Discussion 5: Mutability, Iterators, Generators

Attendance

Your TA will come around during discussion to check you in.

If you miss discussion for a good reason (such as sickness or a scheduling conflict), email cs61a@berkeley.edu within one week to receive attendance credit.

Mutability

There are many built-in methods that we can use to mutate lists. Here are some of the most useful ones:
  • append(el): Appends el to the end of the list and returns None.
  • extend(lst): Extends the list by concatenating it with lst and returns None.
  • remove(el): Removes the first occurence of el from the list and returns None.
  • insert(i, el): Inserts el at index i and returns None.

  • pop(i): Removes and returns the element at index i. If no index is given, removes and returns the last element in the list.

Let's see these methods in action:

>>> l = [3, 5, 6]
>>> l.append(10)    # Append 10 to the end of the list
>>> l
[3, 5, 6, 10]
>>> l.extend([30, 40])
>>> l
[3, 5, 6, 10, 30, 40]
>>> l.remove(5)     # Remove the first occurrence of 5
>>> l
[3, 6, 10, 30, 40]
>>> l.insert(2, -2) # Insert -2 at index 2
>>> l
[3, 6, -2, 10, 30, 40]
>>> l.pop()         # Remove and return the last element
40
>>> l
[3, 6, -2, 10, 30]
>>> l.pop(2)        # Remove and return the element at index 2
-2
>>> l
[3, 6, 10, 30]

Take note of two things:

  • The name l refers to the same list object during this entire session; it is never reassigned. The reason the output looks different each time we call a method is because the list that l evaluates to is being mutated.
  • The only method here that has a return value is pop! All of the other methods return None.
Here are the above method calls displayed in an environment diagram:

Q1: Copying Copies

Draw the environment diagram on paper or a tablet (without having the computer draw it for you)! Then, check your work by stepping through the diagram with PythonTutor

Iterators

An iterable is any value that can be iterated through, or gone through one element at a time. One construct that we've used to iterate through an iterable is a for statement: 
for elem in iterable:
    # do something

In general, an iterable is an object on which calling the built-in iter function returns an iterator. An iterator is an object on which calling the built-in next function returns the next value.

For example, a list is an iterable value.

>>> s = [1, 2, 3, 4]
>>> next(s)       # s is iterable, but not an iterator
TypeError: 'list' object is not an iterator
>>> t = iter(s)   # Creates an iterator
>>> t
<list_iterator object ...>
>>> next(t)       # Calling next on an iterator
1
>>> next(t)       # Calling next on the same iterator
2
>>> next(iter(t)) # Calling iter on an iterator returns itself
3
>>> t2 = iter(s)
>>> next(t2)      # Second iterator starts at the beginning of s
1
>>> next(t)       # First iterator is unaffected by second iterator
4
>>> next(t)       # No elements left!
StopIteration
>>> s             # Original iterable is unaffected
[1, 2, 3, 4]

You can also use an iterator in a for statement because all iterators are iterable. But note that since iterators keep their state, they're only good to iterate through an iterable once:

>>> t = iter([4, 3, 2, 1])
>>> for e in t:
...     print(e)
4
3
2
1
>>> for e in t:
...     print(e)

There are built-in functions that return iterators. These built-in Python sequence operations are said to compute results lazily.

>>> m = map(lambda x: x * x, [3, 4, 5])
>>> next(m)
9
>>> next(m)
16
>>> f = filter(lambda x: x > 3, [3, 4, 5])
>>> next(f)
4
>>> next(f)
5
>>> z = zip([30, 40, 50], [3, 4, 5])
>>> next(z)
(30, 3)
>>> next(z)
(40, 4)

Q2: WWPD: Iterators

What would Python display? 
>>> s = "cs61a"
>>> s_iter = iter(s)
>>> next(s_iter)
>>> next(s_iter)
>>> list(s_iter)
>>> s = [[1, 2, 3, 4]]
>>> i = iter(s)
>>> j = iter(next(i))
>>> next(j)
>>> s.append(5)
>>> next(i)
>>> next(j)
>>> list(j)
>>> next(i)

Generators

A generator is an iterator that is returned by calling a generator function, which is a function that contains yield statements instead of return statements. The ways to use an iterator are to call next on it or to use it as an iterable (for example, in a for statement).

Q3: Big Fib

This generator function yields all of the Fibonacci numbers.

def gen_fib():
    n, add = 0, 1
    while True:
        yield n
        n, add = n + add, n

Explain the following expression to each other so that everyone understands how it works. (It creates a list of the first 10 Fibonacci numbers.)

(lambda t: [next(t) for _ in range(10)])(gen_fib())

Then, complete the expression below by writing only names and parentheses in the blanks so that it evaluates to the smallest Fibonacci number that is larger than 2026.

Talk with each other about what built-in functions might be helpful, such as map, filter, list, any, all, etc.

Q4: Something Different

Implement differences, a generator function that takes t, a non-empty iterator over numbers. It yields the differences between each pair of adjacent values from t. If t iterates over a positive finite number of values n, then differences should yield n-1 times.

Q5: Partitions

Tree-recursive generator functions have a similar structure to regular tree-recursive functions. They are useful for iterating over all possibilities. Instead of building a list of results and returning it, just yield each result.

You'll need to identify a recursive decomposition: how to express the answer in terms of recursive calls that are simpler. Ask yourself what will be yielded by a recursive call, then how to use those results.

Definition. For positive integers n and m, a partition of n using parts up to size m is an addition expression of positive integers up to m in non-decreasing order that sums to n.

Implement partition_gen, a generator function that takes positive n and m. It yields the partitions of n using parts up to size m as strings.

Reminder: For the partitions function we studied in lecture (video), the recursive decomposition was to enumerate all ways of partitioning n using at least one m and then to enumerate all ways with no m (only m-1 and lower).

Q6: Squares

Implement the generator function squares, which takes positive integers total and k. It yields all lists of perfect squares greater or equal to k*k that sum to total. Each list is in non-increasing order (large to small).

Q7: Church Generator

Implement church_generator, a generator function that takes in a function f as an argument. church_generator yields functions that apply f to their argument one more time than the previously generated function. (The yielded functions are known as Church numerals.)