scipy.special.xlog1py#

scipy.special.xlog1py(x, y, out=None) = <ufunc 'xlog1py'>#

Compute x*log1p(y) so that the result is 0 if x = 0.

Parameters:

xarray_like: Multiplier
yarray_like: Argument
outndarray, optional: Optional output array for the function results

Returns:

zscalar or ndarray: Computed x*log1p(y)

Notes

Added in version 0.13.0.

Array API Standard Support

xlog1py has experimental support for Python Array API Standard compatible backends in addition to NumPy. Please consider testing these features by setting an environment variable SCIPY_ARRAY_API=1 and providing CuPy, PyTorch, JAX, or Dask arrays as array arguments. The following combinations of backend and device (or other capability) are supported.

Library	CPU	GPU
NumPy	✅	n/a
CuPy	n/a	✅
PyTorch	✅	✅
JAX	✅	✅
Dask	✅	n/a

See Support for the array API standard for more information.

Examples

This example shows how the function can be used to calculate the log of the probability mass function for a geometric discrete random variable. The probability mass function of the geometric distribution is defined as follows:

\[f(k) = (1-p)^{k-1} p\]

where \(p\) is the probability of a single success and \(1-p\) is the probability of a single failure and \(k\) is the number of trials to get the first success.

>>> import numpy as np
>>> from scipy.special import xlog1py
>>> p = 0.5
>>> k = 100
>>> _pmf = np.power(1 - p, k - 1) * p
>>> _pmf
7.888609052210118e-31

If we take k as a relatively large number the value of the probability mass function can become very low. In such cases taking the log of the pmf would be more suitable as the log function can change the values to a scale that is more appropriate to work with.

>>> _log_pmf = xlog1py(k - 1, -p) + np.log(p)
>>> _log_pmf
-69.31471805599453

We can confirm that we get a value close to the original pmf value by taking the exponential of the log pmf.

>>> _orig_pmf = np.exp(_log_pmf)
>>> np.isclose(_pmf, _orig_pmf)
True