Python-Cheatsheet.md

Python Coding Interview Cheatsheet

Lists (Arrays)

Initialization

• Definition: Lists are ordered, mutable collections of items.
• Usage: nums = [1, 2, 3]
  • Creates a list with elements 1, 2, and 3.
  • Lists can hold items of different data types, though typically they hold similar types for consistency.

Dynamic Arrays

• Definition: Lists in Python are dynamic, meaning they can grow and shrink in size.
• Usage:

  nums.append(4)  # Adds element 4 to the end of the list
  nums.pop()  # Removes and returns the last element (4 in this case)

  • append is used to add an item to the end of the list.
  • pop is used to remove the last item from the list.

Slicing

• Definition: Accessing a subset of the list using indices.
• Usage: nums[1:3] (returns elements at index 1 and 2)

  nums = [1, 2, 3, 4]
  sliced = nums[1:3]  # Result: [2, 3]

  • Slicing allows you to create a new list from an existing one, starting at the first index and up to but not including the second index.

List Comprehension

• Definition: A concise way to create lists.
• Usage: [i for i in range(5)] (creates [0, 1, 2, 3, 4])

  squares = [i*i for i in range(5)]  # Result: [0, 1, 4, 9, 16]

  • List comprehensions provide a concise way to create lists from sequences.

Unpacking

• Definition: Assigning elements of a list to multiple variables.
• Usage: a, b, c = [1, 2, 3]

  nums = [1, 2, 3]
  a, b, c = nums  # a=1, b=2, c=3

  • Useful for quickly assigning values without needing to access indices individually.

Strings

Initialization

• Definition: Strings are immutable sequences of characters.
• Usage: s = "hello"
  • Creates a string with the text "hello".

Slicing

• Definition: Accessing a subset of the string.
• Usage: s[1:3] (returns characters at index 1 and 2)

  s = "hello"
  sliced = s[1:3]  # Result: "el"

Immutable

• Definition: Strings cannot be changed after they are created.
• Usage:

  s = "hello"
  s = s + " world"  # Creates a new string "hello world"

  • To change a string, you must create a new one.

Conversion

• Definition: Changing data types to/from strings.
• Usage:

  num = int("123")  # Converts string "123" to integer 123
  s = str(123)  # Converts integer 123 to string "123"

Tuples

Initialization

• Definition: Tuples are immutable, ordered collections of items.
• Usage: t = (1, 2, 3)
  • Creates a tuple with elements 1, 2, and 3.

Immutability

• Definition: Tuples cannot be changed after creation.
• Usage:

  t = (1, 2, 3)
  # t[0] = 0  # This will raise an error because tuples are immutable

Use in Dictionaries

• Definition: Tuples can be used as keys in dictionaries.
• Usage:

  d = {(1, 2): "a pair"}  # Tuples as keys

Dictionaries (HashMaps)

Initialization

• Definition: Dictionaries are mutable collections of key-value pairs.
• Usage: d = {"a": 1, "b": 2}
  • Creates a dictionary with keys "a" and "b" mapped to values 1 and 2, respectively.

Access

• Definition: Retrieve a value by its key.
• Usage: d["a"] (returns 1)

  value = d["a"]  # Result: 1

Modify

• Definition: Change the value associated with a key.
• Usage: d["a"] = 10

  d["a"] = 10  # Changes the value of key "a" to 10

Remove

• Definition: Delete a key-value pair.
• Usage: del d["a"]

  del d["a"]  # Removes the key "a" and its value from the dictionary

Comprehension

• Definition: A concise way to create dictionaries.
• Usage: {i: i*2 for i in range(3)} (creates {0: 0, 1: 2, 2: 4})

  doubled = {i: i*2 for i in range(3)}  # Result: {0: 0, 1: 2, 2: 4}

Sets

Initialization

• Definition: Sets are unordered collections of unique items.
• Usage: s = {1, 2, 3}
  • Creates a set with elements 1, 2, and 3.

Add/Remove

• Definition: Add or remove items from a set.
• Usage:

  s.add(4)  # Adds 4 to the set
  s.remove(3)  # Removes 3 from the set

Comprehension

• Definition: A concise way to create sets.
• Usage: {i for i in range(5)}

  squares = {i*i for i in range(5)}  # Result: {0, 1, 4, 9, 16}