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Chapter 03 | Structured Data | Working with Structured Data

Dictionary

  • Dictionary is a great built-in data structure to manipulate structured data in Python.
  • It stores value in the key/value pair. The Key is always unique.
  • Dictionary is also referred to as associative array.
  • Data which has this form is an easy candidate for storing in a Dictionary.
    • Data with 2 columns and multiple rows.
  • Dictionary is often used as a lookup table.

Dictionary Operations

person = {
    "Name": "Ford Prefect",
    "Gender": "Male",
    "Occupation": "Researcher",
    "Home Planet": "Betelgeuse Seven",
}
  • Each keys is separated from its value with a :.
  • The complete Key-Value pair is wrapped inside a {}
  • pprint is a module which can be used to print a Dictionary.
  • The insertion order is not maintained while retrieving values.
  • We can access value from a Dictionary using the [] notation like this, person["Name"]
  • Dictionary lookup is very fast.
  • The Python Interpreter can access the value associated with a key quickly.
  • A new key-value can be added to an existing Dictionary with just this.
    • person["Age"] = 33
  • Incrementing a value can be done with += operator.
    • found["a"] += 1
  • Python does not have a ++ operator.
  • We can also iterate over the Dictionary using a for loop.
for k in found:
    print(k, "was found", found[k], "times.")

Bullet Points

  • Dictionary is a collection of rows, with each row containing two columns. The first column is called a Key and the next is called a value.
  • Each row is called a key-value pair. Dictionary can grow and shrink in size.
  • Insertion order is not maintained by a dictionary.
  • Accessing data in a dictionary uses the square bracket notation. Put the key inside the square brackets.
  • Python for loop can be used to iterate over a dictionary.

Dictionary loops

  • The output for a dictionary is not in any order, we can create the order by passing the dictionary to a sorted() method, which sorts the dictionary.
    • for k in sorted(found):
    • sorted() does not change the actual ordering of the dictionary.
  • Dictionary have an interesting function called items(), which gives access to a key-value pair as the value for a loop variable.
    • for k, v in found.items():

Dictionary Membership

  • We can check if a key exists in a dictionary with the help of in operator.
  • We can also use the not in operator to check for non membership.
  • setdefault() : initializes with a default value if the key is not available in the dictionary.

Bullet Points

  • Insertion order is not maintained in Dictionary, we can use sorted() method to sort a dictionary.
  • .item() allows to iterate over a Key-Value pair. On each iteration it returns a pair of key-value.
  • When we access non existing Key in dictionary, it leads to KeyError.
  • KeyError can be avoided by using the in, not in operator, or even we can use the setdefault() to initialize the value.

Sets

  • Sets do not allow duplicate values. This is by far the most valuable information about sets.
  • Sets also support mathematical operations like, union intersection etc.
  • Sets are optimize for fast look-up.
  • Sets syntax looks like this.
    • vowels = {'a', 'e', 'i', 'o', u}
      • This declaration is similar to dictionary, with the major difference being, the absence of : for Key-Value pair.
  • Sets do not maintained the insertion order. It can be ordered with the help of sorted() function.

Set Operation.

  • in : it can be used to find the existence of an object inside a set.
  • union() : it combines 2 sets.
  • difference() : tells what is present in 1 set bit not in other.
  • intersection() : Returns the common object in 2 sets.

Bullet Points

  • Sets do not allow duplicates.
  • Sets are enclosed in {} but they are missing the : with key-value pair used in dictionary.
  • Sets also do not maintain the insertion order. sorted() helps in getting the order.
  • set() converts any sequence into a set.
  • Union, intersection, difference are some of the well know built-in function of sets.

Tuples

  • Tuples solves a specific problem, which is its property to not mutate.
  • Tuples are denoted by vowels = ("a", "e", "i", "o", "u")
    • The () are optional, the , is what creates the tuple.
    • A single member tuple is created by vowels = ('a', ), notice the comma.
  • Tuples support the [] notation to access it's member.
  • Tuple's immutability helps when we have to make sure our data does not change at any cost.

Nested Data Structure.

  • We can nest data structure.