python学习笔记(九)杨辉三角的实现

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杨辉三角代码的艺术
杨辉三角定义如下：
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/ \
1 1
/ \ / \
1 2 1
/ \ / \ / \
1 3 3 1
/ \ / \ / \ / \
1 4 6 4 1
/ \ / \ / \ / \ / \
1 5 10 10 5 1
把每一行看做一个list，写一个generator，不断输出下一行的list：

方案一：普通代码

使用前一行pre，导出当前行cur

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def triangles(max):
    triangle = [[1]]
    n = 0
    while n<max:
        pre = triangle[n]
        cur = []
        yield pre
        length = len(pre) + 1
        cur.append(1)
        if length > 2:
            for m in range(1,length-1):
                cur.append(pre[m-1]+pre[m])
        cur.append(1)
        triangle.append(cur)
        n = n+1
    return 'Done'

方案二：使用zip内置函数

代码的艺术：

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>>> def triangle2():
...     L = [1]
...     while True:
...         yield L
...         L = [x+y for x,y in zip([0]+L, L+[0])]
...
>>> t = triangle2()
>>> next(t)
[1]
>>> next(t)
[1, 1]
>>> next(t)
[1, 2, 1]
>>> next(t)
[1, 3, 3, 1]
>>> next(t)
[1, 4, 6, 4, 1]
>>> next(t)
[1, 5, 10, 10, 5, 1]
>>> next(t)
[1, 6, 15, 20, 15, 6, 1]
>>> next(t)
[1, 7, 21, 35, 35, 21, 7, 1]
>>> next(t)
[1, 8, 28, 56, 70, 56, 28, 8, 1]

仅一行关键性代码L = [x+y for x,y in zip([0]+L, L+[0])]

zip()函数

原型为：zip([iterable, ...])
参数Iterable为可迭代对象，并且可以返回多个参数。该函数返回一个以元组为元素的列表，其中第i个元组包含每个参数序列的第i个元素。返回的列表长度被截断为最短参数序列的长度。只有一个序列参数时，他返回一个1元组的列表，没有参数时，它返回一个空列表。

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>>> a = [1,2,3]
>>> b = (5,6,7)
>>> c = 'abc'
>>> d = 'f'

>>> for n in (zip(a,b)):
...     print(n)
...
(1, 5)
(2, 6)
(3, 7)

>>> for n in (zip(a,c)):
...     print(n)
...
(1, 'a')
(2, 'b')
(3, 'c')

>>> for n in (zip(a,d)):
...     print(n)
...
(1, 'f')