2.Control Flow

Control Flow

if,elif,else

if phone_balance < 5:
    phone_balance += 10
    bank_balance -= 10
           

if season == 'spring':
    print('plant the garden!')
elif season == 'summer':
    print('water the garden!')
elif season == 'fall':
    print('harvest the garden!')
elif season == 'winter':
    print('stay indoors!')
else:
    print('unrecognized season')
           

• 以下等同于false
  • None
  • 任何數值型，值為0
    • 0,0.0,0j,Decimal(0),Fraction(0,1)
  • 空容器
    • “”,[],{},(),set(),range(0)

Loops

for

• 疊代器 iteration
  • 可以是任何容器，檔案以及字典
  • 每次取一個元素
  • 明确疊代
    • 預定義疊代次數
• 利用for loop我們可以建立一個lists或者修改一個lists

cities = ['new york city', 'mountain view', 'chicago', 'los angeles']
for city in cities:
    print(city)
print("Done!")
           

new york city
mountain view
chicago
los angeles
Done!
           

• Operation and Method
  • range(start,end,step)
  • 三個參數必須全部是整數
  • 預設
    • start = 0
    • step = 1
  • 三種情況
    • range(3)
      • 0 到 3-1 步長為1
    • range(2,6)
      • 2 到 6-1 步長為1
    • range(1,10,2)
      • 1 到 10-1 步長為2
• c++或java中for的使用

for i in range(start,end):
    arr[i] #Operation
           

• 容器使用

cities = ['new york city', 'mountain view', 'chicago', 'los angeles']

for index in range(len(cities)):
    cities[index] = cities[index].title()
           

New York City
Mountain View
Chicago
Los Angeles

           

• For loop for dictionary

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

for key in cast:
    print(key)

           

Jerry Seinfeld
Julia Louis-Dreyfus
Jason Alexander
Michael Richards
           

隻能通路key

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

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
           

通過item擷取字典的鍵值對，然後利用unpacking 指派給key和value，以既通路key，又通路value

• while
  • 無需明确疊代次數

card_deck = [4, 11, 8, 5, 13, 2, 8, 10]
hand = []

# adds the last element of the card_deck list to the hand list
# until the values in hand add up to 17 or more
while sum(hand)  < 17:
    hand.append(card_deck.pop())
           

Zip and Enumerate

• Zip
  • return 疊代器
  • 需要轉換成一個list
  • method and Operation:
    • zip(list1,list2)

      傳回了一個疊代器

      list(zip(list1,list2))

      或者for item,weight in zip(items,weights)

items = ['banana','mattresses','dog kennels','machine','cheese']
weights = [15,34,42,120,5]

print(list(zip(items,weights)))
           

[('banana', 15), ('mattresses', 34), ('dog kennels', 42), ('machine', 120), ('cheese', 5)]
           

• • zip(*manifest)
    • 解包

      items,weights = zip(*manifest)

items = ['banana','mattresses','dog kennels','machine','cheese']
weights = [15,34,42,120,5]

manifest = []
for item, weight in zip(items,weights):
    manifest.append((item,weight))

print(manifest)
items1,weights1 = zip(*manifest)

print(items1)
print(weights1)

           

[('banana', 15), ('mattresses', 34), ('dog kennels', 42), ('machine', 120), ('cheese', 5)]
('banana', 'mattresses', 'dog kennels', 'machine', 'cheese')
(15, 34, 42, 120, 5)

Process finished with exit code 0

           

items = ['banana','mattresses','dog kennels','machine','cheese']
for i,item in zip(range(len(items),items))
    print(i,' ',item)
           

• Enumerate

items = ['banana','mattresses','dog kennels','machine','cheese']
for i,item in zip(range(len(items)),items):
    print("{} {}".format(i,item))
           

輸出

id	item
banana
1	mattresses
2	dog kennels
3	machine
4	cheese