Is it possible to index nested lists using tuples in python?

问题

I just started with python and very soon wondered if indexing a nested list with a tuple was possible. Something like: elements[(1,1)]

One example where I wanted to do that was something similar to the code below in which I save some positions of the matrix that I will later need to access in a tuple called index.

index = ( (0,0), (0,2), (2,0), (2,2) )

elements = [ [ 'a', 'b', 'c'],
             [ 'c', 'd', 'e'],
             [ 'f', 'g', 'h'] ]

for i in index:
    print (elements [ i[0] ] [ i[1] ])

    # I would like to do this:
    # print(elements[i])

It seems like a useful feature. Is there any way of doing it? Or perhaps a simple alternative?

回答1:

Yes, you can do that. I wrote a similar example:

index = [ [0,0], [0,2], [2,0], [2,2] ]

elements = [ [ 'a', 'b', 'c'],
             [ 'c', 'd', 'e'],
             [ 'f', 'g', 'h'] ]

for i,j in index:
    print (elements [ i ] [ j ])

a c f h

回答2:

If you really want to use tuples for indexing you can implement your own class that extends list and redefines __getattr__ to work with tuples and use that:

class TList(list):
    def __getitem__(self, index):
        if hasattr(index, "__iter__"):
            # index is list-like, traverse downwards
            item = self
            for i in index:
                item = item[i]
            return item
        # index is not list-like, let list.__getitem__ handle it
        return super().__getitem__(index)

elements = TList([ [ 'a', 'b', 'c'],
                   [ 'c', 'd', 'e'],
                   [ 'f', 'g', 'h'] ])
index = ( (0,0), (0,2), (2,0), (2,2) )
for i in index:
    print(elements[i])

a
c
f
h

回答3:

# I would like to do this:
# print(elements[i])

No you cannot index a specific value of a nested list in this way.

The only slightly better way would be to "unpack" the tuples are you're iterating over them:

Example:

for i, j in index:
    print(elements[i][j])

See: Tuples ans Sequences

回答4:

If you want to print everything in elements

index = ( (0,0), (0,2), (2,0), (2,2) )

elements = [ [ 'a', 'b', 'c'],
             [ 'c', 'd', 'e'],
             [ 'f', 'g', 'h'] ]

for row in elements:
    for i in range(len(row)):
        print (row[i])

回答5:

You can use list comprehensions:

index = ((0, 0), (0, 2), (2, 0), (2, 2))

elements = [['a', 'b', 'c'],
            ['c', 'd', 'e'],
            ['f', 'g', 'h']]

tmp = [print(elements[i][j]) for i,j in index]

来源：https://stackoverflow.com/questions/30341008/is-it-possible-to-index-nested-lists-using-tuples-in-python

标签

python

tuples

nested-lists