jax.numpy.compress#

jax.numpy.compress(condition, a, axis=None, *, size=None, fill_value=0, out=None)[source]#

Compress an array along a given axis using a boolean condition.

JAX implementation of numpy.compress().

Parameters:

  • condition (ArrayLike) – 1-dimensional array of conditions. Will be converted to boolean.

  • a (ArrayLike) – N-dimensional array of values.

  • axis (int | None | None) – axis along which to compress. If None (default) then a will be flattened, and axis will be set to 0.

  • size (int | None | None) – optional static size for output. Must be specified in order for compress to be compatible with JAX transformations like jit() or vmap().

  • fill_value (ArrayLike) – if size is specified, fill padded entries with this value (default: 0).

  • out (None | None) – not implemented by JAX.

Returns:

An array of dimension a.ndim, compressed along the specified axis.

Return type:

Array

See also

Notes

This function does not require strict shape agreement between condition and a. If condition.size > a.shape[axis], then condition will be truncated, and if a.shape[axis] > condition.size, then a will be truncated.

Examples

Compressing along the rows of a 2D array:

>>> a = jnp.array([[1,  2,  3,  4],
...                [5,  6,  7,  8],
...                [9,  10, 11, 12]])
>>> condition = jnp.array([True, False, True])
>>> jnp.compress(condition, a, axis=0)
Array([[ 1,  2,  3,  4],
       [ 9, 10, 11, 12]], dtype=int32)

For convenience, you can equivalently use the compress() method of JAX arrays:

>>> a.compress(condition, axis=0)
Array([[ 1,  2,  3,  4],
       [ 9, 10, 11, 12]], dtype=int32)

Note that the condition need not match the shape of the specified axis; here we compress the columns with the length-3 condition. Values beyond the size of the condition are ignored:

>>> jnp.compress(condition, a, axis=1)
Array([[ 1,  3],
       [ 5,  7],
       [ 9, 11]], dtype=int32)

The optional size argument lets you specify a static output size so that the output is statically-shaped, and so this function can be used with transformations like jit() and vmap():

>>> f = lambda c, a: jnp.extract(c, a, size=len(a), fill_value=0)
>>> mask = (a % 3 == 0)
>>> jax.vmap(f)(mask, a)
Array([[ 3,  0,  0,  0],
       [ 6,  0,  0,  0],
       [ 9, 12,  0,  0]], dtype=int32)