Dictionary

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今天想分享如何用 dictionary 字典方式存資料。

key and Value 如何使用宣槓取直

  • collection of of {key:value} pairs
  • no duplicate
  • allow changeable and ordered

declare dictionary 宣告 dict

It's an unorder , it's not next to each other, so we can't use index like we use in list. Dictionary will have a a key and value look like this:

syntax:

declare: dictname= {key:value}
get key: print(dictname['key'])

Access and Get value 取我們的直:

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dict={
    'keyA':1,
    'keyB':2
}

取 key 的直: print(dict['keyA']) => 1
取所有直 print(dict) => {'keyA': 1, 'keyB': 2}

宣告不同 data type

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dict={
    'keyA':[1,2,3], #list
    'keyB':'hello', #tuple
    'keyC': True
}
  • 取 keyA 的直

print(dict)

output
{‘keyA’: [1, 2, 3], ‘keyB’: ‘hello’, ‘keyC’: True}

  • 取 keyA 的第 2 直:

print(dict['keyA'][1]) => 2

  • list 裡面放 dict
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dict=[{
    'keyA':[1,2,3], #list
    'keyB':'hello', #tuple
    'keyC': True
},
{
    'keyA':[4,5,6], #list
    'keyB':'hello', #tuple
    'keyC': True
}]

print(dict[0]['keyA'][2]) => 2

Method

Method Description
clear() Removes all items from the dictionary.
copy() Returns a shallow copy of the dictionary.
fromkeys(seq[, v]) Returns a new dictionary with keys from seq and value equal to v (defaults to None).
get(key[,d]) Returns the value of the key. If the key does not exist, returns d (defaults to None).
items() Return a new object of the dictionary’s items in (key, value) format.
keys() Returns a new object of the dictionary’s keys.
pop(key[,d]) Removes the item with the key and returns its value or d if key is not found. If d is not provided and the key is not found, it raises KeyError.
popitem() Removes and returns an arbitrary item (key, value). Raises KeyError if the dictionary is empty.
setdefault(key[,d]) Returns the corresponding value if the key is in the dictionary. If not, inserts the key with a value of d and returns d (defaults to None).
update([other]) Updates the dictionary with the key/value pairs from other, overwriting existing keys.
values() Returns a new object of the dictionary’s values

Declare Dict

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country = {
     "USA": "WDC",
    "China":"BJN",
    "Taiwan":"TP",
    "Thailand":"BKK"
}
user={
 "name":"James",
 "phone":"123",
 123:[1,2,3], #list
 True:'hello',
 'hi': True,
}

key

key need to be immutable, which mean can not change.

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print(user['phone']) #123
print(user[123]) #[1, 2, 3]
print(user[True]) # hello
print(user['hi']) #True
  • print(dict[123]) #[1,2,3]
  • print(dict[True]) #hello
  • print(dict['hi']) #True

If add [100]:True will occur error because it’s a immutable, can’t change key. list can be change, so we can’t chnage it. Dictionary often use string as it’s key

multiply key

如果有相同的 key，最後一個會被取代 overwrite，因此 key 要 unquie。
As you can see below, if we declare same key, the last key will overwrite the first one, this is why when I print it the last value I show up.

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dict={
 "123":"hello",
 "123":"world"
}
print(dict['123']) #world

get()

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#get value
print(country.get("USA"))
#WDC

get key avoid error when no key exisit

如果我們有 key 用原本語法會有 error，我們可以用 get()方法就不會跳出 error。

get a nonexist key(key 不存在)

print(user['age']) => pop error > > print(user.get("age")) => #none

create key with default value

if user don’t have key age, add a default value。建立 key，給予預設直。

print(user.get("age", 55)) => 55

But if if we have key age, then it will ignore 55。

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user={
 "name":"James",
 "phone":"123",
 "age":60,
}
print(user.get("age", 55)) => `60`
print(user.get("age")) => `60`

keys values items

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user={
 "name":"James",
 "phone":"123"
}

print('name' in user) #false
print('id' in user) #false
print('id' in user.keys()) #false
print('James' in user.values()) #True
print(user.items()) #dict_items([('name', 'James'), ('phone', '123')])

Get() 取 value 的值

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country = {
     "USA": "WDC",
    "China":"BJN",
    "Taiwan":"TP",
    "Thailand":"BKK"
}

  • get the value

    print(country.get("USA"))

    output:
    WDC

Update 新增與修改

  • 新增 item key and Value，add new item key and value

    country.update({"Japan":"TKY"}) > print(country)

    output:
    {‘USA’: ‘WDC’, ‘China’: ‘BJN’, ‘Taiwan’: ‘TP’, ‘Thailand’: ‘BKK’, ‘Japan’: ‘TYY’}

  • 修改 Value，update exist value change exists key

    country.update({"Japan":"TTT"}) > print(country)

    output:
    {‘USA’: ‘WDC’, ‘China’: ‘BJN’, ‘Taiwan’: ‘TP’, ‘Thailand’: ‘BKK’, ‘Japan’: ‘TTT’}

pop and popitem 移除 item

  • pop 移除

    country.pop("Taiwan") > print(country)

    output:
    {‘USA’: ‘WDC’, ‘China’: ‘BJN’, ‘Thailand’: ‘BKK’, ‘Japan’: ‘TTT’}

  • popitem:移除最後一組
    pop item will remove the latest item

    country.popitem() > print(country)

    output:
    {‘USA’: ‘WDC’, ‘China’: ‘BJN’, ‘Thailand’: ‘BKK’}

clear 清除 empty or clear

country.clear() > print(country)

output
{}

keys and values (get value or key)

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country = {
     "USA": "WDC",
    "China":"BJN",
    "Taiwan":"TP",
    "Thailand":"BKK"
}

  • keys
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#get the keys
keys= country.keys()
print(keys)
#dict_keys(['USA', 'China', 'Taiwan', 'Thailand'])
  • values
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#get the value
values= country.values()
print(values)
# dict_values(['WDC', 'BJN', 'TP', 'BKK'])

copy dictionary

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countrybk=country.copy()
print(countrybk)
print(country)
#{'USA': 'WDC', 'China': 'BJN', 'Taiwan': 'TP', 'Thailand': 'BKK'}
#{'USA': 'WDC', 'China': 'BJN', 'Taiwan': 'TP', 'Thailand': 'BKK'}

user2=user.copy()
print(user) #{'name': 'James', 'phone': '123', 'age': 60}
print(user2) #{'name': 'James', 'phone': '123', 'age': 60}

Method Example

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# Dictionary Methods
marks = {}.fromkeys(['Math', 'English', 'Science'], 0)
print(marks)
# Output: {'English': 0, 'Math': 0, 'Science': 0}

for item in marks.items():
    print(item)

# Output: ['English', 'Math', 'Science']
print(list(sorted(marks.keys())))

iterate key and value

keys

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for key in country.keys():
    print(key)
#USA
#China
#Taiwan
#Thailand

value

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for value in country.values():
    print(value)
#WDC
#BJN
#TP
#BKK

items() keys and values

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for key, value in country.items():
    print(f"{key}:{value}")
#USA:WDC
#China:BJN
#Taiwan:TP
#Thailand:BKK

Dictionary Comprehension

  • dictionary = {key: expression for (key,value) in iterable}
  • dictionary = {key: expression for (key,value) in iterable if conditional}
  • dictionary = {key: (if/else) for (key,value) in iterable}
  • dictionary = {key: function(value) for (key,value) in iterable}

using for condition

Syntax:

dictionry={key:expression for (key, value) in iterate}

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citiesinF={"Beijing": 20, "Boston":10, "New York":30, "Califorina": 35}
citiesinC= {key:round((value-32)*(5/9)) for (key,value) in citiesinF.items()}
print(citiesinC)

output:

{‘Beijing’: -7, ‘Boston’: -12, ‘New York’: -1, ‘Califorina’: 2}

using if condition

Syntax:

dictionry={key:expression for (key, value) in iterate if conditional}

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weather={"Beijing": "snowing", "Boston":"cloudy", "New York":"sunny"}
sunnyweather={key: value for (key,value) in weather.items() if value =="sunny"}
print(sunnyweather)

output:

{‘New York’: ‘sunny’}

using if else condition

Syntax:

dictionry={key:if/else for (key, value) in iterate}

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citiestemp={"Beijing": 20, "Boston":10, "New York":30, "Califorina": 35}
desccities={key: ("warm"if value >= 20 else "cold") for (key,value) in citiestemp.items()}
print(desccities)

output:

{‘Beijing’: ‘warm’, ‘Boston’: ‘cold’, ‘New York’: ‘warm’, ‘Califorina’: ‘warm’}

using function

Syntax:

dictionry={key:function(value)) for (key, value) in iterate}

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def checktemp(value):
    if value >= 25:
        return "cool"
    elif 69 >= value >=40:
        return "warm"
    else:
        return "cold"

citiestemp={"Beijing": 20, "Boston":10, "New York":30, "Califorina": 35}
desccities={key: checktemp(value) for (key,value) in citiestemp.items()}
print(desccities)

output:

{‘Beijing’: ‘cold’, ‘Boston’: ‘cold’, ‘New York’: ‘cool’, ‘Califorina’: ‘cool’}

Example

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# Dictionary Comprehension
squares = {x: x*x for x in range(6)}

print(squares)
#{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

squares = {}
for x in range(6):
    squares[x] = x*x
print(squares)
#{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Build-In function

Function Description
all() Return True if all keys of the dictionary are True (or if the dictionary is empty).
any() Return True if all keys of the dictionary are True (or if the dictionary is empty).
len() Return the length (the number of items) in the dictionary.
sorted() Compares items of two dictionaries. (Not available in Python 3)

Build-In Example

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# Dictionary Built-in Functions
squares = {0: 0, 1: 1, 3: 9, 5: 25, 7: 49, 9: 81}

# Output: False
print(all(squares))

# Output: True
print(any(squares))

# Output: 6
print(len(squares))

# Output: [0, 1, 3, 5, 7]

ALL Examples

Creating Python Dictionary Example

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# empty dictionary
my_dict = {}

# dictionary with integer keys
my_dict = {1: 'apple', 2: 'ball'} #{1: 'apple', 2: 'ball'}

# dictionary with mixed keys
my_dict = {'name': 'John', 1: [2, 4, 3], 'age': 26} #{1: 'apple', 2: 'ball'}

print(my_dict['name']) #John
print(my_dict.get('age')) #26

Changing and Adding Dictionary elements

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# Changing and adding Dictionary Elements
my_dict = {'name': 'Jack', 'age': 26}
#{'name': 'Jack', 'age': 26}

# update value
my_dict['age'] = 27
#{'name': 'Jack', 'age': 27}

# add item
my_dict['address'] = 'Downtown'
#{'name': 'Jack', 'age': 27, 'address': 'Downtown'}

Removing elements from Dictionary

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# create a dictionary
squares = {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

# remove a particular item, returns its value
print(squares.pop(4)) #16
print(squares) #{1: 1, 2: 4, 3: 9, 5: 25}

# remove an arbitrary item, return (key,value)
print(squares.popitem()) # (5, 25)
print(squares)#{1: 1, 2: 4, 3: 9}

# remove all items
squares.clear()
print(squares) #{}

# delete the dictionary itself
del squares

# Throws Error
print(squares)

Dictionary Operations

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# Membership Test for Dictionary Keys
squares = {1: 1, 3: 9, 5: 25, 7: 49, 9: 81}
print(1 in squares) #True
print(2 not in squares) #True

# membership tests for key only not value
print(49 in squares) #False

Iterating Through a Dictionary

  • Iterating through a Dictionary
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# Iterating through a Dictionary
squares = {1: 1, 3: 9, 5: 25, 7: 49, 9: 81}
for i in squares:
    print(squares[i], end=" ")
# 1 9 25 49 81
  • Iterating all keys and values
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account = {'eric': 17, 'ben':30}
for name, age in account.items():
    print("Hi " + name+ "your age: "+str(age))
# Hi ericyour age: 17
# Hi benyour age: 30
  • Iterating all keys
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account = {'eric': 17, 'ben':30}
for name in account.keys():
    print("hi " + name)
# hi eric
# hi ben
  • Iterating all values
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account = {'eric': 17, 'ben':30}
for age in account.values():
    print("age " + str(age))
# age 17
# age 30

[Amazon test] read file from text file

  • Question: Read file into datastucture and search like logfile
    If we have a log.txt, as below, you need to and search specfic string, like cat.
  • log.txt
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1 cat
2 lion
3 apple
  • Answer2: using dictionary
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d={}
with open("log.txt", "r") as textfile:
   #lines = textfile.readlines()
   lines = textfile.read().splitlines()
   #print(lines)
for i in lines:
    #sli = i.split(' ')
    (key,val)=i.split(" ")
    d[int(key)]=val
   # print(key,"",val)
print(d)