Understanding recursion using all possible combinations in an array

I'm trying to understand recursion with the help of few examples. I found this example of printing all possible combinations of r elements in a given array of size n using recursion.

Print all possible combinations of r elements in a given array of size n.

They are using the idea behind the expression:

enter image description here

What i'm trying to understand here is the conceptual meaning of this expression. I have read different articles but couldn't find a satisfactory explanation.

A mathematical or practical example which uses this expression would be really helpful.

Answer:1

I need to create some directories and files and all of them should have permission 0600. When I Run from NetBeans Debug: After creating directory when i try to store some files there I get IOException ...

I am writing a program which reads in files and then processes them. At the moment, I am using a BufferedReader to read in a list of files from a txt file. This txt file was generated in a command ...

Question about ScheduledExecutorService.shceduleAtFixedRate - I schedule taskA to run every 500 millis, which blocks for 1000 millis. Now subsequent executions aren't gonna wait the extra 500 millis, ...

Take any OOPs language with referencing as basic structure like java and c#. For fast execution, they also support primitive types like int and char. I think this is done by storing them on the stack....