Ping Pong style animation in Cinema 4D can be achieved pretty easily even without Xpresso. Tools like GreyScaleGorilla's Signal Plugin even let you easly setup a Ping Pong Style Animation without keyframes. Tools are built into the timeline to assist in these types of repetitive animations. In the example below, the cube's Z size is oscillating back and forth to a size of 200cm, however there are only two keyframes in the entire animation.
After setting up your initial keyframes, go to the Functions Menu in the Timeline Manager and select Track After/Oscillate After. The default action is to just oscillate (Ping Pong) once, but if you select your object in the Timeline manager, it's properties become available in the Attribute Manager where you can change the number of repetitions to whatever you need.
The Timeline's menus and tools are often forgotten because they are not part of the Standard Cinema 4D Layout, and if they aren't shown to you, you will probably never know they are there and you will find yourself duplicating keyframes for many years to come.
You could use a Random Effector set to Object and use Noise to Oscillate Scale, but you get only a Random result.
What techniques do you use to create repetitive animations in Cinema 4D? Share them in the comments below.