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Control Flow

Iterating through Dictionaries with For loops

The iteration through Dictionaries in pyhton is a different than Swift, in this case to iterate and get back both key and value you need to use a built-in method item.

For example:

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cast = {
           "Jerry Seinfeld": "Jerry Seinfeld",
           "Julia Louis-Dreyfus": "Elaine Benes",
           "Jason Alexander": "George Costanza",
           "Michael Richards": "Cosmo Kramer"
       }

for key,value in cast.items():
    print("Actor: {}   Role: {}".format(key,value))

which will result in something like:

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Actor: Jerry Seinfeld    Role: Jerry Seinfeld
Actor: Julia Louis-Dreyfus    Role: Elaine Benes
Actor: Jason Alexander    Role: George Costanza
Actor: Michael Richards    Role: Cosmo Kramer

for Loops vs. while Loops

for loops are idea when the number of iteration are known or finite.

Examples:

  • When you have an iterable collection (list, string, set, tuple, dictionary): 

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    for name in names:

  • When you want to iterate through a loop for a definite number of times, using range() 

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    for i in range(5):

while loop are ideal when the iterations are to continue until a condition is met

Examples:

  • When you want to use comparison operations: 

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    while count <= 100:

  • When you want to loop base on receiving specific user input 

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    while user_input == 'y'

the following are required to build a correct while loop:

  • The condition for existing the while loop should be included.
  • Check if the iteration conditions is met.
  • Body of the loop should change the value of condition variables.

Break and continue.

  • break terminates a loop.
  • continue skips one iteration of a loop.

Zip and Enumerate

zip and enumerate are build-in functions that can come handy when dealing with loops.

Zip

zip will return an iterator that combine multiples iterables into one sequence of tuples, this will be more clear with an example:

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list(zip(['a','b','c'], [1,2,3]))

which out put will be:

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[('a',1),('b',2),('c',3)]

In this case we can see that zip create a iterator that combine the two provided iterables and each iterator is a tuple with items in the position of the original iterable.

the reverse or Unzip process can be done using the *

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letters = ['a','b','c']
nums = [1,2,3]

for letter, num in zip(letters,nums):
    print("{}:{}".format(letter,num))

some_list = [('a',1),('b',2),('c',3)]
letters, nums = zip(*some_list)
In Python3, zip methods returns a zip object instead of a list. This zip object is an iterator. Iterators are lazily evaluated.

Lazy evaluation, or call-by-need is an evaluation strategy which delays the evaluation of an expression until its value is needed and which also avoids repeated evaluations

Iterators returns only element at a time. len function cannot be used with iterators. We can loop over the zip object or the iterator to get the actual list

Consider the below example:

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list_a = [1, 2, 3]
list_b = [4, 5, 6]

zipped = zip(a, b) # Output: Zip Object. <zip at 0x4c10a30>

len(zipped) # TypeError: object of type 'zip' has no len()

zipped[0] # TypeError: 'zip' object is not subscriptable

list_c = list(zipped) #Output: [(1, 4), (2, 5), (3, 6)]

list_d = list(zipped) # Output []... Output is empty list becuase by the above statement zip got exhausted.

Enumerate enumerate is a built in function that return an iterator of tuples containing indexes and values of a list, example:

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letters = ['a','b','c','d','e']

for i, letter in enumerate(letters):
    print(i, letter)

this will be the output:

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0 a
1 b
2 c
3 d
4 e

an example of enumerate, here:

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cast = ["Barney Stinson", "Robin Scherbatsky", "Ted Mosby", "Lily Aldrin", "Marshall Eriksen"]
heights = [72, 68, 72, 66, 76]

for i, character in enumerate(cast):
    cast[i] = character + " " + str(heights[i])

print(cast)

It will take 2 list and it will out put a single list of items that include a both original list as a single string

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['Barney Stinson 72', 'Robin Scherbatsky 68', 'Ted Mosby 72', 'Lily Aldrin 66', 'Marshall Eriksen 76']

List Comprehensions

List comprehensions are just present in python and not in other languages, this are normally use to create a list in a quickly and concisely way, for example:

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capitalized_cities = []
for city in cities:
    capitalized_cities.append(city.title())

can be reduce to:

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capitalized_cities = [city.title() for city in cities]

Conditional can be added to this list comprehensions (listcomps). be aware that if the conditional has a else statement the syntax will be a bit different.

Lets start with a simply conditional.

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squares = [x**2 for x in range(9) if x % 2 == 0]

this will create a list with the power of the even numbers

if you want to add a else, you will get a syntax error

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squares = [x**2 for x in range(9) if x % 2 == 0 else x + 3] # this will produce a syntax error

in this case, it is necessary move all the block to the beginning.

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squares = [x**2 if x % 2 == 0 else x + 3 for x in range(9)]

Exercise using the dictionaries and for loops

Provide a list with the name(s) of the director(s) with the most Oscar wins. We are asking for a list because there could be more than 1 director tied for the most Oscar wins.

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winners = {1931: ['Norman Taurog'], 1932: ['Frank Borzage'], 1933: ['Frank Lloyd'], 1934: ['Frank Capra'], 1935: ['John Ford'], 1936: ['Frank Capra'], 1937: ['Leo McCarey'], 1938: ['Frank Capra'], 1939: ['Victor Fleming'], 1940: ['John Ford'], 1941: ['John Ford'], 1942: ['William Wyler'], 1943: ['Michael Curtiz'], 1944: ['Leo McCarey'], 1945: ['Billy Wilder'], 1946: ['William Wyler'], 1947: ['Elia Kazan'], 1948: ['John Huston'], 1949: ['Joseph L. Mankiewicz'], 1950: ['Joseph L. Mankiewicz'], 1951: ['George Stevens'], 1952: ['John Ford'], 1953: ['Fred Zinnemann'], 1954: ['Elia Kazan'], 1955: ['Delbert Mann'], 1956: ['George Stevens'], 1957: ['David Lean'], 1958: ['Vincente Minnelli'], 1959: ['William Wyler'], 1960: ['Billy Wilder'], 1961: ['Jerome Robbins', 'Robert Wise'], 1962: ['David Lean'], 1963: ['Tony Richardson'], 1964: ['George Cukor'], 1965: ['Robert Wise'], 1966: ['Fred Zinnemann'], 1967: ['Mike Nichols'], 1968: ['Carol Reed'], 1969: ['John Schlesinger'], 1970: ['Franklin J. Schaffner'], 1971: ['William Friedkin'], 1972: ['Bob Fosse'], 1973: ['George Roy Hill'], 1974: ['Francis Ford Coppola'], 1975: ['Milos Forman'], 1976: ['John G. Avildsen'], 1977: ['Woody Allen'], 1978: ['Michael Cimino'], 1979: ['Robert Benton'], 1980: ['Robert Redford'], 1981: ['Warren Beatty'], 1982: ['Richard Attenborough'], 1983: ['James L. Brooks'], 1984: ['Milos Forman'], 1985: ['Sydney Pollack'], 1986: ['Oliver Stone'], 1987: ['Bernardo Bertolucci'], 1988: ['Barry Levinson'], 1989: ['Oliver Stone'], 1990: ['Kevin Costner'], 1991: ['Jonathan Demme'], 1992: ['Clint Eastwood'], 1993: ['Steven Spielberg'], 1994: ['Robert Zemeckis'], 1995: ['Mel Gibson'], 1996: ['Anthony Minghella'], 1997: ['James Cameron'], 1998: ['Steven Spielberg'], 1999: ['Sam Mendes'], 2000: ['Steven Soderbergh'], 2001: ['Ron Howard'], 2002: ['Roman Polanski'], 2003: ['Peter Jackson'], 2004: ['Clint Eastwood'], 2005: ['Ang Lee'], 2006: ['Martin Scorsese'], 2007: ['Ethan Coen', 'Joel Coen'], 2008: ['Danny Boyle'], 2009: ['Kathryn Bigelow'], 2010: ['Tom Hooper']}

first step, is "open the dictionary", i will need to list the directors as a key and the number of wins as a value, for that i can use the for loops and the get method of dictionaries

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most_win_director = [] # i will use this list later to print the results

win_dict = {} # this Dictionary will hold the directors and the values ( number of wins)

for year, winner_name in winner.item():
    for winner in winner_name:
        win_dict[winner] = win_dict.get(winner,0) + 1

so in this step i get a dictionary that holds the name and the number of wins

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{'Peter Jackson': 1, 'Anthony Minghella': 1, 'Robert Redford': 1, 'Clint Eastwood': 2, 'Ron Howard': 1, 'Billy Wilder': 2, 'Steven Spielberg': 2, 'Richard Attenborough': 1, 'Mel Gibson': 1, 'Leo McCarey': 2, 'William Friedkin': 1, 'Barry Levinson': 1, 'Oliver Stone': 2, 'Warren Beatty': 1, 'Ang Lee': 1, 'Joseph L. Mankiewicz': 2, 'Sydney Pollack': 1, 'Robert Wise': 2, 'Woody Allen': 1, 'John Ford': 4, 'Bob Fosse': 1, 'Jerome Robbins': 1, 'Robert Benton': 1, 'Elia Kazan': 2, 'Frank Lloyd': 1, 'John G. Avildsen': 1, 'Tom Hooper': 1, 'Frank Borzage': 1, 'Sam Mendes': 1, 'John Huston': 1, 'Carol Reed': 1, 'Francis Ford Coppola': 1, 'Joel Coen': 1, 'Fred Zinnemann': 2, 'William Wyler': 3, 'Jonathan Demme': 1, 'Kathryn Bigelow': 1, 'Delbert Mann': 1, 'Danny Boyle': 1, 'George Cukor': 1, 'Norman Taurog': 1, 'Tony Richardson': 1, 'George Roy Hill': 1, 'James L. Brooks': 1, 'Martin Scorsese': 1, 'David Lean': 2, 'Franklin J. Schaffner': 1, 'Bernardo Bertolucci': 1, 'John Schlesinger': 1, 'Ethan Coen': 1, 'Michael Cimino': 1, 'Milos Forman': 2, 'Mike Nichols': 1, 'Michael Curtiz': 1, 'Steven Soderbergh': 1, 'Robert Zemeckis': 1, 'Kevin Costner': 1, 'Frank Capra': 3, 'Vincente Minnelli': 1, 'James Cameron': 1, 'George Stevens': 2, 'Roman Polanski': 1, 'Victor Fleming': 1}

now I will need to create a variable highest_wins I will change the value of this variable every time I encounter a director with a higher number of wins, and I will clean the most_win_director and append the a new name.

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highest_wins = 0

for director, wins in win_dict.items():
    if wins > highest_wins:
        highest_wins = wins
        most_win_director.clear()
        most_win_director.append(director)
    elif wins == highest_wins:
        most_win_director.append(director)
    else:
        continue
finally print the solution:

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 print("most_win_director = {}".format(most_win_director))

and here a more compact solution using max and list comprehension:

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highest_count = max(win_count_dict.values())

most_win_director = [key for key, value in win_count_dict.items() if value == highest_count]