How to remove specific elements in a numpy array

Question

How can I remove some specific elements from a numpy array  Say I have  import numpy as np  a   np array  1 2 3 4 5 6 7 8 9     I then want to remove 3 4 7 from a  All I know is the index of the values  index  2 3 6

User · Answer

To delete by value     modified array   np delete original array  np where original array    value to delete

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Not being a numpy person  I took a shot with    gt  gt  gt  import numpy as np  gt  gt  gt  import itertools  gt  gt  gt    gt  gt  gt  a   np array  1 2 3 4 5 6 7 8 9    gt  gt  gt  index  2 3 6   gt  gt  gt  a   np array list itertools compress a   i not in index for i in range len a        gt  gt  gt  a array  1  2  5  6  8  9     According to my tests  this outperforms numpy delete    I don t know why that would be the case  maybe due to the small size of the initial array   python -m timeit -s  import numpy as np  -s  import itertools  -s  a   np array  1 2 3 4 5 6 7 8 9    -s  index  2 3 6    a   np array list itertools compress a   i not in index for i in range len a        100000 loops  best of 3  12 9 usec per loop  python -m timeit -s  import numpy as np  -s  a   np array  1 2 3 4 5 6 7 8 9    -s  index  2 3 6    np delete a  index   10000 loops  best of 3  108 usec per loop   That s a pretty significant difference  in the opposite direction to what I was expecting   anyone have any idea why this would be the case   Even more weirdly  passing numpy delete   a list performs worse than looping through the list and giving it single indices   python -m timeit -s  import numpy as np  -s  a   np array  1 2 3 4 5 6 7 8 9    -s  index  2 3 6    for i in index        np delete a  i   10000 loops  best of 3  33 8 usec per loop   Edit  It does appear to be to do with the size of the array  With large arrays  numpy delete   is significantly faster   python -m timeit -s  import numpy as np  -s  import itertools  -s  a   np array list range 10000     -s  index  i for i in range 10000  if i   2    0    a   np array list itertools compress a   i not in index for i in range len a        10 loops  best of 3  200 msec per loop  python -m timeit -s  import numpy as np  -s  a   np array list range 10000     -s  index  i for i in range 10000  if i   2    0    np delete a  index   1000 loops  best of 3  1 68 msec per loop   Obviously  this is all pretty irrelevant  as you should always go for clarity and avoid reinventing the wheel  but I found it a little interesting  so I thought I d leave it here

User · Answer

Remove specific index i removed 16 and 21 from matrix   import numpy as np mat   np arange 12 26  a    4 9  del map   np delete mat  a  del map reshape 3 4    Output   array   12  13  14  15          17  18  19  20          22  23  24  25

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You can also use sets   a   numpy array  10  20  30  40  50  60  70  80  90   the index list    2  3  6   the big set   set numpy arange len a    the small set   set the index list  the delta row list   list the big set - the small set   a   a the delta row list

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list comprehension could be an interesting approach as well  a   np array  1  2  3  4  5  6  7  8  9   index   np array  2  3  6    index is changed to an array    out    val for i  val in enumerate a  if all i    index    gt  gt  gt   1  2  5  6  8  9

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If you don t know the index  you can t use logical and  x   10 np random randn 1 100  low   5 high   27 x 0 np logical and x 0    gt low x 0    lt high

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A Numpy array is immutable  meaning you technically cannot delete an item from it  However  you can construct a new array without the values you don t want  like this   b   np delete a   2 3 6

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Using np delete is the fastest way to do it  if we know the indices of the elements that we want to remove  However  for completeness  let me add another way of  removing  array elements using a boolean mask created with the help of np isin  This method allows us to remove the elements by specifying them directly or by their indices   import numpy as np a   np array  1  2  3  4  5  6  7  8  9     Remove by indices   indices to remove    2  3  6  a   a  np isin np arange a size   indices to remove     Remove by elements  don t forget to recreate the original a since it was rewritten in the previous line    elements to remove   a indices to remove      3  4  7  a   a  np isin a  elements to remove

User · Answer

In case you don t have the indices of the elements you want to remove  you can use the function in1d provided by numpy  The function returns True if the element of a 1-D array is also present in a second array  To delete the elements  you just have to negate the values returned by this function  Notice that this method keeps the order from the original array  In  1   import numpy as np          a   np array  1  2  3  4  5  6  7  8  9           rm   np array  3  4  7             np in1d return true if the element of  a  is in  rm          idx   np in1d a  rm          idx  Out 1   array  False  False   True   True  False  False   True  False  False    In  2     Since we want the opposite of what  in1d  gives us             you just have to negate the returned value         a  idx   Out 2   array  1  2  5  6  8  9

User · Answer

There is a numpy built-in function to help with that   import numpy as np  gt  gt  gt  a   np array  1  2  3  4  5  6  7  8  9    gt  gt  gt  b   np array  3 4 7    gt  gt  gt  c   np setdiff1d a b   gt  gt  gt  c array  1  2  5  6  8  9

User · Answer

Use numpy delete   - returns a new array with sub-arrays along an axis deleted  numpy delete a  index    For your specific question   import numpy as np  a   np array  1  2  3  4  5  6  7  8  9   index    2  3  6   new a   np delete a  index   print new a   Prints   1  2  5  6  8  9     Note that numpy delete   returns a new array since array scalars are immutable  similar to strings in Python  so each time a change is made to it  a new object is created  I e   to quote the delete   docs       A copy of arr with the elements specified by obj removed  Note that   delete does not occur in-place       If the code I post has output  it is the result of running the code

[python] How to remove specific elements in a numpy array

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