  Nonuniform structure of low-density nuclear matter, known as nuclear pasta,
is expected to appear not only in the inner crust of neutron stars but also in
core-collapse supernova explosions and neutron-star mergers. We perform fully
three-dimensional calculations of inhomogeneous nuclear matter and neutron-star
matter in the low-density region using the Thomas-Fermi approximation. The
nuclear interaction is described in the relativistic mean-field approach with
the point-coupling interaction, where the meson exchange in each channel is
replaced by the contact interaction between nucleons. We investigate the
influence of nuclear symmetry energy and its density dependence on pasta
structures by introducing a coupling term between the isoscalar-vector and
isovector-vector interactions. It is found that the properties of pasta phases
in the neutron-rich matter are strongly dependent on the symmetry energy and
its slope. In addition to typical shapes like droplets, rods, slabs, tubes, and
bubbles, some intermediate pasta structures are also observed in cold stellar
matter with a relatively large proton fraction. We find that nonspherical
shapes are unlikely to be formed in neutron-star crusts, since the proton
fraction obtained in $\beta$ equilibrium is rather small. The inner crust
properties may lead to a visible difference in the neutron-star radius.
