  We investigated the magnetic structure of the heavy fermion compound
CePt$_2$In$_7$ below $T_N~=5.34(2)$ K using magnetic resonant X-ray diffraction
at ambient pressure. The magnetic order is characterized by a commensurate
propagation vector ${k}_{1/2}~=~\left( \frac{1}{2} , \frac{1}{2},
\frac{1}{2}\right)$ with spins lying in the basal plane. Our measurements did
not reveal the presence of an incommensurate order propagating along the high
symmetry directions in reciprocal space but cannot exclude other incommensurate
modulations or weak scattering intensities. The observed commensurate order can
be described equivalently by either a single-${k}$ structure or by a
multi-${k}$ structure. Furthermore we explain how a commensurate-only ordering
may explain the broad distribution of internal fields observed in nuclear
quadrupolar resonance experiments (Sakai et al. 2011, Phys. Rev. B 83 140408)
that was previously attributed to an incommensurate order. We also report
powder X-ray diffraction showing that the crystallographic structure of
CePt$_2$In$_7$ changes monotonically with pressure up to $P~=~7.3$ GPa at room
temperature. The determined bulk modulus $B_0~=~81.1(3)$ GPa is similar to the
ones of the Ce-115 family. Broad diffraction peaks confirm the presence of
pronounced strain in polycrystalline samples of CePt$_2$In$_7$. We discuss how
strain effects can lead to different electronic and magnetic properties between
polycrystalline and single crystal samples.
