Because only one class (let's call it i) is the correct one, the vector y takes the value 1 only for this value of k, i.e., has y i = 1 and y j = 0 ∀ j = i. A vector like this, with one value=1 and the rest 0, is called a one-hot vector. The terms in the sum in Eq. 5.34 will thus be 0 except for the term corresponding to the true class, i.e.: