CS 61A

Time: Wed 10/18/17 6 pm

Extra Midterm 2 Practice

Vulcans (sp15-mt2-2)

1. Draw the environment diagram that results from executing the code after the entire program is finished. No errors occur during the execution of this example.

   def scramble(egg):
     return [egg, over(egg)]
   
   def over(easy):
     easy[1] = [[easy], 2]
     return list(easy[1])
   
   egg = scramble([12, 24])

Pointers (sp16-mt2-1)

For each of the following code fragments, draw the final state of the program in boxes and arrows.

• Code:

  L = Link(1, Link(2))
  P = L
  Q = Link(L, Link(P))
  P.rest.rest = Q

• Code:

  L = Link.empty
  for i in range(3):
    L = Link(i, L)

• Code:

  P = Link(0, Link(1, Link(2)))
  def crack1(L):
    if L is Link.empty:
      return (Link.empty, Link.empty)
    L1, L2 = crack1(L.rest)
    return (Link(L.first, L2), L1)
  Q, R = crack1(P)

• Code:

  P = Link(0, Link(1, Link(2)))
  def crack2(L):
    if L is Link.empty:
      return (Link.empty, Link.empty)
    L1, L2 = crack2(L.rest)
    L.rest = L2
    return (L, L1)
  Q, R = crack2(P)

Extra Notes on __repr__()

Several scenarios while you are interacting with the Python interpreter.

'abc' # 'abc' - There are quotation marks around it
1 # 1 - There's no quotation mark around it

However, things can become interesting working with custom objects in terms of __repr__()!

class Foo:

  def __repr__(self):
    return 'abc'

f = Foo()
f # abc - There's no quotation mark around it

Compared to the following:

class Bar:

  def __repr__(self):
    return "'abc'"

b = Bar()
b # 'abc' - There are quotation marks around it

Take-Aways

However, when you are simply printing something out with print(), Python will display the __str__() of the object you passed to it.

When you are querying an statement/expression in REPL, Python will display the __repr__() of the object (if not None). You can think the following...

'howdy' # 'howdy'

As print(repr('howdy')), meaning you are printing out the repr for the string 'howdy'.

NB: Both __repr__() and __str__() needs to return strings.