Summary

In this lesson, you learned that:

• A function is a named piece of code that you write once and can run as many times as you need. When you change it, the change applies to every call.
• A function definition has a return type, a name, parameters in parentheses and a body in curly braces: int add(int a, int b) { ... }. void as the return type means that the function returns nothing, and (void) in the parentheses means that it takes nothing. A definition alone doesn't run anything.
• You call a function with its name, the arguments in parentheses and a semicolon: print_line(5);. The program starts in main, and other functions are called from there. printf, strlen and toupper are functions too.
• The compiler must know a function before it is called: define the function above main, or write its prototype above main, such as int add(int a, int b);. Otherwise, you get the error call to undeclared function. Header files such as stdio.h hold the prototypes of library functions.
• A parameter is a variable in the definition, an argument is a value in the call. The arguments go into the parameters in order, and their number and types must fit.
• return gives a value back to the place of the call, so you can use the call like a value: int total = add(2, 3) * 2;. It also ends the function right away. A function that isn't void must return a value in every case, otherwise the program doesn't compile here. In a void function, return; ends it early.
• printf only shows a value; the program can't work with it any further. A function that calculates something should return the result and leave printing to the code that called it.
• The variables of a function and its parameters are local: they exist only in that function, and that applies to main too. A parameter gets a copy of the argument, so changing it doesn't change the caller's variable. Values go into a function through its parameters and come out with return.
• An array parameter is written int numbers[], and the function also needs the number of elements as another parameter. The function works on the caller's array itself, so its changes stay. A string that the function only reads is a const char text[], and the function finds its end by the '\0'.
• The library has ready-made functions for math in math.h: sqrt, pow, round and fabs, which return a double. abs for whole numbers is in stdlib.h.

In the next lesson, on pointers, you will find out what addresses in memory are, how a function can change a variable of the code that called it, and why arrays are not copied.