The Null Statement

The Null Statement

As you may notice, the for statement does not end with a semicolon. The for statement has within it either a statement block that ends with the closing brace (}) or a single statement that ends with a semicolon. The following for statement contains a single statement:

for (i=0; i<8; i++)
      sum += i;

Now consider a statement such as this:

for (i=0; i<8; i++);

Here the for statement is followed by a semicolon immediately.

In the C language, there is a special statement called the null statement. A null statement contains nothing but a semicolon. In other words, a null statement is a statement with no expression.

Therefore, when you review the statement for (i=0; i<8; i++);, you can see that it is actually a for statement with a null statement. In other words, you can rewrite it as

for (i=0; i<8; i++)
   ;

Because the null statement has no expression, the for statement actually does nothing but loop. You'll see some examples of using the null statement with the for statement later in the book.

WARNING

    Because the null statement is perfectly legal in C, you should pay attention to placing semicolons in your for statements. For example, suppose you intended to write a for loop like this:

    for (i=0; i<8; i++)
       sum += i;

    for (i=0; i<8; i++);
       sum += i;

    your C compiler will still accept it, but the results from the two for statements will be quite different. (See exercise 1 in this lesson for an example.)