Zeni Study

Pages
- Learn Python In 7 Days
  Day 1 : Variable + if/else
  Day 2: Loops
  Day 3 : Array + String
  Day 4 — Functions
  Day 5 — Dictionaries - Hashmap
  Day 6 — Files & Errors
  Day 7 — Mini Project & Programmer Thinking
- Machine Coding:
  Day 1 — Bank account + Wallet :
  Day 2 : RateLimiter + seat booking
  Day 3 : Library Management + Classroom Attendance System
  Day 4 : Bank account and notification system.
  Day 5 : Key-value and payment processor.
  Day 6 : Discount engine + Seat booking( in depth)
  Day 7: Parking lot System :
- Machine coding extended:
  Day 7.5 — Parking Lot (Constraint-Heavy Variant)
  Day 8 : Order + Payment System
  Day 8 — Extension Round (Mid-Interview Change)
  Day 9 — Ticket Booking with Expiry (Simulated Time)
  Day 9 — Extension Round (Seat Lock Ownership + Forced Unlock)
- SQL + DB Thinking course
  Day 1 — Tables, Rows & Thinking in Data
  Day 2 — Relationships & Foreign Keys (Thinking in Connections)
  Day 3 — Filtering, Aggregation & Answering Data Questions
  Day 4 — JOINs (Combining Tables the Right Way)
  Day 5 — Subqueries, EXISTS & NOT EXISTS (Thinking in Layers)
  Day 6 — Indexes, Constraints & How Databases Think
  Day 7 — Full SQL Interview Simulation (SDE-1 Ready)
  Extended Day 1 — Core Business Systems (Deep Practice)
  Extended Day 2 — Booking & Platform Systems (High-Depth Practice)
  Extended Day 3 — Content & Learning Platforms (Capstone Practice)
- LLD: Java
  DAY 1 — Core Foundations (SDE-2 Level)
  DAY 2 — State, Time & Domain Correctness
  DAY 3 — Extensibility & Failure Handling (SDE-2 Core Signal)
  DAY 4 — Concurrency, Scheduling & Consistency (SDE-2 Critical)
  DAY 5 — Workflow Design & Config-Driven Systems (SDE-2+)
  DAY 6 — Eventing & Usage-Based Systems (Advanced SDE-2)
  DAY 7 — Interview Simulation (Hard, Production-Grade)
  Hidden Edge Cases Interviewers Expect (Day-wise)
  Detailed Testing file:
- Assignment:
  If else Practice Set

Zeni Study

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Day 2: Loops

Explore

Day 2: Loops

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🟢 Day 2 — Repeating Work (Loops & Logical Flow)

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1️⃣ Why Do We Need Loops?

Imagine this task:

Print numbers from 1 to 100.

Option 1 (Not Possible)

print(1)

print(2)

print(3)

...

print(100)

❌ This is not programming. This is typing.

Real-World Example

Washing 10 plates

Attendance for 50 students

Sending reminders to 100 users

💡 Whenever same work repeats, we need a loop.

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2️⃣ How Repetition Works in Real Life

Think like this:

“Start from 1 Keep increasing by 1 Stop when you reach 10”

That is exactly how a loop works.

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3️⃣ for Loop — When You Know How Many Times

Basic Syntax

for i in range(1, 6):

print(i)

How Computer Thinks (VERY IMPORTANT)

Line-by-line:

range(1, 6) generates numbers → 1, 2, 3, 4, 5

First time: i = 1

Print 1

Go back to loop

i = 2

Print 2 …and so on

Stops automatically when numbers finish.

🧠 The loop controls i, not you.

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4️⃣ Understanding range() (Critical Concept)

range(start, end)

start → included

end → NOT included

range(1, 5) → 1, 2, 3, 4

Common Beginner Confusion

“Why doesn’t it include 5?”

Answer:

Because computer counts how many steps, not numbers.

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5️⃣ Using Loop for Calculation

Example: Sum of Numbers

total = 0

for i in range(1, 6):

total = total + i

print(total)

How to Think

total remembers result so far

Each loop adds something

This is called accumulation

🧠 Very important thinking pattern: “Start with zero → keep adding”

⁠

6️⃣ while Loop — When You Don’t Know How Many Times

Real-World Example

Keep asking password until correct

Keep reading input until user types ‘exit’

Example

i = 1

while i <= 5:

print(i)

i = i + 1

How Computer Thinks

Check condition

If True → run body

After body → check again

Stop when condition becomes False

⁠

7️⃣ Infinite Loop (Danger Zone 🚨)

i = 1

while i <= 5:

print(i)

❌ This never stops.

Why?

i never changes

Condition always True

🧠 Every while loop must move toward stopping

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8️⃣ break — Force Stop

for i in range(1, 10):

if i == 5:

break

print(i)

Output:

break exits loop immediately.

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9️⃣ continue — Skip One Step

for i in range(1, 6):

if i == 3:

continue

print(i)

Output:

continue skips current iteration.

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🔁 Key Thinking Pattern (Very Important)

Whenever you see a loop, ask:

Where does it start?

When does it stop?

What changes each time?

What is remembered across iterations?

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⚠️ Common Beginner Mistakes

Off-by-one errors (range)

Forgetting to update loop variable

Writing logic outside loop

Confusing for and while

Expecting loop to “understand intent”

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🧪 Practice Questions (DO NOT RUSH)

Think → write steps → then code.

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Q1

Print numbers from 1 to 10.

⁠

Q2

Print all even numbers between 1 and 20.

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Q3

Take a number n and print sum of numbers from 1 to n.

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Q4

Print multiplication table of a given number.

Example:

5 x 1 = 5

5 x 2 = 10

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Q5

Count how many digits are there in a given number.

Example:

Input: 12345

Output: 5

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Q6

Reverse a number.

Example:

Input: 123

Output: 321

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Q7 (Thinking Question)

What happens if n = 0 or n is negative? Does your logic still work?

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🔑 HINTS (Read Together, Only If Stuck)

Use % 10 to extract last digit

Use // 10 to remove last digit

Initialize counters properly

For sum problems → start with 0

For reverse → start with 0

Check loop condition carefully

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✅ What You Should Be Able to Do Now

After Day 2, you should:

Repeat tasks using loops

Predict how many times loop runs

Avoid infinite loops

Use loop to compute results

Think step-by-step like computer

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🧠 Reflection Questions (Answer in Words)

What controls when a loop stops?

Difference between for and while?

Why do we need to update variables inside loops?

What causes infinite loops?

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⭐ Golden Rule from Day 2

If you don’t control repetition, repetition will control your program.

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