Index all *except* one item in python

Question

Is there a simple way to index all elements of a list (or array, or whatever) except for a particular index? E.g.,

• mylist[3]

Answer 1

The simplest way I found was:

mylist[:x] + mylist[x+1:]

that will produce your mylist without the element at index x.

Example

mylist = [0, 1, 2, 3, 4, 5]
x = 3
mylist[:x] + mylist[x+1:]

Result produced

mylist = [0, 1, 2, 4, 5]

Answer 2

>>> l = range(1,10)
>>> l
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> l[:2] 
[1, 2]
>>> l[3:]
[4, 5, 6, 7, 8, 9]
>>> l[:2] + l[3:]
[1, 2, 4, 5, 6, 7, 8, 9]
>>> 

See also

Explain Python's slice notation

Answer 3

Use np.delete ! It does not actually delete anything inplace

Example:

import numpy as np
a = np.array([[1,4],[5,7],[3,1]])                                       

# a: array([[1, 4],
#           [5, 7],
#           [3, 1]])

ind = np.array([0,1])                                                   

# ind: array([0, 1])

# a[ind]: array([[1, 4],
#                [5, 7]])

all_except_index = np.delete(a, ind, axis=0)                                              
# all_except_index: array([[3, 1]])

# a: (still the same): array([[1, 4],
#                             [5, 7],
#                             [3, 1]])

Answer 4

If you want to cut out the last or the first do this:

list = ["This", "is", "a", "list"]
listnolast = list[:-1]
listnofirst = list[1:]

If you change 1 to 2 the first 2 characters will be removed not the second. Hope this still helps!

Answer 5

If you don't know the index beforehand here is a function that will work

def reverse_index(l, index):
    try:
        l.pop(index)
        return l
    except IndexError:
        return False

Answer 6

If you are using numpy, the closest, I can think of is using a mask

>>> import numpy as np
>>> arr = np.arange(1,10)
>>> mask = np.ones(arr.shape,dtype=bool)
>>> mask[5]=0
>>> arr[mask]
array([1, 2, 3, 4, 5, 7, 8, 9])

Something similar can be achieved using itertools without numpy

>>> from itertools import compress
>>> arr = range(1,10)
>>> mask = [1]*len(arr)
>>> mask[5]=0
>>> list(compress(arr,mask))
[1, 2, 3, 4, 5, 7, 8, 9]