➕ added notes IITM 🐍 🏫

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content/notes/iit-madras/data-science-and-application/foundational-level/python/_index.md

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  keywords: ["Python","Programming","Notes","IITM"]
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content/notes/iit-madras/data-science-and-application/foundational-level/python/week 5/Lec1-Dictionaries.md

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 title: Dictionaries
 date: 2025-05-08
 weight: 51
----
+---
+
+Let’s dive into **Dictionaries in Python** 🗝️📚 with clear explanations, emojis, and practice questions (with solutions)!
+
+## What is a Dictionary? 🤔
+
+A **dictionary** in Python is a **mutable** (changeable) data structure that stores data as **key-value pairs**. Think of it like a real dictionary: you look up a word (key) to get its definition (value)[^1][^2].
+
+- **Keys**: Must be unique and hashable (like strings, numbers, or tuples of immutables).
+- **Values**: Can be any data type (even lists or other dictionaries).
+- **Syntax**: Curly braces `{}` and colon `:` separate keys and values.
+
+```python
+# Example dictionary
+student = {
+    "name": "Alice",
+    "age": 20,
+    "major": "CS"
+}
+```
+
+📝 *Here, "name", "age", and "major" are keys; "Alice", 20, and "CS" are values.*
+
+## Key Features of Dictionaries 🏆
+
+- **Unordered**: Items are not stored in a specific order (though Python 3.7+ preserves insertion order).
+- **Mutable**: You can add, change, or remove items after creation.
+- **No Duplicate Keys**: Each key must be unique. If you assign a value to an existing key, it will overwrite the old value[^2].
+- **Fast Lookups**: Accessing a value by key is very quick.
+
+
+## Creating Dictionaries 🛠️
+
+**Empty dictionary:**
+
+```python
+my_dict = {}
+# or
+my_dict = dict()
+```
+
+**With initial values:**
+
+```python
+person = {"name": "Bob", "age": 25}
+```
+
+**From a list of tuples:**
+
+```python
+pairs = [("a", 1), ("b", 2)]
+my_dict = dict(pairs)
+```
+
+
+## Accessing Items 🔎
+
+- **By key:**
+
+```python
+print(person["name"])  # Output: Bob
+```
+
+If the key doesn’t exist, you get a `KeyError`.
+- **Using `.get()` method:**
+
+```python
+print(person.get("age"))         # Output: 25
+print(person.get("height", 0))   # Output: 0 (default value)
+```
+
+Safer, as it returns `None` or a default value if the key is missing[^2].
+
+
+## Modifying Dictionaries ✏️
+
+**Add or update a key-value pair:**
+
+```python
+person["city"] = "Delhi"  # Adds a new key
+person["age"] = 26        # Updates existing key
+```
+
+**Remove an item:**
+
+```python
+del person["city"]         # Removes key 'city'
+removed = person.pop("age") # Removes 'age' and returns its value
+```
+
+
+## Dictionary Methods 🛠️
+
+| Method | Description | Example |
+| :-- | :-- | :-- |
+| `.keys()` | Returns all keys | `person.keys()` |
+| `.values()` | Returns all values | `person.values()` |
+| `.items()` | Returns all key-value pairs (tuples) | `person.items()` |
+| `.update()` | Updates dictionary with another dict | `person.update({"age": 27})` |
+| `.get()` | Gets value for key (safe) | `person.get("age", 0)` |
+| `.pop()` | Removes key and returns its value | `person.pop("age")` |
+
+## Looping Through Dictionaries 🔄
+
+**Loop through keys:**
+
+```python
+for key in person:
+    print(key)
+```
+
+**Loop through values:**
+
+```python
+for value in person.values():
+    print(value)
+```
+
+**Loop through key-value pairs:**
+
+```python
+for key, value in person.items():
+    print(f"{key}: {value}")
+```
+
+
+## Practice Questions with Solutions 🏋️‍♂️
+
+### 1. Create a dictionary with your name, age, and city. Print each value.
+
+```python
+my_info = {"name": "Rahul", "age": 21, "city": "Mumbai"}
+print(my_info["name"])  # Rahul
+print(my_info["age"])   # 21
+print(my_info["city"])  # Mumbai
+```
+
+
+### 2. Add a new key "country" with value "India" to your dictionary. Then, remove the "city" key.
+
+```python
+my_info["country"] = "India"
+del my_info["city"]
+print(my_info)  # {'name': 'Rahul', 'age': 21, 'country': 'India'}
+```
+
+
+### 3. Given the dictionary: `fruits = {"apple": 2, "banana": 3, "orange": 1}`
+
+Write code to:
+
+- Print all keys
+- Print all values
+- Print all key-value pairs
+
+```python
+fruits = {"apple": 2, "banana": 3, "orange": 1}
+print(fruits.keys())   # dict_keys(['apple', 'banana', 'orange'])
+print(fruits.values()) # dict_values([2, 3, 1])
+print(fruits.items())  # dict_items([('apple', 2), ('banana', 3), ('orange', 1)])
+```
+
+
+### 4. Write a function that counts the frequency of each letter in a string and returns a dictionary.
+
+```python
+def letter_count(s):
+    freq = {}
+    for char in s:
+        if char in freq:
+            freq[char] += 1
+        else:
+            freq[char] = 1
+    return freq
+
+print(letter_count("banana"))  # {'b': 1, 'a': 3, 'n': 2}
+```
+
+
+### 5. Check if "apple" is a key in the `fruits` dictionary.
+
+```python
+if "apple" in fruits:
+    print("Yes, apple is in the dictionary!")
+else:
+    print("No, apple is not found.")
+```
+
+
+## Step-by-Step Example: Counting Unique Characters
+
+**Problem:** Count the number of unique characters in a string using a dictionary.
+
+**Step 1:** Create an empty dictionary
+**Step 2:** Loop through each character in the string
+**Step 3:** If character is not in dictionary, add it
+**Step 4:** Print the length of the dictionary (number of unique characters)
+
+```python
+s = "hello world"
+unique_chars = {}
+for char in s:
+    unique_chars[char] = 1  # Value doesn't matter here
+print(len(unique_chars))  # Output: 8 (h, e, l, o,  , w, r, d)
+```
+
+
+## Common Pitfalls ⚠️
+
+- **Keys must be hashable**: You can't use lists or other dictionaries as keys.
+- **Accessing a missing key with `[]` raises KeyError**: Use `.get()` to avoid errors.
+- **Duplicate keys are not allowed**: The last value assigned to a key will overwrite previous ones.
+
+
+## Dictionary Comprehensions 🧠
+
+You can create dictionaries in a single line using a comprehension:
+
+```python
+squares = {x: x*x for x in range(1, 6)}
+print(squares)  # {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
+```
+
+
+## Summary Table: Lists vs Dictionaries
+
+| Feature | List | Dictionary |
+| :-- | :-- | :-- |
+| Access by | Index (position) | Key (any hashable object) |
+| Syntax | `[^1][^3][^2]` | `{"a": 1, "b": 2}` |
+| Mutable | Yes | Yes |
+| Ordered | Yes (since Python 3.7) | Yes (since Python 3.7) |
+| Duplicate keys? | N/A | No |
+
+## Try It Yourself! 🚀
+
+**Q:** Create a dictionary for three students with their names as keys and their grades as values. Then, print the average grade.
+
+**Solution:**
+
+```python
+grades = {"Anil": 85, "Bina": 92, "Chetan": 78}
+average = sum(grades.values()) / len(grades)
+print(f"Average grade: {average}")
+```
+
+**Dictionaries** are super useful for organizing and accessing data quickly by a unique key! Practice using them in your projects and you'll master them in no time! 😃🔑[^1][^3][^2]
+
+<div style="text-align: center">⁂</div>
+
+[^1]: itpacs_cafiero.pdf
+
+[^2]: Introduction_to_Python_Programming_-_WEB.pdf
+
+[^3]: Learning_Python.pdf
+
+[^4]: thinkpython2.pdf
+
+[^5]: Python-Cheatsheet-2024.pdf
+
+[^6]: OER-202301_Wang_2023-Introduction-to-Computer-Programming-with-Python.pdf
+
+[^7]: pythonlearn.pdf
+