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, count etc.
int num;
Here, num is a
variable of integer type.
Rules for
writing variable name in C:
Variable name
can be composed of letters (both uppercase and lowercase letters), digits and
underscore '_' only.
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.
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.
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:
You can use
small caps a, b, c, d, e, f instead of uppercase letters while writing a
hexadecimal constant.
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.
Escape
Sequences:
Sometimes, it
is necessary to use newline(enter), tab, quotation mark etc. in the program
which either cannot be typed or has special meaning in C programming. In such
cases, escape sequence are used. For example: \n is used for newline. The
backslash( \ ) causes "escape" from the normal way the characters are
interpreted by the compiler.
Escape
Sequences:
Escape
Sequences Character
\b Backspace
\f Form feed
\n Newline
\r Return
\t Horizontal tab
\v Vertical tab
\\ Backslash
\' Single quotation mark
\" Double quotation mark
\? Question mark
\0 Null character
\a bell(sound)
String
constants:
String
constants are the constants which are enclosed in a pair of double-quote marks.
For example:
"good" //string constant
""
//null
string constant
" "
//string
constant of three white space
"x"
//string
constant having single character.
"Earth is
round\n" //prints string with newline
Enumeration
constants:
Keyword enum
is used to declare enumeration types. For example:
enum color
{yellow, green, black, white};
Here, the
variable name is color and yellow, green, black and white are the enumeration
constants having value 0, 1, 2 and 3 respectively by default.
