Getting True values in a boolean list from indices - python

Question

How do I compute the inverse of what is described here: Getting indices of True values in a boolean list ? That above link always comes up when I try searching for "how to obtain the true values in a boolean list from integer indices," but it gives me the indices from the true values in a boolean list, which is the inverse of what I want...

For example, from:

t = [4, 5, 7]
count = 16

I want to obtain:

[False, False, False, False, True, True, False, True, False, False, False, False, False, False, False, False]

The values are all 0 indexed, as expected with Python. I'm guessing that my question is a duplicate, but it's so annoying to not be able to find what I'm looking for every time I try to remember how to do this operation, I decided to ask a new question so my Google search will hopefully bring up this post next time.

jpp · Accepted Answer · 2018-12-11 12:35:09Z

4

You can use a list comprehension. I recommend you turn t into a set for O(1) lookup:

t_set = set(t)
res = [i in t_set for i in range(count)]

edited Dec 11, 2018 at 12:35

answered Dec 11, 2018 at 3:42

jpp

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U13-Forward · Accepted Answer · 2018-12-11 03:39:53Z

3

Use a list comprehension with conditions:

print([True if i in t else False for i in range(count)])

Shorter:

print([i in t else False for i in range(count)])

answered Dec 11, 2018 at 3:39

U13-Forward

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Frank AK · Accepted Answer · 2018-12-11 03:47:29Z

2

How about this:

In [6]: holderplace =[False for i in range(count)]

In [7]: for i in t:
   ...:     holderplace[i-1]=True
   ...:     

In [8]: holderplace
Out[8]: 
[False,
 False,
 False,
 True,
 True,
 False,
 True,
 False,
 False,
 False,
 False,
 False,
 False,
 False,
 False,
 False]

In [9]:

answered Dec 11, 2018 at 3:47

Frank AK

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RoadRunner · Accepted Answer · 2018-12-11 04:01:31Z

1

You could also try using map():

list(map(lambda x: x in t, range(count)))
# [False, False, False, False, True, True, False, True, False, False, False, False, False, False, False, False]

It might also be worth converting t to a set, since lookup is O(1) instead of O(N).

You could also use __contains__():

list(map(t.__contains__, range(count)))

edited Dec 11, 2018 at 4:01

answered Dec 11, 2018 at 3:42

RoadRunner

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