C Programming Variables and Constants

Variables

Variables are memory location in computer's memory to store data. To indicate the memory location, each variable should be given a unique name called identifier. Variable names are just the symbolic representation of a memory location. Examples of variable name: sum, car_no, count etc.

int num;

Here, num is a variable of integer type.

Rules for writing variable name in C

1. Variable name can be composed of letters (both uppercase and lowercase letters), digits and underscore '_' only.
2. The first letter of a variable should be either a letter or an underscore. But, it is discouraged to start variable name with an underscore though it is legal. It is because, variable name that starts with underscore can conflict with system names and compiler may complain.
3. There is no rule for the length of length of a variable. However, the first 31 characters of  a variable are discriminated by the compiler. So, the first 31 letters of two variables in a program should be different.

In C programming, you have to declare variable before using it in the program.

Types of Variables

The Programming language C has two main variable types

• Local Variables
• Global Variables

Local Variables

• Local variables scope is confined within the block or function where it is defined. Local variables must always be defined at the top of a block.
• When a local variable is defined - it is not initalised by the system, you must initalise it yourself.
• When execution of the block starts the variable is available, and when the block ends the variable 'dies'.

Check following example's output

main() { int i=4; int j=10; i++; if (j > 0) { /* i defined in 'main' can be seen */ printf("i is %d\n",i); } if (j > 0) { /* 'i' is defined and so local to this block */ int i=100; printf("i is %d\n",i); }/* 'i' (value 100) dies here */ printf("i is %d\n",i); /* 'i' (value 5) is now visable.*/ } This will generate following output i is 5 i is 100 i is 5

Here ++ is called incremental operator and it increase the value of any integer variable by 1. Thus i++ is equivalent to i = i + 1;

You will see -- operator also which is called decremental operator and it idecrease the value of any integer variable by 1. Thus i-- is equivalent to i = i - 1;

Global Variables

Global variable is defined at the top of the program file and it can be visible and modified by any function that may reference it.

Global variables are initalised automatically by the system when you define them!

Data Type	Initialser
int	0
char	'\0'
float	0
pointer	NULL

If same variable name is being used for global and local variable then local variable takes preference in its scope. But it is not a good practice to use global variables and local variables with the same name.

int i=4; /* Global definition */ main() { i++; /* Global variable */ func(); printf( "Value of i = %d -- main function\n", i ); } func() { int i=10; /* Local definition */ i++; /* Local variable */ printf( "Value of i = %d -- func() function\n", i ); } This will produce following result Value of i = 11 -- func() function Value of i = 5 -- main function

i in main function is global and will be incremented to 5. i in func is internal and will be incremented to 11. When control returns to main the internal variable will die and and any reference to i will be to the global.

Constants

Constants are the terms that can't be changed during the execution of a program. For example: 1, 2.5, "Programming is easy." etc. In C, constants can be classified as:

Integer constants

Integer constants are the numeric constants(constant associated with number) without any fractional part or exponential part. There are three types of integer constants in C language: decimal constant(base 10), octal constant(base 8) and hexadecimal constant(base 16) .

Decimal digits: 0 1 2 3 4 5 6 7 8 9

Octal digits: 0 1 2 3 4 5 6 7

Hexadecimal digits: 0 1 2 3 4 5 6 7 8 9 A B C D E F.

For example:

Decimal constants: 0, -9, 22 etc
Octal constants: 021, 077, 033 etc
Hexadecimal constants: 0x7f, 0x2a, 0x521 etc

Notes:

1. You can use small caps a, b, c, d, e, f instead of uppercase letters while writing a hexadecimal constant.
2. Every octal constant starts with 0 and hexadecimal constant starts with 0x in C programming.

Floating-point constants

Floating point constants are the numeric constants that has either fractional form or exponent form. For example:

-2.0
0.0000234
-0.22E-5

Note:Here, E-5 represents 10^-5. Thus, -0.22E-5 = -0.0000022.

Character constants

Character constants are the constant which use single quotation around characters. For example: 'a', 'l', 'm', 'F' etc.