  Even at tree level, the first quantized string theory suffers from apparent
short distance singularities associated with collision of vertex operators that
prevent us from straightforward numerical computation of various quantities.
Examples include string theory S-matrix for generic external momenta and
computation of the spectrum of string theory under a marginal deformation of
the world-sheet theory. The former requires us to define the S-matrix via
analytic continuation or as limits of contour integrals in complexified moduli
space, while the latter requires us to use an ultraviolet cut-off at
intermediate steps. In contrast, string field theory does not suffer from such
divergences. In this paper we show how string field theory can be used to
generate an explicit algorithm for computing tree level amplitudes in any
string theory that does not suffer from any short distance divergence from
integration over the world-sheet variables. We also use string field theory to
compute second order mass shift of string states under a marginal deformation
without having to use any cut-off at intermediate steps. We carry out the
analysis in a broad class of string field theories, thereby making it manifest
that the final results are independent of the extra data that go into the
formulation of string field theory. We also comment on the generalization of
this analysis to higher genus amplitudes.
