Chapter 6 Dictionaries

6.1 A Simple Dictionary

花括号

alien_0 = {'color': 'green', 'points': 5}

print(alien_0['color'])
print(alien_0['points'])

6.2 Working with Dictionaries

key-value pairs
Each key is connected to a value.

6.2.1 Adding New 增加

 alien_0['x_position'] = 0

6.2.2 Starting with an Empty Dictionary

alien_0 = {}

6.2.3 Modifying Values 调整值

alien_0 = {'color': 'green'} #初始

alien_0['color'] = 'yellow'

6.2.4 Removing 移除

alien_0 = {'color': 'green', 'points': 5}

del alien_0['points']

6.2.5 A Dictionary of Similar Objects

favorite_languages = { #然后换行，一个pair一行。注意缩进
	'jen': 'python',
	'sarah': 'c',
	'edward': 'ruby',
	'phil': 'python', #在最后一个key-value pair后也加逗号，便于以后添加新pair
	}

6.2.6 Using get() to Access Values

不确定该pair是否存在于字典中时

alien_0 = {'color': 'green', 'speed': 'slow'}

point_value = alien_0.get('points', 'No point value assigned.')
print(point_value)

#输出
No point value assigned.

get括号中的第二个声明：如果字典中没有该pair，返回的值。

6.3 Looping Through a Dictionary

6.3.1 Looping Through All Key-Value Pairs

user_0 = {
	'username': 'efermi',
	'first': 'enrico',
	'last': 'fermi',
	}
	
for key, value in user_0.items(): #any name for the variables
	print(f"\nKey: {key}")
	print(f"Value: {value}")

6.3.2 Looping Through All the Keys in a Dictionary

for name in favorite_languages.keys():

for name in favorite_languages: #在loop a dictionary的时候，默认是loop keys

6.3.3 Looping Through a Dictionary’s Keys in a Particular Order

for name in sorted(favorite_languages.keys()):

6.3.4 Set 集合

集合（set）是一个无序的不重复元素序列。

可以使用大括号 { } 或者 set() 函数创建集合，注意：创建一个空集合必须用 set() 而不是 { }，因为 { } 是用来创建一个空字典。

创建格式：

(){}[]区别
()tuple
{}set     dictionary
[]list

6.3.5 Looping Through All Values in a Dictionary

for language in favorite_languages.values():

避免输出重复：使用set

 for language in set(favorite_languages.values()):

6.4 Nesting 嵌套

A List of Dictionaries 字典列表

alien_0 = {'color': 'green', 'points': 5}
alien_1 = {'color': 'yellow', 'points': 10}
alien_2 = {'color': 'red', 'points': 15}

aliens = [alien_0, alien_1, alien_2]

for alien in aliens:
	print(alien)

更换字典中的key-value pairs:

# Make an empty list for storing aliens.
aliens = []

# Make 30 green aliens.
for alien_number in range (30):
	new_alien = {'color': 'green', 'points': 5, 'speed': 'slow'}
	aliens.append(new_alien)
 
for alien in aliens[:3]:
	if alien['color'] == 'green':
		alien['color'] = 'yellow'
		alien['speed'] = 'medium'
		alien['points'] = 10
 
# Show the first 5 aliens.
for alien in aliens[:5]:
	print(alien)
print("...")

A List in a Dictionary 在字典中存储列表

# Store information about a pizza being ordered.
pizza = {
	'crust': 'thick',
	'toppings': ['mushrooms', 'extra cheese'],
	}

# Summarize the order.
print(f"You ordered a {pizza['crust']}-crust pizza "	
	"with the following toppings:")
for topping in pizza['toppings']:
	print("\t" + topping)

A Dictionary in a Dictionary 在字典中存储字典

users = {      #缩进使得代码更易读
 	'aeinstein': {
 		'first': 'albert',
		'last': 'einstein',
		'location': 'princeton',
		},
		
	'mcurie': {
		'first': 'marie',
		'last': 'curie',
		'location': 'paris',
		},
		
	}