Index of element in NumPy array

Question

In Python we can get the index of a value in an array by using  index    But with a NumPy array  when I try to do  decoding index i   I get   AttributeError   numpy ndarray  object has no attribute  index   How could I do this on a NumPy array

User · Answer

You can use the function numpy nonzero    or the nonzero   method of an array  import numpy as np  A   np array   2 4              6 2    index  np nonzero A gt 1         OR  A gt 1  nonzero     Output    array  0  1    array  1  0      First array in output depicts the row index and second array depicts the corresponding column index

User · Answer

I m torn between these two ways of implementing an index of a NumPy array   idx   list classes  index var  idx   np where classes    var    Both take the same number of characters  but the first method returns an int instead of a numpy ndarray

User · Answer

This problem can be solved efficiently using the numpy indexed library  disclaimer  I am its author   which was created to address problems of this type  npi indices can be viewed as an n-dimensional generalisation of list index  It will act on nd-arrays  along a specified axis   and also will look up multiple entries in a vectorized manner as opposed to a single item at a time   a   np random rand 50  60  70  i   np random randint 0  len a   40  b   a i   import numpy indexed as npi assert all i    npi indices a  b     This solution has better time complexity  n log n at worst  than any of the previously posted answers  and is fully vectorized

User · Answer

Use np where to get the indices where a given condition is True   Examples   For a 2D np ndarray called a   i  j   np where a    value    when comparing arrays of integers  i  j   np where np isclose a  value     when comparing floating-point arrays   For a 1D array   i    np where a    value    integers  i    np where np isclose a  value     floating-point   Note that this also works for conditions like  gt     lt       and so forth     You can also create a subclass of np ndarray with an index   method   class myarray np ndarray       def   new   cls   args    kwargs           return np array  args    kwargs  view myarray      def index self  value           return np where self    value    Testing   a   myarray  1 2 3 4 4 4 5 6 4 4 4   a index 4    array   3   4   5   8   9  10

User · Answer

You can convert a numpy array to list and get its index    for example   tmp    1 2 3 4 5   python list a   numpy array tmp   numpy array i   list a  index 2    i will return index of 2  which is 1   this is just what you wanted

User · Answer

If you are interested in the indexes  the best choice is np argsort a   a   np random randint 0  100  10  sorted idx   np argsort a

[python] Index of element in NumPy array

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