Filtering a list based on a list of booleans

Question

I have a list of values which I need to filter given the values in a list of booleans   list a    1  2  4  6  filter    True  False  True  False    I generate a new filtered list with the following line   filtered list    i for indx i in enumerate list a  if filter indx     True    which results in   print filtered list  1 4    The line works but looks  to me  a bit overkill and I was wondering if there was a simpler way to achieve the same     Advices  Summary of two good advices given in the answers below   1- Don t name a list filter like I did because it is a built-in function   2- Don t compare things to True like I did with if filter idx   True   since it s unnecessary  Just using if filter idx  is enough

User · Answer

With numpy   In  128   list a   np array  1  2  4  6   In  129   filter   np array  True  False  True  False   In  130   list a filter   Out 130   array  1  4     or see Alex Szatmary s answer if list a can be a numpy array but not filter  Numpy usually gives you a big speed boost as well  In  133   list a    1  2  4  6  10000 In  134   fil    True  False  True  False  10000 In  135   list a np   np array list a  In  136   fil np   np array fil   In  139    timeit list itertools compress list a  fil   1000 loops  best of 3  625 us per loop  In  140    timeit list a np fil np  10000 loops  best of 3  173 us per loop

User · Answer

With python 3 you can use list a filter  to get True values  To get False values use list a  filter

User · Answer

Like so   filtered list    i for  i  v  in zip list a  filter  if v    Using zip is the pythonic way to iterate over multiple sequences in parallel  without needing any indexing  This assumes both sequences have the same length  zip stops after the shortest runs out   Using itertools for such a simple case is a bit overkill      One thing you do in your example you should really stop doing is comparing things to True  this is usually not necessary  Instead of if filter idx   True       you can simply write if filter idx

User · Answer

filtered list    list a i  for i in range len list a   if filter i

User · Answer

To do this using numpy  ie  if you have an array  a  instead of list a   a   np array  1  2  4  6   my filter   np array  True  False  True  False   dtype bool  a my filter   gt  array  1  4

User · Answer

You re looking for itertools compress    gt  gt  gt  from itertools import compress  gt  gt  gt  list a    1  2  4  6   gt  gt  gt  fil    True  False  True  False   gt  gt  gt  list compress list a  fil    1  4    Timing comparisons py3 x     gt  gt  gt  list a    1  2  4  6   gt  gt  gt  fil    True  False  True  False   gt  gt  gt   timeit list compress list a  fil   100000 loops  best of 3  2 58 us per loop  gt  gt  gt   timeit  i for  i  v  in zip list a  fil  if v    winner 100000 loops  best of 3  1 98 us per loop   gt  gt  gt  list a    1  2  4  6  100  gt  gt  gt  fil    True  False  True  False  100  gt  gt  gt   timeit list compress list a  fil                 winner 10000 loops  best of 3  24 3 us per loop  gt  gt  gt   timeit  i for  i  v  in zip list a  fil  if v  10000 loops  best of 3  82 us per loop   gt  gt  gt  list a    1  2  4  6  10000  gt  gt  gt  fil    True  False  True  False  10000  gt  gt  gt   timeit list compress list a  fil                 winner 1000 loops  best of 3  1 66 ms per loop  gt  gt  gt   timeit  i for  i  v  in zip list a  fil  if v   100 loops  best of 3  7 65 ms per loop   Don t use filter as a variable name  it is a built-in function

[python] Filtering a list based on a list of booleans

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