Unit 00 — Before We Write Code

Before we write a single line of code, we need to answer four honest questions. What is DSA? Why do companies test it? How does a computer actually store your data? And why are we using C instead of Python or JavaScript?

This unit is for everyone — especially those who have never written code before, or who come from a non-IT background. If you already know some programming, you will still find the memory and computer internals section useful. Read it anyway.

What is Data Structures & Algorithms?

Let's start with the two words separately — because most explanations combine them too early and create confusion.

Why Does Every Tech Company Test DSA?

If you have ever looked at a software job description, you have seen this: "Data Structures and Algorithms round." You may have wondered — why? I just want to build websites. Why do I need to know how to reverse a linked list?

Here is the honest answer.

How Your Computer Actually Stores Data

This section is something most tutorials completely skip. They just say "create a variable" and move on. We are not going to do that — because if you do not understand what happens in memory, a huge part of DSA will always feel like magic tricks instead of logic.

Think of RAM as a street full of houses

Your computer's RAM (Random Access Memory) is where all active data lives while a program is running. Think of it as a very long street with millions of tiny houses. Each house has a unique address and can hold exactly one small piece of information.

When you write int age = 25 in a program, your computer does three things:

Why Are We Using C?

You might be wondering — everyone online uses Python or Java for DSA. Why are we doing this in C? Good question. Here is the full honest answer.

Your First C Program — Line by Line

Let us write actual code. This is the most classic program in all of programming — printing "Hello, World!" on the screen. We are going to explain every single line so nothing is a mystery.

#include <stdio.h>
 
int main() {
    printf("Hello, World!\n");
    return 0;
}

Now let us work with variables

A variable is a named box in memory that holds a value. In C, you must always say what type of value the box will hold. This is called a data type.

#include <stdio.h>
 
int main() {
    int age = 20;           /* whole number */
    float height = 5.9;     /* decimal number */
    char grade = 'A';       /* single character — use single quotes */
 
    printf("Age: %d\n", age);       /* %d for integers */
    printf("Height: %f\n", height); /* %f for floats */
    printf("Grade: %c\n", grade);   /* %c for characters */
 
    return 0;
}

The %d, %f, and %c are format specifiers — placeholders that tell printf what type of value to print. The actual variable comes after the comma.

Taking input from the user

#include <stdio.h>
 
int main() {
    int number;
 
    printf("Enter a number: ");
    scanf("%d", &number);   /* read an integer from the user */
 
    printf("You entered: %d\n", number);
 
    return 0;
}

Notice the &number — that ampersand (&) means "the memory address of number." scanf needs to know where to store the value you type, so you give it the address.

Decisions and loops — the last basics you need

#include <stdio.h>
 
int main() {
    int marks;
 
    printf("Enter your marks: ");
    scanf("%d", &marks);
 
    /* Decision — if/else */
    if (marks >= 60) {
        printf("Result: Pass\n");
    } else {
        printf("Result: Fail\n");
    }
 
    /* Repetition — for loop: print 1 to 5 */
    int i;
    for (i = 1; i <= 5; i++) {  /* i++ means add 1 to i each time */
        printf("%d ", i);
    }
    printf("\n");
 
    return 0;
}

You Are Ready for Unit 01

You now know what DSA is, why it matters, how memory works, why we use C, and the basics of writing C programs. That is the complete foundation.

In Unit 01, we tackle Complexity — how to measure the speed and memory usage of any piece of code. This is the skill that separates engineers who write code from engineers who write good code.