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Day 1 : Variable + if/else


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🟢 Day 1 — Thinking Like a Computer (Foundations)

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1️⃣ Why Are We Learning This?

Imagine you are talking to a very obedient but very dumb assistant.
It never guesses
It never assumes
It never understands meaning
It only follows exact instructions, step by step
That assistant is a computer.
💡 Programming = learning how to give clear, exact, unambiguous instructions
Python is just the language we use to talk to it.
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2️⃣ How a Computer Thinks (Very Important)

A computer:
Reads code top to bottom
Executes one line at a time
Never skips lines
Never understands intent
Example (read carefully):
x = 10
x = 20
print(x)
👉 Output will be 20 Because the latest instruction wins.
🧠 A computer does not remember history. It only knows the current value.
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3️⃣ Variables — Storing Information

Real-world analogy

Think of a labeled box.
Label = variable name
Value = what is inside the box
name → "Amit"
age → 22

Python example

name = "Amit"
age = 22

print(name)
print(age)

Important Rules

Variable names should describe the data
Python decides the type automatically
You don’t declare types like other languages
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4️⃣ Data Types (Only What We Need Today)→ i love my wife

Numbers

x = 10 # integer
price = 99.5 # float

Text (String)

city = "Delhi"

Boolean (True / False)

is_active = True
🧠 Think of types as different kinds of boxes
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5️⃣ Taking Input from User

So far, you are giving values.
Now let the user give values.
name = input("Enter your name: ")
print(name)

Important Truth (Critical for beginners)

❗ input() always gives text, even if user types a number.
age = input("Enter age: ")
print(type(age)) # this is string, not number

Converting to number

age = int(input("Enter age: "))
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6️⃣ Making Decisions — if / else

Real life example:
If traffic light is red → stop Else → go
Python works the same way.
age = int(input("Enter age: "))

if age >= 18:
print("Adult")
else:
print("Minor")

How computer evaluates this:

Check condition (age >= 18)
If True → run first block
If False → run else block
🧠 Computer does not know what “adult” means. It only checks true or false.
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7️⃣ Indentation — Python’s Superpower

Python uses spaces, not brackets.
❌ Wrong
if age >= 18:
print("Adult")
✅ Correct
if age >= 18:
print("Adult")
🧠 Indentation tells Python which code belongs together
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8️⃣ Common Beginner Mistakes (Read Carefully)

Forgetting to convert input to number
Using = instead of ==
Wrong indentation
Assuming computer “understands intention”
Example mistake:
if age = 18: # ❌ wrong
Correct:
if age == 18:
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9️⃣ Practice Questions (Do These Seriously)

Do not jump to code immediately. Think first. Write steps on paper.

Q1

Print "Hello, Python" on the screen.
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Q2

Store your name and age in variables and print them.
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Q3

Take a number from user and print whether it is positive or negative.
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Q4

Take a number and print whether it is even or odd.
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Q5

Take two numbers and print the larger one.
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Q6

Take age as input and print:
"Child" if age < 13
"Teen" if age between 13–19
"Adult" otherwise
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Q7 (Thinking Question)

What happens if the user enters "abc" instead of a number? Why does the program crash?
(No fixing yet. Just observe.)
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🔑 HINTS (Read Only If Stuck)

Use % operator to check even/odd
if, elif, else are mutually exclusive
Convert input using int()
Comparison operators: >, <, >=, <=, ==
Think in true / false, not English meaning
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🔟 What You Should Be Able to Do Now

By end of Day 1, you should:
Understand how computer executes code
Store and update values
Take input from user
Make decisions using conditions
Predict output before running code
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🧠 Reflection (VERY IMPORTANT)

Answer in words, not code:
Why does computer need exact instructions?
Why is input always a string?
What happens if indentation is wrong?
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