  In this article we investigate the pressure function and affinity dimension
for iterated function systems associated to the "box-like" self-affine fractals
investigated by D.-J. Feng, Y. Wang and J.M. Fraser. Combining previous results
of V. Yu. Protasov, A. K\"aenm\"aki and the author we obtain an explicit
formula for the pressure function which makes it straightforward to compute the
affinity dimension of box-like self-affine sets. We also prove a variant of
this formula which allows the computation of a modified singular value pressure
function defined by J.M. Fraser. We give some explicit examples where the
Hausdorff and packing dimensions of a box-like self-affine fractal may be
easily computed.
