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Day 2 — Variables, Data Types & Writing Your First Program

DSA Partner Challenge | Week 1: Language Basics + Maths for DSA

📌 Topic of the Day

##Notes: https://drive.google.com/file/d/1GWlAVWke6TmOTyRqjnvbPNQzvtUP6TUa/view?usp=sharing

Variables, Data Types, Input/Output & Debugging

Today you write actual code for the first time in this challenge. By the end of the day you should be able to take input, process it, and print output — the skeleton of every single DSA solution you'll ever write.

🎥 Resources

Language Resource
☕ Java Kunal Kushwaha — Java Architecture, I/O & Debugging
🐍 Python CampusX — 100 Days of Python (Variables, Data Types & I/O section)
⚙️ C++ Love Babbar / Striver — Variables, Data Types & I/O

🧠 What to Learn Today

Variables

  • A named container in memory that holds a value
  • Every variable has a name, a type, and a value
  • Declaration → int age; | Initialization → age = 21; | Both → int age = 21;
  • Java convention: camelCase → myAge not my_age

Primitive Data Types (Java)

Type Size Example Use
int 4 bytes int x = 10; Whole numbers — most common in DSA
long 8 bytes long x = 100000L; Large whole numbers
double 8 bytes double pi = 3.14; Decimals (more precise)
float 4 bytes float x = 3.14f; Decimals (less precise)
char 2 bytes char c = 'A'; Single character (single quotes)
boolean 1 bit boolean b = true; true or false only
byte 1 byte byte b = 127; Small integers
short 2 bytes short s = 300; Integers -32768 to 32767

String is not a primitive — it's an object. String name = "Tripti"; (capital S, double quotes)

Type Casting

  • Widening (automatic): int → long → double — no data loss
  • Narrowing (manual): must cast explicitly — int x = (int) 9.99; → gives 9 (decimal lost)

Output (Java)

System.out.println("Hello");           // prints + newline
System.out.print("Hello");             // prints, no newline
System.out.printf("Name: %s, Age: %d%n", name, age);  // formatted

Format specifiers: %d (int), %f (double), %s (String), %c (char), %n (newline)

Input using Scanner (Java)

import java.util.Scanner;

Scanner sc = new Scanner(System.in);
int num      = sc.nextInt();         // reads integer
double d     = sc.nextDouble();      // reads decimal
String word  = sc.next();            // reads one word
String line  = sc.nextLine();        // reads full line
char c       = sc.next().charAt(0);  // reads character
sc.close();

⚠️ After nextInt(), calling nextLine() reads the leftover newline. Fix: add an extra sc.nextLine() to flush it.

Python I/O

name = input("Enter name: ")       # always returns string
age  = int(input("Enter age: "))   # cast to int
print("Hello", name)
print(f"Hello {name}, you are {age} years old")  # f-string

C++ I/O

#include <iostream>
using namespace std;

int x;
cin >> x;
cout << "Value: " << x << endl;

Debugging Basics

  • Syntax Error → wrong grammar, caught at compile time (missing ;, wrong spelling)
  • Runtime Error → valid code but crashes during execution (divide by zero, null pointer)
  • Logic Error → runs fine but gives wrong output — hardest to find
  • Debug by: reading the full error message, adding print statements to trace values, using breakpoints in IDE
  • Check first: wrong variable name? Off by one? = instead of ==?

💻 Practice Questions

Solve in your language. Push solutions to your fork of the repo.

1. Write a program to print whether a number is even or odd — take input from the user.

2. Take a name as input and print a greeting message for that name.

Input: Tripti
Output: Hello Tripti, welcome to the DSA Challenge!

3. Input Principal (P), Time (T), Rate (R) from the user and calculate Simple Interest.

SI = (P * T * R) / 100

4. Take two numbers and an *operator (+, -, , /) as input and calculate the result using if conditions.

Input: 10 + 5
Output: 15

5. Take 2 numbers as input and print the largest.

6. Input an amount in Indian Rupees and convert it to USD. (Use: 1 USD = 83.5 INR — update with current rate)

7. Print the Fibonacci Series up to n numbers.

Input: 7
Output: 0 1 1 2 3 5 8

8. Check whether a given String is a Palindrome or not.

Input: "racecar" → Palindrome
Input: "hello"   → Not a Palindrome

9. Find all Armstrong Numbers between two given numbers.

An Armstrong number = sum of each digit raised to the power of number of digits e.g. 153 = 1³ + 5³ + 3³ = 153 ✅

✅ Checklist Before You Sleep

  • Watched the resource video for my language
  • I know all 8 primitive types in Java (or equivalents in my language)
  • I can take input and print output without looking it up
  • I understand the difference between print and println
  • I know what widening and narrowing type casting mean
  • I can explain the 3 types of errors
  • Solved at least questions 1–5
  • Attempted questions 6–9

💬 Community

Stuck on any question? Drop the question number in the community — someone will help.

Solved all 9? Drop a 🔥 in the community.

Next up → Day 3: Conditional Statements (if, else if, else, switch, ternary)