C basics: main, printf, and functions

C is a compiled language

• C is one of the oldest and most important languages. Linux, databases, and even Python are built with C.
• Unlike Python, C is compiled: the computer first turns your code into a program, then runs it.
• C is strict: declarations and expression statements end with a semicolon ;, and every variable has a type. A block such as if (...) { ... } does not need an extra semicolon.

The C skeleton

• Every C program starts in a function called main. Your code goes inside main.
• #include <stdio.h> gives you printf, which prints text.
• return 0; at the end of main means "the program finished without an error".

#include <stdio.h>

int main(void) {
    printf("C runs from inside main.\n");
    int year = 11;
    printf("Grade %d\n", year);
    return 0;
}

printf and types

• printf prints text. The \n inside the text means "start a new line".
• To print a value, use a placeholder: %d for an int, %f for a double, %c for one character.
• printf("Grade %d\n", year); puts the value of year where the %d is.
• Every variable needs a type first: int year = 11; or double pi = 3.14;.

Common mistakes

• End declarations and expression statements with ;. Blocks use braces; do not add a stray ; after an if condition. main returns int (return 0;).
• printf needs the right format (%d, %f) and \n for a new line.

Now you try

• Each task pre-fills the code you need. Write inside main, or complete the function shown.
• Some tasks ask for a function only — do not write your own main; the checker adds one.
• For a complete program, Run compiles and runs it. For a function task, Check answer compiles your code with the supplied tests and shows the results. Your code is compiled and run on the server.