Lesson 05 — Foundation

EK VALUE NAHI.
POORA COLLECTION.

Aapke paas sirf ek learner nahi, learners ki list hai. Ek task nahi, daily tasks hain. List ordered, mutable collection hai — yani values ki sequence jise aap later change kar sakte ho.

WHAT: list square brackets mein banti hai

skills = ["Python", "SQL", "Pandas"]

print(skills[0]) # Python
print(len(skills)) # 3

WHEN: collection ko update karna

append() end mein add karta hai; remove() matching value ko hatata hai; pop() index se item nikalta hai. Production code mein item exists karta hai ya nahi, woh check karna bhi zaruri hota hai.

tasks = ["Variables revise", "Strings practice"]
tasks.append("Lists exercise")
tasks[0] = "Variables project"

print(tasks)
# ['Variables project', 'Strings practice', 'Lists exercise']

WHERE: simple task tracker

List aapke first mini-project ka building block hai. Abhi loop complete nahi hua, fir bhi aap collection ko create/update aur display kar sakte ho.

Useful list operations

• items.sort() mutates the list; sorted(items) new sorted list deta hai.
• items[:2] slicing ke saath first two items deta hai.
• "Python" in skills membership check karta hai.
• Nested list tab use hoti hai jab collection ke andar collection ho; soon matrices aur tabular data se connect karoge.

Access karna: index se nikalo

Har item ka number (index) hota hai — 0 se start. Aakhri item -1 se milta hai, aur tukda [1:3] se. Range ka end include nahi hota.

fruits = ["aam", "kela", "seb", "angoor"]
print(fruits[0])   # aam
print(fruits[-1])  # angoor (aakhri)
print(fruits[1:3])  # [kela, seb] (3 include nahi)
print(len(fruits))  # 4
print("seb" in fruits)  # True

Add karna: teen tareeke

End me jodna ho to append(), beech me insert(), poori list milani ho to extend(). Teenon ki jagah fix hai.

cart = ["doodh"]
cart.append("bread")  # end me
cart.insert(0, "anda")  # shuru me
cart.extend(["makkhan", "chai"])  # poori list
print(cart)

Remove karna: value, index ya sab

Value pata ho to remove(), position pata ho to pop() (wapas bhi deta hai!), index range udana ho to del, sab saaf karna ho to clear().

cart = ["anda", "doodh", "bread", "makkhan"]
cart.remove("doodh")  # value se
nikala = cart.pop(0)  # index se + wapas
print(nikala, cart)
del cart[0]  # index uda do
cart.clear()  # khaali
print(cart)  # []

Loop chalana: har item par kaam

List ka asli power loop me hai — har item par same kaam. Index bhi chahiye to enumerate(), chhota kaam ho to comprehension ek line me.

marks = [72, 91, 65]
for m in marks:
    print(m + 5)  # grace marks

for i, m in enumerate(marks):
    print(i, m)

naye = [m + 5 for m in marks]
print(naye)

Sort karna: order lagao

Marks ki list ho ya naam — sort() chhota-se-bada lagata hai. Yaad rakho: sort() kuch wapas nahi deta (None), list andar hi badal jati hai. Bina badle nayi sorted list chahiye to sorted().

marks = [72, 91, 65]
marks.sort()
print(marks)  # [65, 72, 91]

names = ["Ramesh", "Aman"]
print(sorted(names, reverse=True))  # ulte order me, original same

Copy aur join: do list ek saath

b = a copy NAHI hai — dono same list ko pakadte hain! Asli copy a.copy() se banti hai. Do list jodne ke liye + ya extend().

a = [1, 2]
b = a
b.append(3)
print(a)  # [1, 2, 3] — a bhi badal gaya!

c = a.copy()
c.append(4)
print(a, c)  # [1, 2, 3] [1, 2, 3, 4]

full = [1, 2] + [3, 4]
print(full)  # [1, 2, 3, 4]

Methods quick table

# Unpythonic: khaali list me loop se bharna
squares = []
for i in range(5):
    squares.append(i * i)

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

MNC tip: Comprehension vs loop — dono likhna aana chahiye, padhna comprehension aani chahiye.

Common mistakes

• Invalid index: list ke 3 items hain to items[3] error karega; last valid index 2 hai.
• remove() missing item ke saath error deta hai — external input par check karna seekho.
• sort() ka result variable mein assign mat karo; woh list ko in-place change karta hai aur None return karta hai.
• sort() kuch wapas nahi deta — marks = marks.sort() likhoge to marks me None aa jayega.
• b = a copy nahi hai — asli copy a.copy() se banao.