Indexing with List of Indices to Exclude

Question

This is similar to some other questions (Explicitly select items from a Python list or tuple, Grabbing specific indices of a list in Python), but I'm looking to do the opposite:

What is a clean way to specify a list/tuple of indices to exclude, instead of to select? I'm thinking of something similar to R or MATLAB where you can specify indices to exclude, like:

vector1 <- c('a', 'b', 'c', 'd')
vector2 <- vector1[-1] # ['b', 'c', 'd']
vector3 <- vector1[c(-1, -2)] # ['c', 'd']

Is there a good way to accomplish the same thing in Python? Apologizes if this is a dupe, I wasn't sure exactly what to search for.

Answer 1

>>> to_exclude = {1, 2}
>>> vector = ['a', 'b', 'c', 'd']
>>> vector2 = [element for i, element in enumerate(vector) if i not in to_exclude]

The tricks here are:

• Use a list comprehension to transform one list into another. (You can also use the filter function, especially if the predicate you're filtering on is already lying around as a function with a nice name.)
• Use enumerate to get each element and its index together.
• Use the in operator against any Set or Sequence* type to decide which ones to filter. (A set is most efficient if there are a lot of values, and probably conceptually the right answer… But it really doesn't matter much for just a handful; if you've already got a list or tuple with 4 indices in it, that's a "Set or Sequence" too, so you can just use it.)

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* Technically, any Container will do. But most Containers that aren't a Set or Sequence would be silly here.

Answer 2

import numpy
target_list = numpy.array(['1','b','c','d','e','f','g','h','i','j'])
to_exclude = [1,4,5]
print target_list[~numpy.in1d(range(len(target_list)),to_exclude)]

because numpy is fun