否可迭代
False

a = [1,2,3]
b = ['a','b','c']
c = 'ABC'
for i , value in enumerate(a):
    print(i,value)
>>>0 1
>>>1 2
>>>2 3
 

帶if的列表生成式，在生成list時候，進行篩選：

>>>[x*x for i in range(1,11) if i%2 == 0]
[4,16,36,64,100]

>>>[m +n for m in 'ABC' for n in 'XYZ']
['AX','AY','AZ', 'BX', 'BY', 'BZ', 'CX', 'CY', 'CZ']

建立generator，只需把列表生成器的[]改為()即可：

g = ( x*x for x in range(10))
next(g)
>>>0
next(g)
>>>1
...
 

for n in g:
    print(n)
>>>0
1
4
9
16
25
36
49
64
81

def test():
    print("step1:")
    yield 'a'
    print("step2:")
    yield 'b'
    print("step3:")
    yield 'c'
    print("step4:")
    yield 'd'
    return 'over'
    
for x in test():
    print(x)
>>>step1:
a
step2:
b
step3:
c
step4:
d
 

x = test()    
while 1:
    try:
        print(next(x))
    except StopIteration as e:
        print(e.value)
        break
>>>
step1:
a
step2:
b
step3:
c
step4:
d
over