The Scope of Variables

The Scope of Variables

Almost every programming language has:

1. constructs for introducing a new variable x, and

2. scoping rules for determining which uses of the variable name x in the rest of the program refer to this variable.

If the same name x is used for several different variables, a programmer can resolve which variable is really meant by a particular use of the name x.

For example, in Java a loop header may introduce a new loop variable and its scope:

for (int i = 0; i < NumOfElements; i++) ... ;

The scope of the variable i is the program text following the = all the way to the end of the loop body. If the loop contains a break construct and if the loop is followed by some statement like

System.out.println("the index where we stopped searching is " + i);

then we know from the scoping rules for Java that the program is almost certainly erroneous. Since the use of i in the invocation of println does not refer to the variable i that was introduced as the loop variable, the reference to variable i in the print statement either is illegal or refers to a variable with a scope that encloses the loop and the subsequent print statement.

Note: We analyzed the preceding program fragment with a rough understanding of its execution semantics but with a precise understanding of the scoping rules. These scoping rules are crucial for building a good mental execution model.

Other examples of scoping constructs in Java and C++ include

* class definitions,

* method definitions,

* parameter declarations, and

* sequences of statements (tails of blocks)