softmax_cross_entropy(x, t, normalize=True, cache_score=True, class_weight=None, ignore_label=-1, reduce='mean', enable_double_backprop=False)¶
Computes cross entropy loss for pre-softmax activations.
Variableor N-dimensional array) – Variable holding a multidimensional array whose element indicates unnormalized log probability: the first axis of the variable represents the number of samples, and the second axis represents the number of classes. While this function computes a usual softmax cross entropy if the number of dimensions is equal to 2, it computes a cross entropy of the replicated softmax if the number of dimensions is greater than 2.
normalize (bool) – If
True, this function normalizes the cross entropy loss across all instances. If
False, it only normalizes along a batch size.
cache_score (bool) – When it is
True, the function stores result of forward computation to use it on backward computation. It reduces computational cost though consumes more memory. If
True, this option is forcibly turned off and the function does not cache the intermediate value.
class_weight (N-dimensional array) – An array that contains constant weights that will be multiplied with the loss values along with the second dimension. The shape of this array should be
(x.shape,). If this is not
None, each class weight
class_weight[i]is actually multiplied to
y[:, i]that is the corresponding log-softmax output of
xand has the same shape as
xbefore calculating the actual loss value.
ignore_label (int) – Label value you want to ignore. Its default value is
-1. See description of the argument t.
reduce (str) – A string that determines whether to reduce the loss values. If it is
'mean', it computes the sum of the individual cross entropy and normalize it according to
normalizeoption. If it is
'no', this function computes cross entropy for each instance and does not normalize it (
normalizeoption is ignored). In this case, the loss value of the ignored instance, which has
ignore_labelas its target value, is set to
enable_double_backprop (bool) – If
True, this function uses implementation that supports higher order differentiation. If
False, it uses single-backprop implementation. This function use the single-backprop version because we expect it is faster. So, if you need second or higher derivatives, you need to turn it on explicitly.
A variable holding a scalar array of the cross entropy loss. If
'mean', it is a scalar array. If
'no', the shape is same as that of
- Return type
This function is differentiable only by
>>> x = np.array([[-1, 0, 1, 2], [2, 0, 1, -1]]).astype(np.float32) >>> x array([[-1., 0., 1., 2.], [ 2., 0., 1., -1.]], dtype=float32) >>> t = np.array([3, 0]).astype(np.int32) >>> t array([3, 0], dtype=int32) >>> y = F.softmax_cross_entropy(x, t) >>> y variable(0.44018972) >>> log_softmax = -F.log_softmax(x) >>> expected_loss = np.mean([log_softmax[row, column].data for row, column in enumerate(t)]) >>> y.array == expected_loss True