Python Iterators

Iterators

• Python iterator syntax is used to perform repetition

  for i in range(10):
      print(i, end=' ')

• The range object is an iterator, which provides the functionality required by the for loop.

• A Python iterator has a built-in function next

  >>> I = iter([2, 4, 6, 8, 10])
  >>> print(next(I))
  2
  >>> print(next(I))
  4

Useful Iterators

• Python includes some useful iterators:

  • All the built-in data structures

  • range

  • enumerate

  • zip

  • map

  • filter

enumerate

• Sometimes you need to keep track of the index when iterating over a list:

  L = [2, 4, 6, 8, 10]
  for i in range(len(L)):
      print(i, L[i])

• The enumerate iterator provides this information

  for i, val in enumerate(L):
      print(i, val)

zip

• The zip iterator allows you to iterate over multiple lists simultaneously

  L = [2, 4, 6, 8, 10]
  R = [3, 6, 9, 12, 15]
  for lval, rval in zip(L, R):
      print(lval, rval)

• Any number of iterables can be zipped together

• The shortest length iterable determines the length of the zipped iterable.

map

• The map iterator takes a function and applies it to the values in an iterator:

  # find the first 10 square numbers
  square = lambda x: x ** 2
  for val in map(square, range(10)):
      print(val, end=' ')

filter

• The filter iterator takes a boolean returning function and passes through the values for the filter function evaluates to True

  # find values up to 10 that are even
  is_even = lambda x: x % 2 == 0
  for val in filter(is_even, range(10)):
      print(val, end=' ')

Iterators as Function Arguments

• Recall that *args and **kwargs can be used to pass sequences and dictionaries to functions.

  >>> print(*range(10))
  0 1 2 3 4 5 6 7 8 9
  >>> print(*map(lambda x: x **2, range(10)))
  0 1 4 9 16 25 36 49 64 81

• This means the inverse of the zip function is the zip function

  >>> L1 = (1, 2, 3, 4)
  >>> L2 = ('a', 'b', 'c', 'd')
  >>> z = zip(L1, L2)
  >>> print(*z)
  (1, 'a') (2, 'b') (3, 'c') (4, 'd')
  >>> z = zip(L1, L2)
  >>> new_L1, new_L2 = zip(*z)
  >>> print(new_L1, new_L2)
  (1, 2, 3, 4) ('a', 'b', 'c', 'd')

Specialized Iterators: itertools

• The Python itertools module contains many more useful iterators

• Example:

  >>> from itertools import combinations
  >>> c = combinations(range(4), 2)
  >>> print(*c)
  (0, 1) (0, 2) (0, 3) (1, 2) (1, 3) (2, 3)