Python Select subset from list based on index set

Question

I have several lists having all the same number of entries  each specifying an object property    property a    545   656   5 4  33   property b     1 2   1 3  2 3  0 3        and list with flags of the same length  good objects    True  False  False  True     which could easily be substituted with an equivalent index list   good indices    0  3    What is the easiest way to generate new lists property asel  property bsel      which contain only the values indicated either by the True entries or the indices   property asel    545   33   property bsel     1 2  0 3

User · Accepted Answer

You could just use list comprehension   property asel    val for is good  val in zip good objects  property a  if is good    or  property asel    property a i  for i in good indices    The latter one is faster because there are fewer good indices than the length of property a  assuming good indices are precomputed instead of generated on-the-fly     Edit  The first option is equivalent to itertools compress available since Python 2 7 3 1  See  Gary Kerr s answer   property asel   list itertools compress property a  good objects

User · Answer

I see 2 options    Using numpy   property a   numpy array  545   656   5 4  33    property b   numpy array   1 2   1 3  2 3  0 3   good objects    True  False  False  True  good indices    0  3  property asel   property a good objects  property bsel   property b good indices   Using a list comprehension and zip it   property a    545   656   5 4  33   property b     1 2   1 3  2 3  0 3  good objects    True  False  False  True  good indices    0  3  property asel    x for x  y in zip property a  good objects  if y  property bsel    property b i  for i in good indices

User · Answer

Use the built in function zip  property asel    a for  a  truth  in zip property a  good objects  if truth      EDIT  Just looking at the new features of 2 7  There is now a function in the itertools module which is similar to the above code   http   docs python org library itertools html itertools compress  itertools compress  ABCDEF    1 0 1 0 1 1     gt    A  C  E  F

User · Answer

Matlab and Scilab languages offer a simpler and more elegant syntax than Python for the question you re asking  so I think the best you can do is to mimic Matlab Scilab by using the Numpy package in Python  By doing this the solution to your problem is very concise and elegant   from numpy import   property a   array  545   656   5 4  33    property b   array   1 2   1 3  2 3  0 3   good objects    True  False  False  True  good indices    0  3  property asel   property a good objects  property bsel   property b good indices    Numpy tries to mimic Matlab Scilab but it comes at a cost  you need to declare every list with the keyword  array   something which will overload your script  this problem doesn t exist with Matlab Scilab   Note that this solution is restricted to arrays of number  which is the case in your example

User · Answer

Assuming you only have the list of items and a list of true required indices  this should be the fastest   property asel     property a index  for index in good indices     This means the property selection will only do as many rounds as there are true required indices  If you have a lot of property lists that follow the rules of a single tags  true false  list you can create an indices list using the same list comprehension principles   good indices     index for index  item in enumerate good objects  if item     This iterates through each item in good objects  while remembering its index with enumerate  and  returns only the indices where the item is true     For anyone not getting the list comprehension  here is an English prose version with the code highlighted in bold   list the index for every group of index  item that exists in an enumeration of good objects  if  where  the item is True

[python] Python: Select subset from list based on index set

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