Constants in C

The quantities involved in programming are not all of variable types alone. Some of the quantities do not change their values as variables do. They are called constants.
A constant is an entity with fixed value that does not change. It can be stored at a location in the memory of the computer and can be referenced through that memory address. There are four basic types of constants in C, viz. integer constants, floating- point constants, character constants and string constants. Composite types may also have constants.
Integer and floating-point constants represent numbers. They are often referred to collectively as numeric-type constants.
C imposes the following rules while creating a numeric constant type data item:
Commas and blank spaces cannot be included within the constants
• The constant can be preceded by a minus(-) sign if desired.
• Value of a constant cannot exceed specified maximum and minimum bounds.
For each type of constant, these bounds will vary from one C-compiler to
another.
Constants are the fixed values that remain, unchanged during the execution of a program and are used in assignment statements Constants can also be stored in variables.
The declaration for a constant in C language takes the following form:
const datatype var_name = value;

Exapmle: const float pi = 22/7;

This declaration defines a constant named pi whose value remains 22/7 throughout the program in which it is defined.
C language facilitates five different types of constants.
1. Character
2. Integer
3. Real
4. String
5. Logical
Character Constants: A character constant consists of a single character, single digit, or a single special symbol enclosed within a pair of single inverted commas. The maximum length of a character constant is one character.
Ex.:'a' is a character constant
Ex.: 'd' is a character constant
Ex.: 'P' is a character constant
Ex.: '7' is a character constant
Ex.: '*' is a character constant
Integer Constants: An integer constant refers to a sequence of digits and has a numeric value. There are three types of integers in C language: decimal, octal, hexadecimal.
Decimal integers : 1,56, 7657,-34 etc.
Octal integers : 076, -076,05 etc. (preceded by zero, 0)
Hexadecimal integers : 0x56, -0x5D etc. (preceded by zero, Ox)
No commas or blanks are allowed in integer constants.
Real or floating point Constants: A number with a decimal point and an optional preceding sign represents a real constant.
Exapmle: 34.8, -655.33, .2, -.56, 7.
Note that 7 is an integer while 7. or 7.0 is real.
Another notation (called scientific notation) for real constants consists of three parts:
i) A sign (+ or 0) preceding the number portion (optional).
ii) A number portion
iii) An exponent portion following the number portion (optional). It
starts with E or e followed by an integer. Which may or may not
be preceded by a sign.
Example:

Valid Representations	Invalid Representations
+ . 72	12 (no decimal)
+ . 72	12 (no decimal)
+ 72.	7.6 E + 2.2(non integer exponent)
+ 7.6 E + 2	1.2 E9229892(very large exponent)
24.4e-5

String Constants: A string constant is a sequence of one or more characters enclosed with in a pair of double quotes (" "). If a single character is enclosed within a pair of double quotes, it will also be interpreted as a string constant and not a character constant.
Exapmle:
1. "Welcome To C Programming \ n"
2. "a"
Actually, a string is an array of characters terminated by a NULL character. Thus, "a" is a string consisting of two characters, viz. 'a' and NULL('\0')
Logical Constants:A logical constant can have either true value or false value. In C, a non zero value is treated as true while 0 is treated as false.

Variables in C

A variable is an entity whose value can change during program execution. A variable can be thought of as a symbolic representation of address of the memory space where values can be stored, accessed and changed. A specific location or address in the memory is allocated for each variable and the value of that variable is stored in that location.

Each variable has a name, data-type, size, and the value it stores. All the variables must have their type indicated so that the compiler can record all the necessary information about them, generate the appropriate code during translation and allocating required space in memory.

Every programming language has its own set of rules that must be observed while
writing the names of variables. If the rules are not followed, the compiler reports
compilation error. Rules for Constructing Variable Name in C language are listed
below:

a. Variable name may be a combination of alphabets, digits or underscores.
Sometimes, an additional constraint on the number of characters in the
name is imposed by compilers in which case its length should not exceed
8 characters.
b. First character must be an alphabet or an underscore (_).
c. No commas or blank spaces are allowed in a variable name.
d. Among the special symbols, only underscore can be used in a variable name.
e.g.: emp_age, item_4, etc.
e. No word, having a reserved meaning in C can be used for variable name.

Note that C language is a case-sensitive language which differentiates between lower case and upper case. Thus, CAT is different from Cat, cAT, CaT. Although any word complying with the rules cited above can be used as the variable name, it is advised tha you create variable names that have some meaning. Thus, you may chose sum as the variable name for storing sum of numbers rather than choosing X.

Type Declaration

C language is strongly typed language, meaning that all the variables must be declared before their use. Declaration does two things:
It tells the compiler what the variable name is.
It specifies what type of data the variable will hold.
In C language, a variable declaration has the form:

Here is one of the valid data types (e.g. int, float, char, etc.). List-of-variables is a comma-separated list of identifiers representing the program variables.
e.g.-
a. int i, j, k; //creates integer variables i,j and K
b. char ch; //creates a character type variable ch
Once variable has been declared in the above manner, the compiler creates a space in the memory and attaches the given name to this space This variable can now be used in the program.
A value is stored in a variable using assignment operation. Assignment is of the form:
Variable-name = ; Obviously, before assignment, the variable must be declared. e.g.: int i, j;
j = 5; i = 0 ;
C also allows assignment of a value to a variable at the time of declaration. It takes the following form:

e.g. : int I = 5;
This declaration not only creates an interior type variable I, but also stores a value 5 at the same time.
The process of assigning initial values to the variable is known as initialization. More than one variable can be initialized in one statement using multiple assignment operators.
e.g.: i. int i = 5, ]=3;
ii . int j, m; j = m = 2; However, there is an exception worth noting. Consider the following example:
int i, j = 2, k ; The assignments will be made as follows: i = 0 j = 2
k =1063 (a garbage value, unutilized) Let us consider some of programming examples to illustrate the matter further.
Example 2
/* Example of assignments */ /'* declaration */
int al, bl; /* declaration & assignment */
int var = 5;
int a, b 1 6, c;
/* declaration & multiple" assignment */
int p, q, r, s;
p = q=r = s = 5;

values stored in various variables are:
var = 5
a = 0, b = 6
c = garbage value
p = 5, q = 5, r = 5, s = 5