Index of element in NumPy array [duplicate]

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?

Answer 1

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 >=, <=, != 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]),) 

Answer 2

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.