We reduce the computation of Gromov-Witten invariants of CP^n in
genus 0 to the combinatorial enumeration of floor diagrams. This
formula was announced in \cite{Br7}.
This is a preliminary draft of the extended version of \cite{Br7}. The
final version of the paper will also contain the
computation of Welschinger invariants of RP^3, and will be available soon.