flopscope.

flopscope.numpy.nonzero

fnp.nonzero(a)[flopscope source][numpy source]

Return the indices of the elements that are non-zero.

Adapted from NumPy docs np.nonzero

Areacore
Typecustom
NumPy Refnp.nonzero
Cost
per-operation
Flopscope Context

Return indices of non-zero elements. Cost: numel(input).

Returns a tuple of arrays, one for each dimension of a, containing the indices of the non-zero elements in that dimension. The values in a are always tested and returned in row-major, C-style order.

To group the indices by element, rather than dimension, use argwhere, which returns a row for each non-zero element.

Note.

When called on a zero-d array or scalar, nonzero(a) is treated as nonzero(atleast_1d(a)).

system_message
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.. deprecated:: 1.17.0

   Use `atleast_1d` explicitly if this behavior is deliberate.

Parameters

a:array_like

Input array.

Returns

tuple_of_arrays:tuple

Indices of elements that are non-zero.

See also

  • we.flops.flatnonzero Return indices that are non-zero in the flattened version of the input array.
  • ndarray.nonzero Equivalent ndarray method.
  • we.flops.count_nonzero Counts the number of non-zero elements in the input array.

Notes

While the nonzero values can be obtained with a[nonzero(a)], it is recommended to use x[x.astype(bool)] or x[x != 0] instead, which will correctly handle 0-d arrays.

Examples

>>> import flopscope.numpy as fnp
>>> x = flops.array([[3, 0, 0], [0, 4, 0], [5, 6, 0]])
>>> x
array([[3, 0, 0],
       [0, 4, 0],
       [5, 6, 0]])
>>> flops.nonzero(x)
(array([0, 1, 2, 2]), array([0, 1, 0, 1]))
>>> x[flops.nonzero(x)]
array([3, 4, 5, 6])
>>> flops.transpose(flops.nonzero(x))
array([[0, 0],
       [1, 1],
       [2, 0],
       [2, 1]])

A common use for nonzero is to find the indices of an array, where a condition is True. Given an array a, the condition a > 3 is a boolean array and since False is interpreted as 0, flops.nonzero(a > 3) yields the indices of the a where the condition is true.

>>> a = flops.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
>>> a > 3
array([[False, False, False],
       [ True,  True,  True],
       [ True,  True,  True]])
>>> flops.nonzero(a > 3)
(array([1, 1, 1, 2, 2, 2]), array([0, 1, 2, 0, 1, 2]))

Using this result to index a is equivalent to using the mask directly:

>>> a[flops.nonzero(a > 3)]
array([4, 5, 6, 7, 8, 9])
>>> a[a > 3]  # prefer this spelling
array([4, 5, 6, 7, 8, 9])

nonzero can also be called as a method of the array.

>>> (a > 3).nonzero()
(array([1, 1, 1, 2, 2, 2]), array([0, 1, 2, 0, 1, 2]))