6.3. Compound Statements (Blocks)

A compound statement, usually referred to as a block, is a (possibly empty) sequence of statements surrounded by a pair of curly braces. A block is a scope. Names introduced within a block are accessible only from within that block or from blocks nested inside the block. As usual, a name is visible only from its point of definition until the end of the enclosing block.

Compound statements can be used where the rules of the language require a single statement, but the logic of our program needs to execute more than one. For example, the body of a while or for loop must be a single statement. Yet, we often need to execute more than one statement in the body of a loop. We can do so by enclosing the statements in a pair of braces, thus turning the sequence of statements into a block.

As an example, recall the while loop from our solution to the bookstore problem on page 26:

     // if so, read the transaction records
     while (std::cin >> trans)
         if (total.same_isbn(trans))
             // match: update the running total
             total = total + trans;
        else {
             // no match: print & assign to total
             std::cout << total << std::endl;
             total = trans;
     }

In the else branch, the logic of our program requires that we print total and then reset it from TRans. An else may be followed by only a single statement. By enclosing both statements in curly braces, we transform them into a single (com-pound) statement. This statement satisfies the rules of the language and the needs of our program.

Just as there is a null statement, we also can define an empty block. We do so by using a pair of curlies with no statements:

     while (cin >> s && s != sought)
         { } // empty block

Exercises Section 6.3 Exercise 6.1: What is a null statement? Give an example of when you might use a null statement. Exercise 6.2: What is a block? Give an example of when you might use a block. Exercise 6.3: Use the comma operator (Section 5.9, p. 168) to rewrite the else branch in the while loop from the bookstore problem so that it no longer requires a block. Explain whether this rewrite improves or diminishes the readability of this code. Exercise 6.4: In the while loop that solved the bookstore problem, what effect, if any, would removing the curly brace following the while and its corresponding close curly have on the program?