Dictionary 字典

不像序列是由一個範圍內的數字當作索引，dictionary 是由鍵 (key) 來當索引，鍵可以是任何不可變的類型；字串和數字都可以當作鍵。Tuple 也可以當作鍵，如果他們只含有字串、數字或 tuple；若一個 tuple 直接或間接地含有任何可變的物件，它就不能當作鍵。你無法使用 list 當作鍵，因為 list 可以經由索引指派 (index assignment)、切片指派 (slice assignment) 或是像 append() 和 extend() 等 method 被修改。

思考 dictionary 最好的方式是把它想成是一組鍵值對 (key: value pair) 的 set，其中鍵在同一個 dictionary 裡必須是獨一無二的。使用一對大括號可建立一個空的 dictionary：{}。將一串由逗號分隔的鍵值對置於大括號則可初始化字典的鍵值對。這同樣也是字典輸出時的格式。

Key type:

Numbers
Booleans
Strings
Tuples

資料集合

dictionary1 = {"keyA":valuea, "keyB":value2, "keyC":value3, "KeyD":value4}

dictionary2 = {"keyA":["value1", "value2"], "keyB":["value3", "value4"]}

搜尋鍵-值

NOTE: Dictionary 如果鍵有重複，新的值會覆蓋舊的。

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
file_counts["txt"]
# Output: 14

# 鍵有重複時
file_counts = {"jpg":10, "txt":14, "csv":2, "py":23, "txt":99}
file_counts["txt"]
# Output: 99

檢查索引

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
"jpg" in file_counts
# Output: True

新增元素: dictionary[key] = value

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
file_counts["cfg"] = 8
print(file_counts)
# Output {'jpg': 10, 'txt': 14, 'csv': 2, 'py': 23, 'cfg': 8}

變更指定索引的元素: dictionary[key] = value

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
file_counts["csv"] = 17
print(file_counts)
# Output {'jpg': 10, 'txt': 14, 'csv': 17, 'py': 23}

刪除指定索引的元素

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23, 'cfg':8}
del file_counts["cfg"]
print(file_counts)
# Output {'jpg': 10, 'txt': 14, 'csv': 2, 'py': 23}

Operations

len(dictionary) - Returns the number of items in a dictionary.
for key, in dictionary - Iterates over each key in a dictionary.
for key, value in dictionary.items() - Iterates over each key,value pair in a dictionary.
if key in dictionary - Checks whether a key is in a dictionary.
dictionary[key] - Accesses a value using the associated key from a dictionary.
dictionary[key] = value - Sets a value associated with a key.
del dictionary[key] - Removes a value using the associated key from a dictionary.

字典使用 for loop 迭代時，預設使用 key 存取

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
for extension in file_counts:
  print(extension)

# Output
jpg
txt
csv
py

Methods

dictionary.get(key, default) - Returns the value corresponding to a key, or the default value if the specified key is not present.
dictionary.keys() - Returns a sequence containing the keys in a dictionary.
dictionary.values() - Returns a sequence containing the values in a dictionary.
dictionary[key].append(value) - Appends a new value for an existing key.
dictionary.update(other_dictionary) - Updates a dictionary with the items from another dictionary. Existing entries are updated; new entries are added.
dictionary.clear() - Deletes all items from a dictionary.
dictionary.copy() - Makes a copy of a dictionary.

.item()

.items() 迭代 dictionary 資料時，可存取 key 與 value。

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
for ext, amount in file_counts.items():
  print("There are {} files with the .{} extension".format(amount, ext))

# Output
There are 10 files with the .jpg extension
There are 14 files with the .txt extension
There are 2 files with the .csv extension
There are 23 files with the .py extension

# This function returns the total time, with minutes represented as 
# decimals (example: 1 hour 30 minutes = 1.5), for all end user time
# spent accessing a server in a given day. 


def sum_server_use_time(Server):

    # Initialize the variable as a float data type, which will be used
    # to hold the sum of the total hours and minutes of server usage by
    # end users in a day.
    total_use_time = 0.0

    # Iterate through the "Server" dictionary’s key and value items 
    # using a for loop.
    for key,value in Server.items():

        # For each end user key, add the associated time value to the
        # total sum of all end user use time.
        total_use_time += Server[key]
        
    # Round the return value and limit to 2 decimal places.
    return round(total_use_time, 2)  

FileServer = {"EndUser1": 2.25, "EndUser2": 4.5, "EndUser3": 1, "EndUser4": 3.75, "EndUser5": 0.6, "EndUser6": 8}

print(sum_server_use_time(FileServer)) # Should print 20.1

# This function receives a dictionary, which contains common employee 
# last names as keys, and a list of employee first names as values. 
# The function generates a new list that contains each employees’ full
# name (First_name Last_Name). For example, the key "Garcia" with the 
# values ["Maria", "Hugo", "Lucia"] should be converted to a list 
# that contains ["Maria Garcia", "Hugo Garcia", "Lucia Garcia"].


def list_full_names(employee_dictionary):
    # Initialize the "full_names" variable as a list data type using
    # empty [] square brackets.  
    full_names = []

    # The outer for loop iterates through each "last_name" key and 
    # associated "first_name" values, in the "employee_dictionary" items.
    for last_name, first_names in employee_dictionary.items():

        # The inner for loop iterates over each "first_name" value in 
        # the list of "first_names" for one "last_name" key at a time.
        for first_name in first_names:

            # Append the new "full_names" list with the "first_name" value
            # concatenated with a space " ", and the key "last_name". 
            full_names.append(first_name+" "+last_name)
            
    # Return the new "full_names" list once the outer for loop has 
    # completed all iterations. 
    return(full_names)


print(list_full_names({"Ali": ["Muhammad", "Amir", "Malik"], "Devi": ["Ram", "Amaira"], "Chen": ["Feng", "Li"]}))
# Should print ['Muhammad Ali', 'Amir Ali', 'Malik Ali', 'Ram Devi', 'Amaira Devi', 'Feng Chen', 'Li Chen']

.keys() .values()

.keys() , .values()

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
file_counts.keys()   # Return dict_keys(['jpg', 'txt', 'csv', 'py'])
file_counts.values() # Return dict_values([10, 14, 2, 23])

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
for value in file_counts.values():
  print(value)

# Output
10
14
2
23

Use the dictionary[key] = value operation to associate a value with a key in a dictionary.
Iterate over keys with multiple values from a dictionary, using nested for loops and an if-statement, and the dictionary.items() method.
Use the dictionary[key].append(value) method to add the key, a string, and the key for each item in the dictionary.

def groups_per_user(group_dictionary):
	user_groups = {}
	# Go through group_dictionary
	for group, users in group_dictionary.items():
		# Now go through the users in the group
		for user in users:
			# Now add the group to the the list of
			if user in user_groups:
				user_groups[user].append(group)
			else:
				user_groups[user] = [group]

# groups for this user, creating the entry
# in the dictionary if necessary

	return(user_groups)

print(groups_per_user({"local": ["admin", "userA"],
		"public":  ["admin", "userB"],
		"administrator": ["admin"] }))

# Should print {'admin': ['local', 'public', 'administrator'], 'userA': ['local'], 'userB': ['public']}

.update()

dictionary.update(other_dictionary) - Updates a dictionary with the items from another dictionary. Existing entries are updated; new entries are added.

wardrobe = {'shirt': ['red', 'blue', 'white'], 'jeans': ['blue', 'black']}
new_items = {'jeans': ['white'], 'scarf': ['yellow'], 'socks': ['black', 'brown']}
wardrobe.update(new_items)

# wardrobe: {'shirt': ['red', 'blue', 'white'], 'jeans': ['white'], 'scarf': ['yellow'], 'socks': ['black', 'brown']}

.copy()

# The scores() function accepts a dictionary "game_scores" as a parameter.
def reset_scores(game_scores):

    # The .copy() dictionary method is used to create a new copy of the "game_scores".
    new_game_scores = game_scores.copy() 

    # The for loop iterates over new_game_scores items, with the player as the key
    # and the score as the value. 
    for player, score in new_game_scores.items():
    
        # The dictionary operation to assign a new value to a key is used
        # to reset the grade values to 0.
        new_game_scores[player] = 0
  
    return new_game_scores
 
# The dictionary is defined.
game1_scores = {"Arshi": 3, "Catalina": 7, "Diego": 6}
 
# Call the "reset_scores" function with the "game1_scores" dictionary. 
print(reset_scores(game1_scores))
# Should print {'Arshi': 0, 'Catalina': 0, 'Diego': 0}

Functions

sorted()

sorted(dict.items()) : 預設以 Dictionary 的 key 做排序
sorted(, key=operator.itemgetter(0) : .itemgettor(0) 指 Dictionary 的 key，以此排序
sorted(, key=operator.itemgetter(1) : .itemgettor(1) 指 Dictionary 的 value，以此排序
sorted(, reverse=True) : 反向排序

fruit = {"oranges": 3, "apples": 5, "bananas": 7, "pears": 2}

sorted(fruit.items())
# [('apples', 5), ('bananas', 7), ('oranges', 3), ('pears', 2)]

import operator
sorted(fruit.items(), key=operator.itemgetter(0))
# [('apples', 5), ('bananas', 7), ('oranges', 3), ('pears', 2)]

sorted(fruit.items(), key=operator.itemgetter(1))
# [('pears', 2), ('oranges', 3), ('apples', 5), ('bananas', 7)]

sorted(fruit.items(), key = operator.itemgetter(1), reverse=True)
# [('bananas', 7), ('apples', 5), ('oranges', 3), ('pears', 2)]

Course 1

Dictionaries vs. Lists

Classes and methods

Examples

Course 2