# Online Convex Optimization: meaning of different loss function each time $t$

In the survey of Shai Shalev-Shwartz (see 1), the online convex optimization is presented as:

where one can see that the loss function is chosen. All proofs for regret bounds use (more or less) the loss difference $f_{t}(w_t) - f_{t+1}(w_t)$ and also $f_{t+1}(w_t) - f_{t+1}(w_{t+1})$.

Concretelly,

as part of the proof, the author stets:

I can understand that $f_t(w_t)$ denotes the 'chosen' loss function at time $t$, where the solution vector $w_t$ was given. Yet, what does $f_{t+1}(w_t)$ mean? how can the loss function be chosen one step ahead and use the previous solution vector to the optimization? how can I compute the loss differences $f_t(w_t) - f_t(w_{t+1})$?

What does $f_{t+1}(w_t)$ mean? It means $f_{t+1}$ is evaluated at the argument value $w_t$.
As an illustrative example, let $t = 1, f_1(x) = x^2, w_1 = 1, w_2 = 2$. Then $f_t(w_t) - f_t(w_{t+1}) = 1 - 4 = -3$. At the time of the evaluation (at t+1), $f_t$ is still known, therefore it can be evaluated.