I have to calculate the return of a vector that gives a historical price series of a stock. The vector is of a form:
a <- c(10.25, 11.26, 14, 13.56)
Using your sample data, I think you mean the following:
a <- c(10.25, 11.26, 14, 13.56) > diff(a)/a[-length(a)]  0.09853659 0.24333925 -0.03142857
diff returns the vector of lagged differences and
a[-length(a)] drops the last element of a.