Channel-wise log-softmax function.

This function computes its logarithm of softmax along the second axis. Let \(c = (c_1, c_2, \dots, c_D)\) be the slice of x along with the second axis. For each slice \(c\), it computes the logarithm of the function \(f(c)\) defined as

\[f(c) = {\exp(c) \over \sum_{d} \exp(c_d)}.\]

This method is theoretically equivalent to log(softmax(x)) but is more stable.


log(softmax(x)) may cause underflow when x is too small, because softmax(x) may returns 0. log_softmax method is more stable.

Parameters:x (Variable or numpy.ndarray or cupy.ndarray) – Input variable. A \(n\)-dimensional (\(n \geq 2\)) float array.
Returns:Output variable. A \(n\)-dimensional (\(n \geq 2\)) float array, which is the same shape with x.
Return type:Variable

See also



>>> x = np.array([[0, 1, 2], [0, 2, 4]], np.float32)
>>> x
array([[0., 1., 2.],
       [0., 2., 4.]], dtype=float32)
>>> F.log_softmax(x).data
array([[-2.407606  , -1.4076059 , -0.4076059 ],
       [-4.1429315 , -2.1429315 , -0.14293146]], dtype=float32)
>>> np.allclose(F.log_softmax(x).data, F.log(F.softmax(x)).data)