if - elif - else — syntax, indentation rules

1.2.15 · Coding › Introduction to Programming (Python)

SYNTAX KAISI DIKHTI HAI

if condition1:
    # block A — runs if condition1 is True
    statement_a
elif condition2:
    # block B — runs if condition1 False AND condition2 True
    statement_b
else:
    # block C — runs if both above were False
    statement_c

Exactly ek kyun run karna chahiye (else ke saath)? Conditions saari possibilities partition karti hain: ya toh koi $c_{k}$ pehla True wala hai, ya koi nahi → else. Cases ka set bina overlap ke covered hai. Isliye ek clean if/elif/else chain exhaustive aur exclusive hoti hai.

Figure — if - elif - else — syntax, indentation rules

Derivation: Python indentation kaise padhta hai

Worked examples

Worked example 1 — Pass / Fail

marks = 72
if marks >= 40:
    print("Pass")
else:
    print("Fail")

Output: Pass

Kyun? marks >= 40 → 72 >= 40 → True, toh if block run hota hai aur else skip ho jaata hai.

Worked example 2 — Grade bands (elif chain, order matters)

score = 85
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"
print(grade)

Output: B

Yeh step kyun? 90 wala test fail hota hai ( $85 < 90$ ). elif score >= 80 par jao → True → grade="B". Python ruk jaata hai — 70 wala elif check bhi nahi hota.
Pehle andaaza lagao, phir verify karo: answer padhne se pehle, score = 70 ke liye output predict karo. (Answer: C, kyunki >=70 pehla True hai.)

Worked example 3 — Nesting (block ke andar block)

age = 20
citizen = True
if age >= 18:
    if citizen:
        print("Can vote")
    else:
        print("Not a citizen")
else:
    print("Too young")

Output: Can vote

Kyun? Outer if True → uske block mein enter karo. Andar, citizen True hai → inner if run hota hai. Do indentation levels (8 spaces) Python ko batate hain ki inner if, outer wale se belong karta hai.

Worked example 4 —

if bina else ke

n = -3
if n < 0:
    print("negative")
print("done")

Output:

negative
done

Kyun? print("done") if ke same indentation par hai, isliye woh block ke bahar hai aur hamesha run hoti hai. else optional hai.

Common mistakes (Steel-man + fix)

Common mistake

elif ki jagah kai alag ifs Sahi lagta hai: "Zyada ifs = zyada thorough." Har ek apna alag check lagta hai. Reality:

if score >= 80: grade="B"
if score >= 70: grade="C"   # overwrites! score=85 -> C

Alag ifs sab evaluate hote hain; baad wale pehle ke results ko overwrite kar sakte hain. Fix: elif use karo taaki chain mutually exclusive ho aur pehle match par ruk jaaye.

Recall Feynman: ek 12 saal ke bachche ko samjhao

Ek hallway imagine karo jisme doors hain. Tum chalte ho aur pehle door par likha sawaal poochte ho: "Kya yeh sach hai?" Agar haan, tum us door se andar jaate ho aur hallway khatam ho jaata hai — baaki doors check nahi hote. Agar nahi, tum agli door (elif) par chalte ho aur phir poochte ho. Agar tum end tak pahunch gaye aur kisi door ne haan nahi kaha, toh tum else likha aakhri door se andar jaate ho. Jis tarah tum apne sentences ko thoda right mein push karte ho, usi tarah tum actions ke ek set ko ek khaas door se jodh rahe ho — saath mein push in kiya hua sab kuch us door se jaane ke baad hota hai.

Active recall

Kaun sa punctuation mark har if, elif, aur else line ke end mein hona chahiye?

Colon :

Python ko kaise pata chalta hai ki kaun si lines if block ke andar belong karti hain?

Unke indentation (leading whitespace) se, braces se nahi.

Ek if chain mein kitne elif blocks ho sakte hain?

Zero ya zyada.

Ek if chain mein kitne else blocks ho sakte hain?

Zyada se zyada ek (aur woh optional hai).

Ek if/elif/elif/else chain mein kitne blocks run hote hain?

Exactly ek — pehla jiska condition True ho (ya else agar koi nahi).

Tabs aur spaces mix karne se kaun sa error aata hai?

TabError: inconsistent use of tabs and spaces.

Ek condition mein = aur == mein kya fark hai?

= value assign karta hai; == equality test karta hai. Conditions ko == chahiye.

Kai alag ifs ki jagah elif kyun prefer karein?

elif branches ko mutually exclusive banata hai aur pehle match par ruk jaata hai; alag ifs sab run hote hain aur results overwrite kar sakte hain.

score=70 ke liye ek chain mein jo >=90, >=80, >=70, else check karti hai, kya select hoga?

>=70 branch (pehla True wala).

Python mein indentation ke ek level mein standard kitne spaces hote hain?

4 spaces.

Connections

Booleans and comparison operators — conditions True/False evaluate hoti hain
Logical operators and, or, not — compound conditions banana
Truthiness in Python — if mein non-bool values (0, "", [] are False)
Indentation and code blocks — yahi rule loops aur functions par bhi apply hota hai
while and for loops — : aur indented blocks use karte hain
match-case statement — alternative branching (Python 3.10+)
Ternary conditional expression — a if cond else b one-liner

if - elif - else — syntax, indentation rules

YEH KYUN exist karta hai?

YEH HAI KYA