Lesson 04 — Foundation

TEXT KO
TOOL BANAO.

Names, emails, prompts, URLs, file content — real applications ka bahut data text hota hai. Python string text ko store karne ke saath character-by-character access aur transform karne bhi deta hai.

WHAT: string ek ordered sequence hai

String quotes ke andar banta hai. Uska har character ek index par hota hai, jo 0 se start hota hai. Isliye "AI"[0] ka output A hai.

tool = "Python"
print(tool[0]) # P
print(tool[-1]) # n
print(tool[0:3]) # Pyt

Quotes, multiline aur assign karna

Single ho ya double — dono quotes se string banta hai. Rule sirf ek: andar wala quote bahar wale se alag ho. Lambi address ya paragraph ke liye triple quotes use karo.

print("Dukaan band hai")
print('Dukaan band hai')
print("Bola 'rate kam karo'")

address = """DSWallah,
Lucknow,
UP"""
print(address)

Loop, length aur check karna

String ek sequence hai — for se har character, len() se lambai, aur in se check karo ki word andar hai ya nahi. Order-ID verify karna ho to yehi use hota hai.

order = "SWIGGY-4821"
print(len(order))  # 11

for ch in order:
    print(ch)

print("-" in order)  # True
print("ZOM" in order)  # False
print("ZOM" not in order)  # True

WHEN: messy text ko clean karna

User input aur external data mein extra spaces ya inconsistent capitals normal hote hain. String methods original string ko nahi, usually new transformed string return karte hain.

upi_id = "  RAMESH@OKHDFC  "
upi_id = upi_id.strip().lower()
print(upi_id)  # ramesh@okhdfc

city = "  lucknow  "
print(city.strip().title())  # Lucknow

Useful methods: lower(), upper(), strip(), replace(), split() aur startswith(). Inhe blindly chain karne se pehle input/output ka chhota example socho.

Methods quick table

Roz kaam aane wale methods ek jagah — example ke saath ratne ki zaroorat nahi, samajh lo.

Jodna: + aur f-string

Do string + se judte hain, lekin beech me space khud dalna padta hai. Numbers ke saath message banana ho to f-string sabse saaf hai.

first = "Ramesh"
last = "Kumar"
print(first + " " + last)  # Ramesh Kumar

amount = 250
print(f"Payable: Rs.{amount}")  # Payable: Rs.250

HOW: f-string ke saath readable output

Text aur variable ko join karne ka clean modern way f-string hai. String ke before f, aur variable braces ke andar.

Escape characters

Kuch cheezein seedha nahi likh sakte — new line, quote ke andar quote, backslash khud. Backslash (\) ke baad wala character special ban jata hai.

print("Pehli line\nDoosri line")
print("Order \"SWIGGY-4821\" ready")
print("C:\\new\\order.txt")
# \n = new line, \" = quote, \\ = backslash

# Unpythonic: loop me jodna (slow)
s = ""
for w in ["a", "b", "c"]:
    s = s + w

# Pythonic: join (fast)
s = "".join(["a", "b", "c"])
print(s)  # abc

MNC tip: "String concat loop me kyun slow hai?" — immutability wala jawab do, offer pakki taraf.

Common mistakes

• name[0] = "R" error dega; strings immutable hain.
• Slicing mein end index include nahi hota: text[0:3] first three chars deta hai.
• Quotes ke beech apostrophe use karna ho to outer quotes ka type change karo ya escape use karo.
• in check case-sensitive hai — "swiggy" in "SWIGGY-4821" False dega.
• Index chahiye to range(len(text)) use karo, warna seedha for ch in text likho.