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🔁 Looping in Python — All Data Structures

(Ways · Patterns · Templates · Real Examples · Interview Notes)

Python loops work on iterables.
Each data structure exposes a different iteration behavior.

🧠 Big Picture (Mental Model)

Iterable ──▶ Iterator ──▶ __next__() ──▶ value

1️⃣ Looping Over String

Template

for char in string:
    ...

With index

for i, ch in enumerate("python"):
    print(i, ch)

Use case

• Parsing
• Validation
• Tokenization

📌 Immutable → read-only loop

2️⃣ Looping Over List

Template

for item in list_:
    ...

Index-based

for i in range(len(lst)):
    print(i, lst[i])

Pythonic (recommended)

for i, item in enumerate(lst):
    print(i, item)

Modify while looping ❌ (danger)

# ❌ Don't do this
for x in lst:
    lst.remove(x)

Safe way

lst = [x for x in lst if x > 0]

3️⃣ Looping Over Tuple

Same as list, but immutable

Template

for item in tpl:
    ...

Tuple unpacking

points = [(1, 2), (3, 4)]
for x, y in points:
    print(x, y)

📌 Faster than list (slightly)
📌 Safe for read-only data

4️⃣ Looping Over Set

⚠️ Unordered

Template

for item in my_set:
    ...

Example

emails = {"a@gmail.com", "b@gmail.com"}
for email in emails:
    print(email)

📌 Use when:

• Uniqueness matters
• Order doesn’t

5️⃣ Looping Over Dictionary (VERY IMPORTANT)

5.1 Default (keys)

for key in my_dict:
    print(key)

5.2 Keys explicitly

for key in my_dict.keys():
    print(key)

5.3 Values

for value in my_dict.values():
    print(value)

5.4 Key + Value (🔥 MOST USED)

for key, value in my_dict.items():
    print(key, value)

📌 Interview note:

Dict iteration is O(n) and preserves insertion order (Python 3.7+)

6️⃣ Looping Over range()

Template

for i in range(start, stop, step):
    ...

Examples

for i in range(5):
    print(i)

for i in range(10, 0, -1):
    print(i)

📌 range is lazy (no memory waste)

7️⃣ Looping with zip()

Parallel iteration

names = ["A", "B"]
scores = [90, 80]

for n, s in zip(names, scores):
    print(n, s)

📌 Stops at shortest iterable

8️⃣ Looping with enumerate()

Template

for index, value in enumerate(iterable, start=0):
    ...

Example

for i, ch in enumerate("data", 1):
    print(i, ch)

📌 Cleaner than range(len())

9️⃣ Nested Loops (All Structures)

Matrix (List of Lists)

matrix = [[1, 2], [3, 4]]

for row in matrix:
    for val in row:
        print(val)

Dict of Lists

data = {"a": [1, 2], "b": [3]}

for k, values in data.items():
    for v in values:
        print(k, v)

🔟 Loop Control Keywords

break

for x in data:
    if x == 5:
        break

continue

for x in data:
    if x < 0:
        continue

else with loop (INTERVIEW FAVORITE ⭐)

for x in data:
    if x == 10:
        break
else:
    print("Not found")

🧩 Universal Loop Templates (SAVE THESE)

1️⃣ Read-only traversal

for item in iterable:
    process(item)

2️⃣ Index-aware

for i, item in enumerate(iterable):
    process(i, item)

3️⃣ Filtered loop

for item in iterable:
    if condition(item):
        process(item)

4️⃣ Parallel loop

for a, b in zip(it1, it2):
    process(a, b)

5️⃣ Dict processing

for k, v in dict_.items():
    process(k, v)

⚠️ Common Mistakes (Interview Traps)

❌ Modifying list while looping
❌ Assuming set order
❌ Using range(len()) unnecessarily
❌ Forgetting .items() for dict key-value loop

🎯 Final Summary Table

If you want next:

• 🔬 Behind-the-scenes iterator protocol
• ⚡ Loop performance & Big-O
• 🧠 Loop → comprehension transformation
• 🎯 20 tricky interview questions

Just say continue 🚀