About Week 4

We goes to recursion in week 4. Somehow I think I gradually follow the class of CSC148. 148 focus on general idea and does not do as much practice as 108. Yes, we do have labs, but I do not think I works as well as PCRS.

Recursion, in short, means use the definition itself to define the definition. It has certain pattern:

def mm(L):
    """
    (int or arbitrarily nested list of int) -> int
    
    Mystery method.  Exactly the same as sum_list,
    except for the last line
    """
    
    if isinstance(L, list):
        return sum( [count(x) for x in L] )
    else:
        return 0

Here the isinstance(X, type) is always there because we need to decide whether to recurs L or end L. By this way we peel the L like an onion until it becomes the very basic element. In the example above L may originally be a list, and after recurs L again and again, the lists would be broken and separated into small elements.