Control Statements: Part 2; Logical Operators - Java: How to Program Early Objects - page 157

In Section 5.8, we show a case in which a for statement cannot be represented with an
equivalent while statement. Typically, for statements are used for counter-controlled rep-
etition and while statements for sentinel-controlled repetition. However, while and for
can each be used for either repetition type.
Scope of a for Statement's Control Variable
If the initialization expression in the for header declares the control variable (i.e., the con-
trol variable's type is specified before the variable name, as in Fig. 5.2), the control variable
can be used only in that for statement—it will not exist outside it. This restricted use is
known as the variable's scope . The scope of a variable defines where it can be used in a
program. For example, a local variable can be used only in the method that declares it and
only from the point of declaration through the end of the method. Scope is discussed in
detail in Chapter 6, Methods: A Deeper Look.
Common Programming Error 5.3
When a for statement's control variable is declared in the initialization section of the
for 's header, using the control variable after the for' s body is a compilation error.
Expressions in a for Statement's Header Are Optional
All three expressions in a for header are optional. If the loopContinuationCondition is
omitted, Java assumes that the loop-continuation condition is always true , thus creating an
infinite loop . You might omit the initialization expression if the program initializes the
control variable before the loop. You might omit the increment expression if the program
calculates the increment with statements in the loop's body or if no increment is needed.
The increment expression in a for acts as if it were a standalone statement at the end of
the for 's body. Therefore, the expressions
counter = counter + 1
counter += 1
++counter
counter++
are equivalent increment expressions in a for statement. Many programmers prefer coun-
ter++ because it's concise and because a for loop evaluates its increment expression after
its body executes, so the postfix increment form seems more natural. In this case, the vari-
able being incremented does not appear in a larger expression, so preincrementing and
postincrementing actually have the same effect.
Common Programming Error 5.4
Placing a semicolon immediately to the right of the right parenthesis of a for header makes
that for 's body an empty statement. This is normally a logic error.
Error-Prevention Tip 5.4
Infinite loops occur when the loop-continuation condition in a repetition statement never
becomes false . To prevent this situation in a counter-controlled loop, ensure that the
control variable is modified during each iteration of the loop so that the loop-continuation
condition will eventually become false . In a sentinel-controlled loop, ensure that the sen-
tinel value is able to be input.