Thursday, 17 October 2013

Scope of Identifiers in a Function

Scope of Identifiers

An 'Identifier' means any name that the user creates in his/her program. These names can be of variables, functions and labels. Here the scope of an identifier means its visibility. We will focus Scope of Variables in our discussion.
Suppose we write the function:
void func1() {
int i;
……..      //Some other lines of code
int j = i+2;    //Perfectly alright
. . .
Now this variable ‘i’ can be used in any statement inside the function func1(). But consider this variable being used in a different function like:
void func2() {
int k = i + 4; //Compilation error
. . .
The variable ‘i’ belongs to func1() and is not visible outside that. In other words, ‘i’ is local to func1().To understand the concept of scope further, we have to see what are Code Blocks? A code block begins with ‘{‘ and ends with ‘}’.Therefore, the body of a function is essentially a code block. Nonetheless, inside a function there can be another block of code like 'for loop' and 'while loop' can have their own blocks of code respectively. Therefore, there can be a hierarchy of code blocks.
A variable declared inside a code block becomes the local variable for that for that block. It is not visible outside that block. See the code below:
void func() {
            int outer; //Function level scope
            . . .
                        Int inner;         //Code block level scope
                        inner = outer; //No problem
                        . . .
inner ++;          //Compilation error
Please note that variable ‘outer’ is declared at function level scope and variable ‘inner’ is declared at block level scope.
The ‘inner’ variable declared inside the inner code block is not visible outside it. In other words, it is at inner code block scope level. If we want to access that variable outside its code block, a compilation error may occur.


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