It's possible to use variables for more than one purpose in several subtle ways. You're better off without this kind of subtlety.

Use each variable for one purpose only. It's sometimes tempting to use one variable in two different places for two different activities. Usually, the variable is named inappropriately for one of its uses or a "temporary" variable is used in both cases (with the usual unhelpful name x or temp). Here's an example that shows a temporary variable that's used for two purposes:

C++ Example of Using One Variable for Two Purposes---Bad Practice

Example 10-15. 

// Compute roots of a quadratic equation.
// This code assumes that (b*b-4*a*c) is positive.
temp = Sqrt( b*b - 4*a*c );
root[O] = ( -b + temp ) / ( 2 * a );
root[1] = ( -b - temp ) / ( 2 * a );
...

// swap the roots
temp = root[0];
root[0] = root[1];
root[1] = temp;

Question: What is the relationship between temp in the first few lines and temp in the last few? Answer: The two temps have no relationship. Using the same variable in both instances makes it seem as though they're related when they're not. Creating unique variables for each purpose makes your code more readable. Here's an improvement:

// Compute roots of a quadratic equation.
// This code assumes that (b*b-4*a*c) is positive.
discriminant = Sqrt( b*b - 4*a*c );
root[0] = ( -b + discriminant ) / ( 2 * a );
root[1] = ( -b - discriminant ) / ( 2 * a );
...

// swap the roots
oldRoot = root[0];
root[0] = root[1];
root[1] = oldRoot;

Avoid variables with hidden meanings. Another way in which a variable can be used for more than one purpose is to have different values for the variable mean different things. For example:

Avoid variables with these kinds of hidden meanings. The technical name for this kind of abuse is "hybrid coupling" (Page-Jones 1988). The variable is stretched over two jobs, meaning that the variable is the wrong type for one of the jobs. In the pageCount example, pageCount normally indicates the number of pages; it's an integer. When pageCount is -1, however, it indicates that an error has occurred; the integer is moonlighting as a boolean!

Even if the double use is clear to you, it won't be to someone else. The extra clarity you'll achieve by using two variables to hold two kinds of information will amaze you. And no one will begrudge you the extra storage.

Make sure that all declared variables are used. The opposite of using a variable for more than one purpose is not using it at all. A study by Card, Church, and Agresti found that unreferenced variables were correlated with higher fault rates (1986). Get in the habit of checking to be sure that all variables that are declared are used. Some compilers and utilities (such as lint) report unused variables as a warning.