廖雪峰python3复习总结——day5-2

生成器：generator，若list过大，不必创建完整的list，从而节省大量的空间。

方法一：把一个列表生成式的[]改成()，就创建了一个generator，通过next()函数获得generator的下一个返回值。generator保存的是算法，每次调用next(g)，就计算出g的下一个元素的值，直到计算到最后一个元素，没有更多的元素时，抛出StopIteration的错误。

>>> L = [x * x for x in range(10)]
>>> L
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> g = (x * x for x in range(10))
>>> g
<generator object <genexpr> at 0x1022ef630>


>>> next(g)
0
>>> next(g)
1
>>> next(g)
4
>>> next(g)
9
>>> next(g)
16
>>> next(g)
25
>>> next(g)
36
>>> next(g)
49
>>> next(g)
64
>>> next(g)
81
>>> next(g)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration


>>> g = (x * x for x in range(10))
>>> for n in g:
...     print(n)
... 
0
1
4
9
16
25
36
49
64
81

方法二：如果一个函数定义中包含yield关键字，那么这个函数就不再是一个普通函数，而是一个generator。关于yield理解：参照博客https://blog.csdn.net/lftaoyuan/article/details/78915518

def fib(max):
    n, a, b = 0, 0, 1
    while n < max:
        yield b
        a, b = b, a + b
        n = n + 1
    return 'done'

练习：

错误：
def triangles():
    a = [1]
    while True:
        yield a
        b=a[:]
        a.append(1)
        if len(a) > 2:
            for i in range(len(a)):
                if i >= 1 and i < (len(a) - 1):
                    a[i] = b[i] + b[i - 1]

results = []
for t in triangles():
    print(t)
    results.append(t)
    n = n + 1
    if n == 10:
        break
print(results)
if results == [
    [1],
    [1, 1],
    [1, 2, 1],
    [1, 3, 3, 1],
    [1, 4, 6, 4, 1],
    [1, 5, 10, 10, 5, 1],
    [1, 6, 15, 20, 15, 6, 1],
    [1, 7, 21, 35, 35, 21, 7, 1],
    [1, 8, 28, 56, 70, 56, 28, 8, 1],
    [1, 9, 36, 84, 126, 126, 84, 36, 9, 1]
]:
    print('测试通过!')
else:
    print('测试失败!')


#这个地方，前面输出没有问题，result输出有问题（标记一下、不清楚是为什么）


修改后输出正确，测试通过：
def triangles():
    a = [1]
    while True:
        b=a[:]
        yield b
        a.append(1)
        if len(a) > 2:
            for i in range(len(a)):
                if i >= 1 and i < (len(a) - 1):
                    a[i] = b[i] + b[i - 1]

简化后：

def triangles():
     a=[1]
     while True:
         b=a[:]
         yield b
         for i in range(len(a)-1):
             a[i+1]=b[i]+b[i+1]
         a.append(1)

网友究极进化版：

def triangles():
    L = [1]
    while True:
        yield L
        k = [L[i] + L[i + 1] for i in range(len(L) - 1)]
        L = [1] + k + [1]

廖雪峰python3复习总结——day5-2

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