VARIABLES.

Till now,we have discussed the elements of algorithm.But a program comprises of algorithm and data.Therefore,it is now necessary to understand the concept of data.It is known that data is a symbolic representation of value and that programs set the context that gives data a proper meaning.In programs,data is transformed into information.
The question is ,how is data represented in program?
Almost every algorithm contains data and usually the data is 'contained' in what is called a variable.
The variable is a container for a value that may vary during the execution of program.For example,in the tea-making algorithm,the level of water is a variable and the quantity of tea level is also a variable.
Each variable in a program is given a name,for example,

water_level
water_temperature
tea_leaves_quantity

and at any given time the value,which is represented by water_level,for instance,may be different to its value at some other time.This statement
"if the kettle does not contain water then fill the kettle" could also be written as
"if water_level is 0 then fill the kettle"
or
"if water_level=0 then fill the kettle"
At some point water_level will be at maximum value,whatever that is and the kettle will be full.

VARIABLES AND DATA TYPES:

The data used in algorithms can be of different types.The simplest types of data that an algorithm might use are

numeric data,e.g., 12,11.45,901,27,32,etc.
alphabetic or character data such as 'A','Z',or 'Any String',or 'Any character from the keyboard'.
logical data,that is propositions with true/false values.

A variable is nothing but a name given to a storage area that our programs can manipulate. Each variable in C has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.

The name of a variable can be composed of letters, digits, and the underscore character. It must begin with either a letter or an underscore. Upper and lowercase letters are distinct because C is case-sensitive. Based on the basic types explained in previous chapter, there will be the following basic variable types:

Type	Description
char	Typically a single octet(one byte). This is an integer type.
int	The most natural size of integer for the machine.
float	A single-precision floating point value.
double	A double-precision floating point value.
void	Represents the absence of type.

C programming language also allows to define various other types of variables, which we will cover in subsequent chapters like Enumeration, Pointer, Array, Structure, Union, etc. For this chapter, let us study only basic variable types.

VARIABLE DEFINITION IN C:

A variable definition means to tell the compiler where and how much to create the storage for the variable. A variable definition specifies a data type and contains a list of one or more variables of that type as follows:

type variable_list;

Here, type must be a valid C data type including char, w_char, int, float, double, bool or any user-defined object, etc., and variable_list may consist of one or more identifier names separated by commas. Some valid declarations are shown here:

int    i, j, k;
char   c, ch;
float  f, salary;
double d;

The line int i, j, k; both declares and defines the variables i, j and k; which instructs the compiler to create variables named i, j and k of type int.

Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of an equal sign followed by a constant expression as follows:

type variable_name = value;

Some examples are:

extern int d = 3, f = 5;    // declaration of d and f. 
int d = 3, f = 5;           // definition and initializing d and f. 
byte z = 22;                // definition and initializes z. 
char x = 'x';               // the variable x has the value 'x'.

For definition without an initializer: variables with static storage duration are implicitly initialized with NULL (all bytes have the value 0); the initial value of all other variables is undefined.

VARIABLE DECLARATION IN C:

A variable declaration provides assurance to the compiler that there is one variable existing with the given type and name so that compiler proceed for further compilation without needing complete detail about the variable. A variable declaration has its meaning at the time of compilation only, compiler needs actual variable declaration at the time of linking of the program.

A variable declaration is useful when you are using multiple files and you define your variable in one of the files which will be available at the time of linking of the program. You will use extern keyword to declare a variable at any place. Though you can declare a variable multiple times in your C program but it can be defined only once in a file, a function or a block of code.

EXAMPLE:

Try following example, where variables have been declared at the top, but they have been defined and initialized inside the main function:

#include <stdio.h>

// Variable declaration:
extern int a, b;
extern int c;
extern float f;

int main ()
{
  /* variable definition: */
  int a, b;
  int c;
  float f;
 
  /* actual initialization */
  a = 10;
  b = 20;
  
  c = a + b;
  printf("value of c : %d \n", c);

  f = 70.0/3.0;
  printf("value of f : %f \n", f);
 
  return 0;
}

When the above code is compiled and executed, it produces the following result:

value of c : 30
value of f : 23.333334

Same concept applies on function declaration where you provide a function name at the time of its declaration and its actual definition can be given anywhere else. For example:

// function declaration
int func();

int main()
{
    // function call
    int i = func();
}

// function definition
int func()
{
    return 0;
}

LVALUES AND RVALUES IN C:

There are two kinds of expressions in C:

<> lvalue : An expression that is an lvalue may appear as either the left-hand or right-hand side of an assignment.
<> rvalue : An expression that is an rvalue may appear on the right- but not left-hand side of an assignment.

Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric literals are rvalues and so may not be assigned and can not appear on the left-hand side. Following is a valid statement:

D-HACKED

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