  As known, any numerical simulation is composed of two parts: (1) the initial
part of writing the relevant code and (2) the running of this code on the
computer screen. The second part of running the program is extensively
discussed theoretically and technically in the relevant literature. In this
work we pay special attention to the less discussed first part and show that it
may be discussed in a terminology and notation which describe physical
phenomena. As examples we discuss the two cases of simulating (1) the harmonic
oscillator and (2) the electron-photon interaction which results in the known
Lamb shift.
