while loop — condition-based, infinite loop dangers

1.2.17 · Coding › Introduction to Programming (Python)

WHAT it is

Definition while loop

Ek while loop ek block of code ko baar baar execute karta hai jab tak ek boolean condition True evaluate hoti rahe. Condition ko pehle har iteration se check kiya jaata hai (yeh ek pre-test loop hai).

while <condition>:
    <body>        # sirf tab run hoga jab condition True ho
# control yahan aata hai jab condition False ho jaaye

Teen cheezein jo HAMESHA chahiye:

Initialise — loop se pehle ek variable exist karta ho.
Condition — ek boolean jo us variable ko use kare.
Update — body andar kuch aisa jo condition ko False ki taraf dhakele.

WHY it exists (aur HOW it runs)

HOW Python isko execute karta hai (step trace):

i = 1                # 1. initialise
while i <= 3:        # 2. condition check  -> True?
    print(i)         # 3. body run karo
    i = i + 1        # 4. update -> step 2 par wapas jao

Trace table (Yeh step kyun? = "har check dikhao, including final wala jo fail hota hai"):

Step	`i`	`i <= 3`?	Action
check	1	True	print 1, i→2
check	2	True	print 2, i→3
check	3	True	print 3, i→4
check	4	False	loop exit

Condition 4 baar check hoti hai 3 numbers print karne ke liye — aakhri check wahi hai jo isko end karta hai. Is off-by-one ko bhoolna #1 bug ka source hai.

Figure — while loop — condition-based, infinite loop dangers

Worked examples

Worked example 1 — Countdown (clean update)

n = 5
while n > 0:
    print(n)
    n -= 1      # update
print("Lift off!")

n -= 1 kyun? Iske bina, n hamesha 5 rahega aur n > 0 hamesha True hoga. Update woh engine hai jo humein stop condition ki taraf le jaata hai. 5 4 3 2 1 print karta hai phir Lift off!.

Worked example 2 — Input validation (unknown iteration count)

password = ""
while password != "open":          # pata nahi kitni tries lagenge
    password = input("Password: ")
print("Access granted")

while kyun, for kyun nahi? Humein bilkul idea nahi user ko kitni attempts chahiye — data loop ko drive karta hai. Update hai hi har round ka naya input().

Worked example 3 — Intentional infinite loop with

break

while True:                # condition khud kabhi False nahi hogi
    cmd = input("> ")
    if cmd == "quit":
        break              # *asli* exit
    print("you said", cmd)

Yeh bug kyun nahi hai? while True deliberate hai; break controlled exit hai. Yeh standard "jab tak rokne ko nahi kaha jaaye tab tak loop karo" pattern hai. Danger sirf tab hai jab infinite loop accidental ho.

Worked example 4 — Accumulator (sum until threshold)

total = 0
x = 1
while total < 100:
    total += x
    x += 1
print(x - 1, total)   # 14 105

Yeh ruk kyun jaata hai? total strictly badhta hai har loop mein, toh usse 100 cross karna hi padega → condition False ho jaati hai. Convergence guarantee: updated variable monotonically boundary ki taraf badhti hai.

The infinite-loop danger

Common mistake Steel-man: "Meri condition khud badal jaayegi."

Yeh lagta hai sach hai kyunki math mein variables ke "values hote hain". Lekin code mein ek variable tabhi badalta hai jab code ki koi line use change kare. Teen classic killers:

Bug	Kaisa dikhta hai	Fix
Missing update	`while i < 10:` andar koi `i +=` nahi	update add karo
Update galat jagah	update `continue` ke baad ya loop ke bahar hai	isse live path ke andar move karo
Float kabhi equal nahi	`while x != 1.0:` with `x += 0.1`	`<`/`>` use karo, floats par `==` kabhi nahi

Emergency stop: terminal mein Ctrl + C runaway loop ko kill kar deta hai.

Recall Feynman: 12-saal ke bachche ko samjhao

Socho tum ek bucket ko cup se bhar rahe ho. Har scoop se pehle tum andar jhaankte ho aur poochte ho: "Kya bucket abhi bhi bhari nahi hai?" Agar haan, toh ek aur scoop laate ho. Agar bucket bhar gayi, toh ruk jaate ho. "Jhaankna" hai condition, "scoop lena" hai body, aur paani daalna hai update. Agar tum jhaankate ho lekin paani nahi daalta (koi update nahi), toh bucket kabhi nahi bharegi — toh tum hamesha scoop karte rehte ho aur haath thak jaata hai. Woh endless scooping hai infinite loop.

Flashcards

When is a while loop's condition evaluated relative to the body?

Har iteration se pehle (pre-test loop); body zero baar bhi run ho sakta hai.

What causes an accidental infinite loop?

Ek aisi condition jo kabhi False nahi hoti — usually ek missing ya misplaced update statement.

Difference between for and while in intent?

for = pata iteration count; while = tab tak repeat karo jab tak condition change ho, count pehle se nahi pata.

Why avoid while x == target: with floating-point updates?

Float arithmetic kabhi kabhi exactly kisi value par nahi utarti; condition target ko skip kar sakti hai aur loop hamesha chalata rahe. </> comparisons use karo.

What does break do inside while True:?

Turant loop se bahar nikal jaata hai, intentionally infinite loop ke liye real, controlled stop provide karta hai.

In while i <= 3: print(i); i+=1 starting i=1, how many times is the condition checked?

4 baar (3 True + 1 final False jo isko end karta hai).

What key combo stops a runaway loop in a terminal?

Ctrl + C.

What error does while x = 5: produce?

SyntaxError — = assignment hai, comparison ke liye == chahiye.

Connections

for loop — iterating over sequences — known-count counterpart
break and continue statements — loop flow ko control karna
Boolean expressions and comparison operators — condition kisse banti hai
input() and type conversion — while-based validation ka common driver
Off-by-one errors — final condition check kyun matter karta hai