Processing collections (including sequences)

• A sequence
  
• Counting elements with a particular property
  • To do this requires keeping track of (1) where we are in the sequence and (2) what the current total is
    
• Generating one or more new sequences, for example, sorting
  • To sort requires keeping track of (1) where we are in the sequence and (2) which elements are in each of the current sorted sequences
    

for loops

• You can loop over the elements of a sequence with a while loop.
• An easier way to do it is with a for loop. In its simplest form, a for loop looks like this.

  for var in sequence:
      statements

• This creates a local variable var which is assigned to each of the elements of sequence in turn as statements are executed.

  def print_elements(seq):
      """Print each of the elements of a sequence."""
      for element in seq:
          print(element)

  This function does the same thing as the following one:

  def print_elements_while(seq):
      """Print each of the elements of a sequence."""
      index = 0
      while index < len(seq):
          print(seq[index])
          index += 1

• Practice
  • Define a function that returns the result of concatenating together all of the strings that are in a list or tuple.
  • Define a function that takes a sequence of things and returns a list containing all of the strings in the sequence.
  • Define a function that takes a sequence and a number and returns the first number in the sequence that is greater than the given number.
  • Define a function that finds the last number in a sequence that's greater than a given number.
  • Define a function that counts the items in a sequence with a particular property, as in the example on the last page.
  • Define a function that sorts the items in a sequence by a particular property, as in the example on the last page.

More complex for loops

• If each element of the sequence is itself a sequence, you can create multiple local variables in the header of the for loop.

  def get_first_name(last_name, names):
      '''Get the first name for a last name from a sequence of name pairs.'''
      for last, first in names:
          if last_name == last:
              return first
      print(last_name, 'not found in database')
  
  >>> get_first_name('Tanaka', ( ('Smith', 'Ed'), ('Wang', 'Fei'), ('Portillo', 'Constanza') ))
  Tanaka not found in database
  >>> get_first_name('Portillo', ( ('Smith', 'Ed'), ('Wang', 'Fei'), ('Portillo', 'Constanza') ))
  'Constanza'

• Practice
  • Define a function that takes a point (an x, y coordinate pair) and a list of points and returns the first point in the list that is above and to the left (northwest) of the first point.
  • Define a function that takes a character and a list of strings and returns the first string that the character is contained in.
  • Define a function that takes one sequence and another sequence consisting of character, position pairs, and tells whether each of the characters is in its position in the first sequence.

    >>> are_chars_in_positions('Ministry of Silly Walks', [('n', 2), (' ', 8), ('M', 0)])
    True
    >>> are_chars_in_positions('Ministry of Silly Walks', [('n', 2), (' ', 13), ('M', 0)])
    False

range

• To generate a sequence of integers, the built-in function range is handy. range returns a simple tuple-like sequence of integers.
• With one argument (an integer), range returns a sequence of integers from 0 to one less than its argument. We can see the behavior of range most easily by converting it to a list or tuple.

  >>> list(range(8))
  [0, 1, 2, 3, 4, 5, 6, 7]

• With two arguments, a and b, range returns a sequence of integers from a to b-1.

  >>> list(range(3, 8))
  [3, 4, 5, 6, 7]

• With three arguments, a, b, and c, range returns a sequence of integers from a to b-1, skipping over every c integers.

  >>> list(range(3, 8, 2))
  [3, 5, 7]

• range is useful when you want to do the same thing a certain number of times; you can use range(times) as the sequence in the header of a for loop.
• Practice
  • Define a function that takes two characters, a length, and a list of positions. prints out length number of characters, using the first of the two characters, expect in positions that are in the list of positions, in which case it prints out the second character.
  • Define a function that takes an integer and returns a list of random numbers that long.
  • Define a function that takes a list of pairs of integers and returns a list of integers for which each of the pairs defines the beginning and end of a sublist within the larger list.

    >>> list_with_ranges([(2, 6), (10, 15), (22, 25)])
    [2, 3, 4, 5, 10, 11, 12, 13, 14, 22, 23, 24]

Answers

Answers to problems in these notes