Step 1 of 5

Your first program

A program is a list of instructions the computer follows from top to bottom. C is a compiled language: you write the instructions as text (the source code), a tool called the compiler checks them and translates them into machine code the processor can run, and then the program runs. Every time you press Check, a real C compiler does exactly that in your browser.

The shape of every C program

#include <stdio.h>

int main(void) {
    printf("Hi!\n");
    return 0;
}

Hi!

Line by line:

#include <stdio.h> pulls in the standard input/output library. It tells the compiler what printf is. Without it the compiler doesn't know the name and reports an error.
int main(void) starts a function called main. Every C program begins running at main, no matter how big it is. int says it gives back a whole number when it finishes, and (void) says it takes no inputs.
The curly braces { } hold the function's body: the instructions that run, in order.
printf("Hi!\n"); calls the printf function to print text. The text goes inside double quotes. \n is a newline: it ends the line, like pressing Enter.
return 0; ends main and reports "success" (0) to the operating system. A non-zero number would mean something went wrong.

Common mistakes

Spelling and capitals matter. Printf or print are different names, and the compiler will say something like call to undeclared function 'print'.
Forgetting #include <stdio.h> gives a similar error about printf.
Forgetting the ; at the end of a statement. You'll practice reading that error in step 5.

Your task

Fill in the blank so the program prints Hello, World!.

Challenge 2

Fill in both blanks: the function that prints text, and the keyword that ends main and hands back 0. The program should print GG, well played!.

Challenge 3

This time there is no starter program. Write the whole thing yourself: the #include, main, a printf and return 0;. It should print Welcome to the arena.

Next: Printing your own text