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  Variable Name and Scope

Introduction to Variable Name and Scope

Every  language provides  of variable naming and its  scope  in its  own format  and rules and regulations. As such  according  to Dijkstra the concept of a  variable in a  programming  can be  given as:

Once a person has understand  the way in  which variables are used  in programming  he has understood the quintessence of programming.

To understand what Dujkstra said we will  concentrate on some issue  of a variable  name  definition a  variable name its naming constraints way  of using and  its  scope.

(a) Definition  of A variable  Name : A name  is a string of characters  used by  program statements  to denote  the variable  (or identifier) to provide  such  naming  to the  variable a sequence of lexical rules are applied to name an entity in a program. It  is called  as the variable naming. The variable names must be specific understand  able and meaningful so  that they  can by  easily used in the  program.

Example  28. In C variable names may  be rate  red interest  etc.

(b) Variable  Naming  Constraints : As  such every  programming  language has its  own  specified rules to provide a name  to a variable or identifier.

Example 29. In C  there are some  rules to be followed to construct variable  names .

  • A variable name is any combination of 1 to 8 alphabets digits or underscores some compilers allow variable names whose length could be up to 40 characters. Still it would be sager to stick to the rule to 8 characters.
  • The first character in the variable name must be an alphabet.
  • No commas or blanks are allowed within a variable name.
  • No special symbol other than an underscore can be used in a variable name.

Since  the maximum  length of a variable name  is 8 characters an  enormous  number of variable names  can be constructed using  the above mentioned rules. It is a good  practice to exploit this enormous choice in a naming variables by  using meaningful  variable  names.

(c) Declaration : A variable  name is usually introduced by a special statement called a declaration of a variable  name.

Example 30. Fig 6.18, shows the variable names declaration in C.

(d) Variable Scope : The variable  scope extends form  the declaration of the  variable until some later closing point specified by the language. The  scope of a variable  is the range of program instructions over which   the name is known. Program instructions can manipulate a variable through  its name within its scope. We also  say that  a variable is visible under its name with  its scope and invisible  outside it. Different programming languages adopt different rules for binding a variable name to its scope.

Example 31. Consider the sample C program  illustrate in fig  6.19.

The  declaration int  x ,y. Make variable  named x and y visible throughout  the program main  the program constrains an internal block which groups a declaration and   statements . the  declaration int temp appearing in the block makes name temp visible  within the inner block , and invisible outside. In the example  if the inner block declaration  a new local variable named  x the outer variable  named  y would no longer  be visible in the inner block .

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