Can loops iterate over time? For example, iteration 1 - Frame 1; iteration 2 - Frame 2; …

Yes, except it's called a solver rather than a loop. Use a solver sop, which runs the network inside once each time step, using the results of the previous frame as the starting point for the next iteration.

http://www.sidefx.com/docs/houdini/nodes/sop/solver.html [www.sidefx.com]

you can certainly put a loop inside a solver! it can be brain twisting though, embrace it!

solver wouldn't work for me, because you can't put it in compile blocks. I am looping through two opencl nodes, which only for loop nodes handle efficiently