在列表中设置的python切片

Question

i would like to slice a set within a list, but every time i do so, i get an empty list in return.

what i try to accomplish (maybe there is an easier way):

• i got a list of sets
• each set has 5 items
• i would like to compare a new set against the list (if the set already exists in the list)
• the first and the last item in the set is irrelevant for the comparison, so only the positions 2-4 are valid for the search of already existing sets

here is my code:

result_set = ['1', '2', '3', '4', '5']

result_matrix = []

result_matrix.append(result_set)

slicing the set is no problem:

print result_set[1:4]

['2', '3', '4']

print result_matrix[:][1:4]

[]

i would expect:

[['2', '3', '4']]

Answer 1

>>> result_set = ['1', '2', '3', '4', '5']

>>> print result_set[1:4]
['2', '3', '4']

>>> result_matrix.append(result_set[1:4])
>>> result_matrix
[['2', '3', '4']]

Answer 2

Using result_matrix[:] returns the whole matrix as it is. You need to treat the result you want as a part of the array.

>>> result_matrix.append(result_set)
>>> result_matrix[:]
[['1', '2', '3', '4']]
>>> result_matrix[:][0]
['1', '2', '3', '4']
>>> result_matrix[0][1:4]
['2', '3', '4']

Also, as pointed out by falsetru:

>>> result_matrix.extend(result_set)
>>> result_matrix
['1', '2', '3', '4']
>>> result_matrix[1:4]
['2', '3', '4']

Answer 3

I think this is what you want to do:

>>> target_set = ['2', '3', '4']
>>> any([l for l in result_matrix if target_set == l[1:-1]])
True
>>> target_set = ['1', '2', '3']
>>> any([l for l in result_matrix if target_set == l[1:-1]])
False

Generalising and making that a function:

def is_set_in_matrix(target_set, matrix):
    return any(True for l in matrix if list(target_set) == l[1:-1])

>>> result_matrix = [['1', '2', '3', '4', '5']]
>>> is_set_in_matrix(['1', '2', '3'], result_matrix)
False
>>> is_set_in_matrix(['2', '3', '4'], result_matrix)
True
# a quirk - it also works with strings...`
>>> s = '234'
>>> is_set_in_matrix(s, result_matrix)
True

Note that I have used l[1:-1] to ignore the first and last elements of the "set" in the comparison. This is more flexible should you ever need sets of different lengths.