WEEK08: more top-down design
 F: File I/O (write) and more function practice

OUTPUT to a file:

 - here's how to open a file for writing (note: myfile is a variable
   name that I choose, and "newfile" is the name of the file to write to):

$ python
>>> myfile = open("newfile", 'w')
>>> type(myfile)
<type 'file'>
(a new python type!!)
>>> myfile.write("write this to the file \n")
>>> myfile.write("and this.... \n")
>>> myfile.close()

 - and here are the results of this:

$ ls

$ cat newfile 
write this to the file 
and this.... 

 - what happens if we leave out the \n's on each line??

 - if "newfile" exists, and we open it with mode "w", then "newfile"
   will be wiped out and overwritten!

 - can also use mode "a" to append to a file


 - draw the stack for this one:

def double(L):
  """assume L is a list of numbers, double each"""

  for i in range(len(L)):
    L[i] = 2*L[i]

>>> L = [2,6,9] 
>>> double(L)
>>> print L
[4, 12, 18]