Controlling complexity is the essence of computer programming.

Brian W. Kernighan. in Software Tools (with PJ Plauger), 1976.

THE FUNCTIONS

A C program is a collection of functions. A C function is equivalent to the subroutine in FORTRAN or BASIC and procedures in PASCAL, PERL, or PYTHON programming languages. A portion of the program cannot be executed independently but only as part of another program. The function contains a specific algorithm or a stand-alone procedure. You have already used several library functions in your previous programs. Output commands for printing or reading files (such as printf(), openf()), mathematical functions (sqrt(), cos() are a library or intrinsic functions as well. Other library functions, we can classify as follows

• Input/output functions. Input/output on the computer devices (e.g. output to the terminal, printer, hard disk, input from keyboard). It is usually used with #include <stdio.h>;
• String manipulation functions. This library contains common operations on strings (e.g., concatenation, length, search, and extraction of substrings). It is usually used with #include <string.h>;
• Mathematical functions. Mathematical calculations (e.g. trigonometrics functions, exponentiation, square root extraction). It is usually used with #include <math.h>;
• Graphical functions. Function for graphics operations (open a graphical window and canvas) and drawing graphical primitives (e.g. points, line, curves).
• Operative system control functions. Operation requiriing allocation of the computer resources or devices (e.g. date and time, allocation of memory). It is usually used with, for example, #include <time.h>;
• Data conversion functions. Operation for data conversion (e.g. change characters type, ascii to integer). It is usually used with #include <ctype.h>;

To use these function, you need to use the precompiler instruction #include at the beginning of the program. The compiler use by deafult the standard library #include <stdlib.h>;

You can write your functions, and it is called user-defined functions. The use of function allows the program to structure and makes its organization and reading easier. C language is structured around the use of functions. The main() function is a function that contains calls to other functions, both intrinsic and user-defined functions.

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