Question

Can anyone tell me what exactly happens in a Recursion Program.. ??

Answer 1

A function calls itself over and over, until some condition is met.

A silly example: How to walk 100 steps recursively:

 function walk(steps) {
     if (steps > 0) { // not there yet
        take 1 step
        walk(steps - 1); // we're 1 step closer, now walk the rest of the way
     } else { // we're there already, don't walk any further than requested
        echo "You're there!"
     }
 }

 walk(100); // go!

This is what happens:

 walk(100):
   take 1 step
   walk(99):
     take 1 step
     walk(98):
       take 1 step
       walk(97):
         (...)
                   walk(2):
                     take 1 step
                     walk(1):
                       take 1 step
                       walk(0):
                         // "steps > 0" no longer true, use the "else" branch
                         echo "You're there!"

Note that the same thing could be accomplished with a loop (“iteratively”):

function walk(steps) {
  for (c=steps; c > 0; c--) { // note the condition check - "c > 0"
    take 1 step
  }
  echo "You're there!"
}

walk(100); // go!

The program flow would be a bit different, yet the result is the same:

walk(100):
  c=100
  take 1 step
  c=99
  take 1 step
  c=98
  take 1 step
  (...)
  c=2
  take 1 step
  c=1
  take 1 step
  c=0
  // "c > 0" no longer true, exit loop
  echo "You're there!"

It cannot be said whether recursion or iteration is always better. In this example, iteration (loop) is simpler to write and easier to understand than recursion; in other cases (e.g. traversing tree structures), recursion may be a better solution.