Lesson 1 — Hello World & Your First Programs

📖

What Is the Hello World Program?

A Hello World program is the traditional first step for learning C. It's a sanity check to confirm your environment is set up correctly, and a simple but complete introduction to syntax, header files, functions, and the compilation process — all in five lines.

hello.c

C

#include <stdio.h>

int main() {
    printf("Hello, World!\n");
    return 0;
}

1

Step-by-Step Breakdown

1

#include <stdio.h> — The Preprocessor Directive

Tells the preprocessor to include the Standard Input/Output header before compilation. It brings in the tools needed for input and output. Without it, the compiler wouldn't recognize printf() at all.

2

int main() — The Entry Point

Every C program must have a main() function — execution always begins here. int means the function returns a whole number to the operating system on completion; () marks it as a function. C is case-sensitive — Main or MAIN will not compile.

3

{ and } — Program Block Identifiers

Curly braces mark the start and end of a logical block. All code belonging to main() must sit inside these braces.

4

printf("Hello, World!\n") — The Output Function

printf() ("print formatted") displays text on screen. The text in double quotes is a string literal; \n is an escape sequence that moves the cursor to a new line. The semicolon ; tells the compiler the statement is complete — every statement needs one.

5

return 0 — Graceful Exit

Tells the operating system the program finished successfully. 0 means "no errors"; any non-zero value signals that something went wrong.

💡 Behind the scenes — execution flow:

1. Preprocess#include is replaced with stdio.h's real content

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2. CompileC code becomes machine instructions

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3. Linkconnects to the C standard library so printf() works

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4. ExecuteOS calls main(), runs line by line to return 0;

2

Multiply Two Integers

Variables + scanf()

Building on Hello World, this program introduces variables and user input. It uses int for whole numbers and the asterisk * as the multiplication operator.

multiply_int.c

C

#include <stdio.h>

int main() {
    int num1 = 0;
    int num2 = 0;
    int product = 0;

    printf("Enter first integer: ");
    scanf("%d", &num1);
    printf("Enter second integer: ");
    scanf("%d", &num2);

    product = num1 * num2;   // * is the multiplication operator

    printf("The product is: %d\n", product);
    return 0;
}

3

Multiply Two Decimal Numbers

Same logic, float type

The second version swaps int for float to handle decimals — everything else follows the same structure.

multiply_float.c

C

#include <stdio.h>

int main() {
    float num1 = 0.0;
    float num2 = 0.0;
    float product = 0.0;

    printf("Enter first decimal number: ");
    scanf("%f", &num1);
    printf("Enter second decimal number: ");
    scanf("%f", &num2);

    product = num1 * num2;   // Same * operator, but with decimals

    printf("The product is: %.2f\n", product);   // %.2f = 2 decimal places
    return 0;
}

terminal — sample output

output

Enter first integer: 6
Enter second integer: 7
The product is: 42

Enter first decimal number: 3.5
Enter second decimal number: 2.0
The product is: 7.00

Feature	Integer Version	Float Version
Data type	int (whole numbers)	float (decimals)
Format specifier	`%d`	`%f` / `%.2f`
Initial value	`0`	`0.0`
Operator	`*`	`*`
Example input	4 and 5	4.5 and 2.3
Example output	20	10.35

⚠️ Common mistake — forgetting the & in scanf. Always write scanf("%d", &num1), never scanf("%d", num1). The & (address operator) tells scanf exactly where in memory to store the value. Forgetting it can crash the program or produce garbage output.

Q

Quick Quiz

Question 1 of 4

What does #include <stdio.h> do?

Question 2 of 4

Why must the function be named exactly "main" in lowercase?

Question 3 of 4

What happens if you forget the & in scanf("%d", &num1)?

Question 4 of 4

What does %.2f mean in a printf format string?

✓

Lesson Checklist

I can explain what #include <stdio.h> does
I understand why every program needs int main()
I know what { } and ; are for
I can explain what return 0; means
I can name the 4 stages: preprocess, compile, link, execute
I can write a program that reads two integers and multiplies them
I can do the same with floats and format the output with %.2f
I know why & is required in scanf
I completed the quiz

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Next: Lesson 2 — Data Types & Operators

Coming up

🔢 int, float, char, double explained Lesson 2
➕ Arithmetic & assignment operators Lesson 2
🔍 Type conversion basics Lesson 2