Numpy where function multiple conditions

Question

I have an array of distances called dists  I want to select dists which are between two values  I wrote the following line of code to do that    dists  np where dists  gt   r   and  np where dists  lt   r   dr      However this selects only for the condition     np where dists  lt   r   dr     If I do the commands sequentially by using a temporary variable it works fine  Why does the above code not work  and how do I get it to work    Cheers

User · Answer

Try   np intersect1d np where dists  gt   r  0  np where dists  lt   r   dr  0

User · Answer

I like to use np vectorize for such tasks  Consider the following    gt  gt  gt    function which returns True when constraints are satisfied   gt  gt  gt  func   lambda d  d  gt   r and d lt    r dr    gt  gt  gt   gt  gt  gt    Apply constraints element-wise to the dists array   gt  gt  gt  result   np vectorize func  dists    gt  gt  gt   gt  gt  gt  result   np where result    Get output    You can also use np argwhere instead of np where for clear output  But that is your call     Hope it helps

User · Answer

One interesting thing to point here  the usual way of using OR and AND too will work in this case  but with a small change  Instead of  and  and instead of  or   rather use Ampersand  amp   and Pipe Operator    and it will work   When we use  and    ar   np array  3 4 5 14 2 4 3 7   np where  ar gt 3  and  ar lt 6    yo   ar   Output  ValueError  The truth value of an array with more than one element is ambiguous  Use a any   or a all     When we use Ampersand  amp     ar   np array  3 4 5 14 2 4 3 7   np where  ar gt 3   amp   ar lt 6    yo   ar   Output  array   3    yo    yo    14    2    yo    3    7    dtype   lt U11     And this is same in the case when we are trying to apply multiple filters in case of pandas Dataframe  Now the reasoning behind this has to do something with Logical Operators and Bitwise Operators and for more understanding about same  I d suggest to go through this answer or similar Q A in stackoverflow   UPDATE  A user asked  why is there a need for giving  ar 3  and  ar lt 6  inside the parenthesis  Well here s the thing  Before I start talking about what s happening here  one needs to know about Operator precedence in Python   Similar to what BODMAS is about  python also gives precedence to what should be performed first  Items inside the parenthesis are performed first and then the bitwise operator comes to work  I ll show below what happens in both the cases when you do use and not use            Case1   np where  ar gt 3  amp  ar lt 6   yo   ar  np where  np array  3 4 5 14 2 4 3 7   gt 3  amp  np array  3 4 5 14 2 4 3 7   lt 6   yo   ar    Since there are no brackets here  the bitwise operator  amp   is getting confused here that what are you even asking it to get logical AND of  because in the operator precedence table if you see   amp  is given precedence over  lt  or  gt  operators  Here s the table from from lowest precedence to highest precedence     It s not even performing the  lt  and  gt  operation and being asked to perform a logical AND operation  So that s why it gives that error      One can check out the following link to learn more about  operator   precedence   Now to Case 2   If you do use the bracket  you clearly see what happens   np where   ar gt 3   amp   ar lt 6    yo   ar  np where   array  False   True   True   True  False   True  False   True     amp   array   True   True   True  False   True   True   True  False      yo   ar    Two arrays of True and False  And you can easily perform logical AND operation on them  Which gives you   np where  array  False   True   True  False  False   True  False  False      yo   ar    And rest you know  np where  for given cases  wherever True  assigns first value i e  here  yo   and if False  the other i e  here  keeping the original    That s all  I hope I explained the query well

User · Answer

I have worked out this simple example   import numpy as np  ar   np array  3 4 5 14 2 4 3 7    print  X for X in list ar  if  X  gt   3 and X  lt   6     gt  gt  gt    3  4  5  4  3

User · Answer

The accepted answer explained the problem well enough  However  the more Numpythonic approach for applying multiple conditions is to use numpy logical functions  In this case  you can use np logical and  np where np logical and np greater equal dists r  np greater equal dists r   dr

User · Answer

Try    import numpy as np dist   np array  1 2 3 4 5   r   2 dr   3 np where np logical and dist gt  r  dist lt  r dr     Output   array  2  3      You can see Logic functions for more details

User · Answer

The best way in your particular case would just be to change your two criteria to one criterion   dists abs dists - r - dr 2    lt   dr 2     It only creates one boolean array  and in my opinion is easier to read because it says  is dist within a dr or r   Though I d redefine r to be the center of your region of interest instead of the beginning  so r   r   dr 2    But that doesn t answer your question     The answer to your question  You don t actually need where if you re just trying to filter out the elements of dists that don t fit your criteria   dists  dists  gt   r   amp   dists  lt   r dr     Because the  amp  will give you an elementwise and  the parentheses are necessary    Or  if you do want to use where for some reason  you can do    dists  np where  dists  gt   r   amp   dists  lt   r   dr         Why  The reason it doesn t work is because np where returns a list of indices  not a boolean array   You re trying to get and between two lists of numbers  which of course doesn t have the True False values that you expect   If a and b are both True values  then a and b returns b   So saying something like  0 1 2  and  2 3 4  will just give you  2 3 4    Here it is in action   In  230   dists   np arange 0 10  5  In  231   r   5 In  232   dr   1  In  233   np where dists  gt   r  Out 233    array  10  11  12  13  14  15  16  17  18  19      In  234   np where dists  lt   r dr  Out 234    array   0   1   2   3   4   5   6   7   8   9  10  11  12      In  235   np where dists  gt   r  and np where dists  lt   r dr  Out 235    array   0   1   2   3   4   5   6   7   8   9  10  11  12       What you were expecting to compare was simply the boolean array  for example  In  236   dists  gt   r Out 236    array  False  False  False  False  False  False  False  False  False         False   True   True   True   True   True   True   True   True          True   True   dtype bool   In  237   dists  lt   r   dr Out 237    array   True   True   True   True   True   True   True   True   True          True   True   True   True  False  False  False  False  False         False  False   dtype bool   In  238    dists  gt   r   amp   dists  lt   r   dr  Out 238    array  False  False  False  False  False  False  False  False  False         False   True   True   True  False  False  False  False  False         False  False   dtype bool    Now you can call np where on the combined boolean array   In  239   np where  dists  gt   r   amp   dists  lt   r   dr   Out 239    array  10  11  12      In  240   dists np where  dists  gt   r   amp   dists  lt   r   dr    Out 240   array   5     5 5   6       Or simply index the original array with the boolean array using fancy indexing  In  241   dists  dists  gt   r   amp   dists  lt   r   dr   Out 241   array   5     5 5   6

User · Answer

This should work   dists   dists  gt   r   amp   dists  lt   r dr      The most elegant way

[python] Numpy where function multiple conditions

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