06-Dictionaries

Dictionaries

• Are used to describe data in more detail
• Data structure that consists key-value pairs
• keys are used to describe data and values are used to represent data
• keys are usually num or strings but values can be anything from boolean to nested dictionaries
• There is no fixed order for items in a dictionary, they are unordered

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Accessing Individual Values

# creating a dictionary
cat = {
  "name": "Kitty",
  "age" : 5,
  "isCute": True
}

# accessing properties
cat["name"] # Kitty

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Iterating Over Dictionaries

• use for v in dict.values() to iterate over values
• use for k in dict.keys() to iterate over keys
• use for k,v in dict.items() to iterate over both at the same time

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Using In with Dictionaries

cat = {
  "name": "Kitty",
  "age" : 5,
  "isCute": True
}

###### DOES A DICTIONARY HAVE A KEY?
print("name" in cat) # True
print("nails" in cat) # False

###### DOES A DICTIONARY HAVE A VALUE?
print("Kitty" in cat.values()) # True
print("Pussy" in cat.values()) # False

# ! THE BELOW CODE WILL THROW AN ERROR
if cat["nails"]:
  print("nail")

# ! THE CORRECT WAY
if "nails" in cat:
  print("nail")

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Dictionary Methods

clear()

• empty out the dictionary

    d = dict(a=1,b=2)
    print(d ) # {'a':1, 'b':2}
    d.clear()
    print(d) # {}

copy()

• copies one dictionary to another variable but they don't point to the same in memory

  d = dict(a=1,b=2)
  c = d.copy()
  print(c) # {'a':1, 'b':2}
  print(c == d) # True
  print(c is d) # False

fromkeys()

• used to dynamically add items to an empty dictionary

• used to set default values

  # add a fixed 'unknown' value to all the keys for new user
  new_user = {}.fromkeys(['name', 'age', 'score'], 'unknown')
  
  print(new_user) # {'name': 'unknown', 'age': 'unknown', 'score': 'unknown'}

• if you don't put a list as the first argument and maybe put a string instead then it will iterate over the characters of the strings

• can also use range as an iterable object here

  new_dict = {}.fromkeys(range(1,10), 'None')
  

get()

• if we have the specified key, we get its value otherwise we get None

  d = dict(a=1,b=2)
  
  # NORMAL WAY
  print(d['a']) # 1
  print(d['c']) # KeyError
  
  # USING GET METHOD
  print(d.get('a')) # 1
  print(d.get('c')) # None

pop(key)

• removes the specified key and its value

• gives an error if the key doesn't exist

• returns the value of the item that is removed

  d = dict(a=1,b=2)
  
  print(d) # {'a':1, 'b':2}
  print(d.pop('a')) # 1
  print(d) # {'b': 2}

popitem()

• removes any random key from the the dictionary
• does not take any argument

update()

• add everything from one dictionary to another one

  a = dict(a=1,b=2)
  b = {'c':3}
  
  #  update `b` with `a`
  b.update(a)
  
  print(b) # {'c': 3, 'a': 1, 'b': 2}

• it doesn't overwrite prev existing values

• it does overwrite duplicate key-values

• it only updates and does not remove anything if we pass a dictionary w fewer properties

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Dictionary Comprehension

• iterates over keys by default
• .items() can be used to iterate over both keys and values

Conditional Logic with Dictionary Comprehension

Just add if else according to logic man ughh

new_list = {num: 'even' if num%2==0 else 'odd' for num in range(1, 10)}

# new_list = {1: 'odd', 2: 'even', 3: 'odd', 4: 'even', 5: 'odd', 6: 'even', 7: 'odd', 8: 'even', 9: 'odd'}