how to get subset of list from index of list in python

问题

I had a list of strings, I need to subset from the list based on their indexes list in generic manner

indx=[0,5,7] # index list

a = ['a', 'b', 3, 4, 'd', 6, 7, 8]

I need to get subsets in generic manner in first iteration:

a[0:5]

2nd iteration:

a[5:7]

3rd iteration:

a[7:]

The code I have tried:

for i in indx:
    if len(indx)==i:
        print(a[i:])
    else:
        print(a[i:i+1])

Expected output:

a[0:5]=='a', 'b', 3, 4, 'd'
a[5:7]=6, 7
a[7:]=8

回答1:

You can try this :

indx.append(len(a))
print(*[a[i:j] for i,j in zip(indx, indx[1:])], sep='\n')

OUTPUT :

['a', 'b', 3, 4, 'd']
[6, 7]
[8]

回答2:

Use a comprehension:

>>> [a[x:y] for x,y in zip(indx,[*indx[1:], None])]
[['a', 'b', 3, 4, 'd'], [6, 7], [8]]

回答3:

results = [a[indx[i]:indx[i+1]] for i in range(len(indx)-1)]

Will return a list containing the 3 lists you want.

回答4:

You were right in thinking of iterating through two items at once, but the issue is that for i in lst does not use indexes, and will fail.

One way to iterate and take two items is by using zip.

indx=[0,5,7] # index list

a = ['a', 'b', 3, 4, 'd', 6, 7, 8]

if indx[0] != 0:
    indx = [0] + indx #fixes left indexing in case 0 is not present
if indx[-1] != len(a):
    indx += [len(a)] #fixes right indexing to make sure you get all values from the list.

print(indx) #[0, 5, 7, 8]
for left, right in zip(indx, indx[1:]):
    print(a[left: right])
#Output:
['a', 'b', 3, 4, 'd']
[6, 7]
[8]

回答5:

Here you go.

for i in range(len(indx)):
try:
    print("a[%s:%s] ==" % (indx[i], indx[i + 1]) + " ", end=" ")
    print(a[indx[i]:indx[i + 1]])
except IndexError:
    print("a[%s:] ==" % (indx[i]) + " ", end=" ")
    print(a[indx[i]:])

Output:

a[0:5] ==  ['a', 'b', 3, 4, 'd']
a[5:7] ==  [6, 7]
a[7:] ==  [8]

回答6:

You can use the slice type to convert your index list into list slices. As others have already posted, use zip to create the start-finish pairs:

>>> print(list(zip(indx, indx[1:]+[None])))
[(0, 5), (5, 7), (7, None)]

We use these pairs to define slices:

>>> slices = [slice(a, b) for a, b in zip(indx, indx[1:]+[None])]

Then use the slice object just like you would an integer index, but the slice will select the list substring as defined in the slice start-finish pair:

>>> for slc in slices:
...     print(a[slc])

Gives:

['a', 'b', 3, 4, 'd']
[6, 7]
[8]

来源：https://stackoverflow.com/questions/55949247/how-to-get-subset-of-list-from-index-of-list-in-python