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 3 : Array + String

Explore

Day 3 : Array + String

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🟢 Day 3 — Working With Many Values (Lists & Strings)

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1️⃣ Why Do We Need This?

Until now, we worked like this:

a = 10

b = 20

c = 30

Real-life problem

What if:

You have marks of 50 students

You have prices of 100 products

You have names of 1000 users

Creating separate variables is:

Impossible to manage

Error-prone

Not scalable

💡 When you have many similar values, you use a collection.

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2️⃣ Introducing Lists — One Variable, Many Values

Real-world analogy

Think of a train:

One train (list)

Many compartments (elements)

Each compartment has a number (index)

⁠

Creating a List

numbers = [10, 20, 30, 40]

Here:

numbers → list

10, 20, 30, 40 → elements

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3️⃣ Indexing — How to Access Elements

print(numbers[0]) # 10

print(numbers[1]) # 20

Important Rule (VERY IMPORTANT)

Python starts counting from 0, not 1.

Index: 0 1 2 3

Value: 10 20 30 40

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Why Index Starts From 0?

You don’t need to memorize why, just accept:

Index = how far from the start

First element is 0 steps away.

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4️⃣ What Happens If Index Does Not Exist?

print(numbers[10])

❌ Program crashes.

Why?

Python does not guess

It only accesses what exists

🧠 The computer never assumes. It either finds something or fails.

⁠

5️⃣ Looping Over a List (MOST USED PATTERN)

Instead of accessing elements one by one:

print(numbers[0])

print(numbers[1])

Use a loop:

for num in numbers:

print(num)

How Computer Thinks

Take first element → num

Print it

Take next element

Stop when list ends

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6️⃣ Lists + Logic = Real Programs

Example: Find Largest Number

numbers = [10, 45, 23, 89, 5]

largest = numbers[0]

for num in numbers:

if num > largest:

largest = num

print(largest)

How to Think (THIS IS IMPORTANT)

Assume first number is largest

Compare every number

Update when you find bigger

🧠 This thinking pattern is used everywhere in programming.

⁠

7️⃣ Strings Are Also Collections 🤯

You already know strings:

name = "python"

But a string is actually:

'p' 'y' 't' 'h' 'o' 'n'

Looping Over String

for ch in name:

print(ch)

💡 Strings behave like lists of characters.

⁠

8️⃣ Length of List or String

print(len(numbers))

print(len(name))

len() tells how many items

Works for list and string

⁠

9️⃣ Counting Pattern (Extremely Important)

Example: Count Even Numbers

count = 0

for num in numbers:

if num % 2 == 0:

count += 1

print(count)

Thinking Pattern

Start counter at 0

Increase when condition matches

Print at end

🧠 Counting is one of the most used patterns in programming.

⁠

🔁 Key Thinking Patterns Learned Today

Whenever you see a collection, ask:

Do I need to check every element?

Do I need to remember something while looping?

Do I need to compare values?

Do I need to count or accumulate?

⁠

⚠️ Common Beginner Mistakes

Using wrong index

Modifying list incorrectly

Forgetting loop

Confusing index and value

Assuming list has unlimited size

⁠

🧪 Practice Questions (Very Important)

Do not rush. Think → write steps → then code.

⁠

Q1

Create a list of 5 numbers and print all elements.

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Q2

Find the smallest number in a list.

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Q3

Count how many even numbers are in a list.

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Q4

Check if a given number exists in a list.

Example:

List: [1, 2, 3, 4]

Input: 3

Output: Found

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Q5

Reverse a list without using built-in reverse.

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Q6

Count number of vowels in a string.

Example:

Input: "programming"

Output: 3

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Q7 (Thinking Question)

What happens if the list is empty? How should your program behave?

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🔑 HINTS (Read Together, Only If Stuck)

Assume first element for min/max

Use % 2 for even check

Use a flag variable (found = False)

For reverse: create a new list or loop backwards

Vowels: a, e, i, o, u

Always think about empty input

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✅ What You Should Be Able to Do Now

After Day 3, you should:

Store multiple values in a list

Loop over collections

Apply logic to each element

Process strings as character lists

Build real data-processing logic

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🧠 Reflection Questions (Answer in Words)

Why do we use lists instead of many variables?

What is difference between index and value?

Why do we assume first element for max/min?

Why does string behave like list?

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⭐ Golden Rule from Day 3

Real programs are not about single values. They are about collections of values.

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If you say “Continue with Day 4”, next we’ll cover:

Functions

Reuse & clean thinking

Breaking problems into pieces

First step toward real software design

One day at a time.

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