linalg:gather

(linalg:gather matrix indices)

Returns the per-row elements a[i, idx[i]] of a matrix (numpy's y[np.arange(n), t] fancy-indexing idiom) as a vector of the input's width -- the pick-the-target-class-probability step of a cross-entropy loss. Index values are truncated to integers. It signals an error unless the input is a matrix and the index length matches its rows. For selecting whole rows instead, use linalg:take-rows.