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Python Hash Tables

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A Hash Table is a data structure that stores data in key-value pairs. In Python, the built-in dict (dictionary) is the most common way to implement a hash table.

Hash tables use a hash function to map keys to a specific index where the value is stored.

🔹 Creating a Hash Table in Python

# Create a hash table (dictionary)
students = {
    "101": "Alice",
    "102": "Bob",
    "103": "Charlie"
}

print(students["102"])  # Output: Bob

✅ Keys can be strings, numbers, or tuples
✅ Values can be of any data type

🔹 Adding Items

students["104"] = "David"

🔹 Updating Values

students["103"] = "Eve"

🔹 Deleting Items

del students["101"]

🔹 Looping Through a Hash Table

for key, value in students.items():
    print(key, ":", value)

🔹 Useful Dictionary Methods

Method Description
dict.get(key) Returns value for key (or None)
dict.keys() Returns all keys
dict.values() Returns all values
dict.items() Returns all key-value pairs
dict.pop(key) Removes item by key
key in dict Checks if key exists

🔹 Hashing Example (Behind the Scenes)

print(hash("apple"))  # Outputs an integer (hash value)

Python internally uses this hash value to store keys efficiently.

🔹 Use Cases of Hash Tables

  • Storing user profiles

  • Phonebooks

  • Caching

  • Counting items (frequency maps)

  • Lookup tables

Practice Questions

Q1. Write a Python program to create a dictionary of employee ID → name.

Q2. Write a Python program to add a new employee to the dictionary.

Q3. Write a Python program to update the name of an existing employee.

Q4. Write a Python program to delete an employee using del keyword.

Q5. Write a Python program to check if the key "105" exists in the dictionary.

Q6. Write a Python program to use a loop to print all keys and values in the dictionary.

Q7. Write a Python program to get the value for key "104" using .get().

Q8. Write a Python program to list all keys using the .keys() method.

Q9. Write a Python program to remove an item from the dictionary using .pop().

Q10. Write a Python program to count the number of items in the dictionary using len().

Python Hash Tables Quiz

Q1: Which Python data type is used as a hash table?

A. list
B. dict
C. set
D. array

Q2: What do hash tables store data as?

A. Only values
B. Keys only
C. Key-value pairs
D. Indexed values

Q3: What is the time complexity of accessing a value by key in a dictionary?

A. O(n)
B. O(1)
C. O(log n)
D. O(n^2)

Q4: What does dict.get(key) return if the key doesn't exist?

A. 0
B. -1
C. Error
D. None

Q5: How do you remove a key-value pair from a dictionary?

A. remove()
B. pop()
C. delete()
D. clear()

Q6: What does dict.items() return?

A. Only keys
B. Only values
C. A list of tuples
D. A list of keys

Q7: What is a hash function used for in a hash table?

A. Encrypting keys
B. Sorting values
C. Mapping key to index
D. Reversing keys

Q8: Which keyword checks if a key exists in a dictionary?

A. include
B. has
C. with
D. in

Q9: Which method returns only keys from a dictionary?

A. keys()
B. items()
C. values()
D. index()

Q10: What will len(dict) return?

A. Total characters
B. Number of values
C. Number of key-value pairs
D. Memory used
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