CS50 Lecture 1 — C

From Scratch to C

Last week you saw programming concepts as colorful blocks in Scratch. This week, those same concepts are written as text in C — one of the most important programming languages ever created.

You won't use C in your daily work. So why learn it? Because C shows you exactly what happens inside the computer. Python and JavaScript hide the complexity; C exposes it. Understanding C makes you better at every other language.

Your First C Program

#include <stdio.h>

int main(void)
{
    printf("Hello, world\n");
    return 0;
}

Let's break this down:

• #include <stdio.h> — tells C to load the "standard input/output" library (so you can use printf)
• int main(void) — every C program starts running from a function called main
• printf("Hello, world\n") — prints text. The \n means "new line"
• return 0 — tells the system "I finished successfully" (0 = no error)
• Every statement ends with a semicolon ;
• Code blocks use curly braces {} (not indentation like Python)

Variables and Types

In Python, you just write x = 5. In C, you must declare the type first:

int age = 29;           // integer (whole number)
float price = 19.99;    // decimal number
char grade = 'A';       // single character
char name[] = "Lia";    // string (array of characters)

Why does C make you specify types? Because it needs to know exactly how much memory to reserve. An int takes 4 bytes, a float takes 4 bytes, a char takes 1 byte. This is the reality that Python hides from you.

Conditionals

Same concept as Scratch and Python, different syntax:

if (deal_value >= 100000)
{
    printf("Major deal\n");
}
else if (deal_value >= 50000)
{
    printf("Significant deal\n");
}
else
{
    printf("Standard deal\n");
}

Notice: conditions must be in parentheses (), and blocks use curly braces {}.

Loops

// while loop
int i = 0;
while (i < 5)
{
    printf("Count: %i\n", i);
    i++;    // i++ means i = i + 1
}

// for loop (most common in C)
for (int j = 0; j < 5; j++)
{
    printf("Step: %i\n", j);
}

The for loop packs three things into one line:

1. int j = 0 — initialize
2. j < 5 — condition (keep going while true)
3. j++ — update after each iteration

Compiling vs. Interpreting

Here's a crucial difference from Python:

Python is interpreted: You write code, run it directly. Python reads your code line by line and executes it. Quick to test, but slower to run.

C is compiled: Before running, a special program (compiler) translates your entire source code into machine code (binary) that the processor can execute directly. Slower to develop, but much faster to run.

Source code (.c) → [Compiler] → Machine code (binary) → [Processor] → Output

When you build a Next.js project and run npm run build, a similar compilation step happens — your TypeScript/JSX gets transformed into optimized JavaScript. Now you know why that step exists.

printf and Format Codes

C's printf uses special codes to insert values into strings:

int age = 29;
float gpa = 3.8;
char name[] = "Lia";

printf("Name: %s, Age: %i, GPA: %.1f\n", name, age, gpa);
// Output: Name: Lia, Age: 29, GPA: 3.8

• %s = string
• %i or %d = integer
• %f = float (%.1f = 1 decimal place)
• %c = character

Python's f-strings (f"Hello {name}") are much nicer. But understanding format codes helps you read logs, error messages, and documentation in many languages.

Key Takeaway

C is harder than Python. That's the point. It forces you to think about things Python handles automatically: memory, types, compilation. This deeper understanding makes you a better programmer in every language.

What to Do This Week

1. Watch CS50 Lecture 1 (link above)
2. You don't need to install C or write C code locally (CS50 provides an online environment). But watch the lecture carefully.
3. Focus on understanding the concepts: types, compilation, how C differs from Python.
4. Take the quiz below.