About the 2008 6.00 OCW, in problem set 3 (Q1), how do I count the number of instances of the key in the target string using recursion?

Question

rsmith6559 · Answer

Generally, a recursive function has three parts.

The "base case", which is when to stop calling the function, in this case when the end of the string is reached.

Code for what you want to do as you're recursing.  Before the base case is reached, and more recursion needs to be done.

Code for what you want to do as the recursion "unwinds".  After the base case is reached and the recursions are returning.

Thinking recursively is tough to get used to, but well worth the effort.

anonymous · Answer

@rsmith6559: Well, I do know the logical steps of recursion; I'm unsure of how to count the no. of recursions that occur. Thanks anyway.

rsmith6559 · Answer

The base case returns 0, and the others increment it before returning it.

anonymous · Answer

@rsmith6559: But again when the recursion stops and control falls back to the base step, the returned value is 0!! :(  Could you please check my test file?

rsmith6559 · Answer

def countSubStringMatchRecursive(target,key):
  #  base case
  if( len( target ) == 0 ):
    return 0
  else:
    #  unwind
    return countSubStringMatchRecursive(target[i:],key) + 1