我遇到了这段难以理解的pythonic代码:
paths = [[end]]
while paths and paths[0][0] != start:
paths = [[parent] + path for path in paths for parent in childToParents[path[0]]]
其中childToParents
是:
defaultdict(<class 'set'>, {'cog': {'log', 'dog'},
'dog': {'dot'},
'dot': {'hot'},
'hit': None,
'hot': {'hit'},
'log': {'lot'},
'lot': {'hot'}})
end
是"cog"
,开始是"hit"
。路径的预期输出为:
[["hit","hot","lot","log","cog"],["hit","hot","dot","dog","cog"]]
我尝试了双for
循环的多种变体。一种这样的尝试是:
paths=[[end]]
while paths and paths[0][0] != start:
for i in xrange(len(paths)):
for parent in childToParents[paths[i][0]]:
paths[i] = [parent] + paths[i]
但这只会给我:
[["hit","hot","dot","dog","log","cog"]]
如何将代码转换为标准的双for
循环?
答案 0 :(得分:1)
理解中的嵌套for
循环从左到右起作用,因此最右边的循环是 inner 循环,左边的循环是 outer 。例如-
a = [1,2,3]
b = [8,9,0]
[(a1, b1) for a1 in a for b1 in b]
等效于:
l = []
for a1 in a:
for b1 in b:
l.append((a1, b1))
l
如果运行,两者都会输出以下内容
[(1, 8), (1, 9), (1, 0), (2, 8), (2, 9), (2, 0), (3, 8), (3, 9), (3, 0)]
对于您的示例代码-
paths = [[end]]
while paths and paths[0][0] != start:
paths = [[parent] + path for path in paths for parent in childToParents[path[0]]]
相当于:
paths = [[end]]
while paths and paths[0][0] != start:
paths_, paths = paths, []
for path in paths_:
for parent in childToParents[path[0]]:
paths.append([parent] + path)
paths
请注意,paths_, paths = paths, []
保留paths
的内容以进行迭代,同时仍将其重置为后续循环。用您的输入运行上面的命令给我:
[['hit', 'hot', 'dot', 'dog', 'cog'], ['hit', 'hot', 'lot', 'log', 'cog']]