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

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

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.

# categories (keys) with a list of available resources (values) for a 
# company’s IT Department. The resources belong to multiple categories.
# The function should reverse the keys and values to show which 
# categories (values) each resource (key) belongs to. 


def invert_resource_dict(resource_dictionary):
  # Initialize a "new_dictionary" variable as a dict data type using
  # empty {} curly brackets. 
    new_dictionary = {}
    # The outer for loop iterates through each "resource_group" and 
    # associated "resources" in the "resource_dictionary" items.
    for resource_group, resources in resource_dictionary.items():
        # The inner for loop iterates over each "resource" value in 
        # the list of "resources" for one "resource_group" key at a time.
        for resource in resources:
            # The if-statement checks if the current "resource" value has 
            # been appended as a key to the "new_dictionary" yet.
            if resource in new_dictionary:
                # If True, then append the "resource_group" as a value to the
                # "resource", which is now the key.
                new_dictionary[resource].append(resource_group)
            # If False (else), then add the "resource" as a new key with the 
            # "resource_group" as a value for that key.
            else:
                new_dictionary[resource] = [resource_group]
    # Return the new dictionary once the outer for loop has completed  
    # all iterations.
    return(new_dictionary)


print(invert_resource_dict({"Hard Drives": ["IDE HDDs", "SCSI HDDs"],
        "PC Parts":  ["IDE HDDs", "SCSI HDDs", "High-end video cards", "Basic video cards"], "Video Cards": ["High-end video cards", "Basic video cards"]}))
# Should print {'IDE HDDs': ['Hard Drives', 'PC Parts'], 'SCSI HDDs': ['Hard Drives', 'PC Parts'], 'High-end video cards': ['PC Parts', 'Video Cards'], 'Basic video cards': ['PC Parts', 'Video Cards']}

.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']}

Set 集合

當您想要儲存一堆元素，並確定這些元素只出現一次時，就會使用集合(set)。集合(set)的元素也必須是不可變的。您可以將其視為字典 (dictionary) 中沒有關聯值 (value) 的鍵 (key)

• 符號用大括號
• 內容必須是唯一值，不可重複；如果提供的元素有重複值，程式不會發生錯誤，set 只會存在一個元素
• 建立空白 set 要用函式 set() 
• 資料不是序列，元素之間沒有索引及順序關係

A = {"jlanksy", "drosas", "nmason"}

# Create an empty set
B = set()

# set 不會有重複的元素
basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}
print(basket)                      # show that duplicates have been removed
# Output: {'orange', 'banana', 'pear', 'apple'}