A novel convex necessary and sufficient condition for state-feedback exponential stabilisability in discrete-time switched linear systems, whose modes are described by rank-one matrices, is reported and proved in the present communication. A switched linear system, of this class, is shown to be state-feedback exponentially stabilisable if and only if a set of linear matrix inequalities (LMIs) associated to the system is feasible. And the solvability of this set of LMIs associated to the system is shown to be equivalent to that of a set of (standard) linear inequalities associated to the system. It is also proved that each solution to this set of LMIs (associated to the system) yields, through explicit formulas, to an exponentially stabilising state-feedback mapping, and also to a Lyapunov function for the exponential stability of the trivial solution of the corresponding closed-loop system (obtained by means of that feedback mapping). And such a Lyapunov function is always represented by a number of quadratic functionals that equals the number of modes composing the switched system.