Every time I see a program I really want to know how it works. For example

for(x=0; x<=2; x++)

tracing...

x=0 0<=2 TRUE
x=1 1<=2 TRUE
x=2 2<=2 TRUE
x=3 3<=2 FALSE

There are some programs that I find it difficult to trace and how can I trace it even if it is hard to trace what I mean is how to determine the flow of the program? I just want to understand the algorithm of the program created by me and by others.