C is a general-purpose programming language. It has been closely associated with the UNIX system where it was developed, since both the system and most of the programs that run on it are written in C. The language, however, is not tied to any one operating system or machine; and although it has been called "system programming language" because it is useful for writing compilers and operating systems, it has been used equally well to write major programs in many different domains.

With C, our aim is tho show the essential elements of the language in real programs, but without getting bogged down in details, rules, and exceptions. We want to get you as quickly as possible to the point where you can write useful programs, and to do that we have to concentrate on the basics; variables and constants, data types, declarations, operators, statements, and loops.

The only way to learn a new programming language is by writing programs in it. The first program to write is the same for all languages:

Printing "hello world"

This is the big hurdle; to leap over it you have to be able to create the program text somewhere, compile it successfully, load it, run it, and find out where your output went. In C, the program to print "hello, world" is:

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#include <stdio.h>

                        int main() {
                            printf("Hello World\n");
                        }
                    

Just how to run this program depends on the sysyem you are using. As a specific example, on the UNIX operating system you must create the program in a file whose name ends in ".c" such as hello.c, then compile it with the command

cc hello.c

a.out

In C, all variables must be declared before they are used, usually at the beginning of the function before any executable statements. A declaration announces, the properties of variables; it consists of a type name and a list of variables such as

int fahr, celcius;
                            int lower, upper, step;

Variable names are made up of letters and digits; the first character must be a letter. The underscore "_" counts as a letter; it is sometimes useful for improving the readability of long variable names. It's wise to choose variable names that are related to the purpose of the variable, and that are unlikely to get mixed up typographically. We tend to use short names for local variables, especially loop indices, and longer names for external variables.

C provides several basic data types such as:

All variables must be declared before use, although certain declarations can be made implicitly by context. A declaration specifies a type, and contain a list of one or more variables of that type, as in

int lower, upper, step;
                            char c, line[1000];
                        

Variables can be distributed among declarations in any fashion; the list above could equally well be written as

int lower;
                            int upper;
                            int step;
                            char c;
                            char line[1000];
                        

This latter form takes more space, but is convenient for adding a comment to each declaration or for subsequent modifications.

A Variable may also be initialized in its declaration. If the naem is followed by an equals sign and an expression, the expression serves as an initializer, as in

char esc = '\\';
                            int i = 0;
                            int limit = MAXLINe + 1;
                            float eps = 1.0e-5;
                        

The binary arithmetic operators are +, -, *, /, and the modulus operator %. Integer division truncates any fractional part. The expression

x % y

produces the remainder when x is divided by y, and thus is zero when y divides x exactly. For example, a year is a leap year if it is divisible by 4 but not by 100, except that years divisible by 400 are leap years. Therefore

if((year % 4 == 0 && year % 100 != 0) || year & 400 == 0)
                            printf("%d is a leap year\n", year);
                        else
                            printf("%d is not a leap year\n", year);
                    

The % operator cannot be applied to float or double. The direction of truncation for / and the sign of the result for % are machine-dependent for negative operands, as is the action taken on overflow or underflow. The binary + and - operators have the same precedence, which is lower than the precedence of *, /, and %, which is in turn lower than unary + and -.

The relational operators are

>	>=	<	=

==	!=

The if-else statement is used to express decisions. Formally, the syntax is

if (expression)
                           statement1
                        else
                           statement2
                    

where the else part is optional. The expression is evaluated; if it is true (that is, if expression has a non-zero value), statement₁ is executed. If it is false (expression is zero) and if there is an else part, statement₂ is executed instead.

Take a look at this example.

while (expression)
                       statement

In the code, the expression is evaluated. If it is non-zero, statement is executed and expression is re-evaluated. This cycle continues until expression becomes zero, at which point execution resumes after statement.

This documentation is based on the book titled 'The C Programming Language' by the creators of C programming language themself: Brian W. Kernighan and Dennis M. Ritchie.