Java Reference
In-Depth Information
Error-Prevention Tip 5.2
Using the final value and operator
<=
in a loop's condition helps avoid off-by-one errors.
For a loop that outputs 1 to 10, the loop-continuation condition should be
counter <= 10
rather than
counter < 10
(which causes an off-by-one error) or
counter < 11
(which is cor-
rect). Many programmers prefer so-called zero-based counting, in which to count 10 times,
counter
would be initialized to zero and the loop-continuation test would be
counter
<
10
.
Error-Prevention Tip 5.3
As Chapter 4 mentioned, integers can overflow, causing logic errors. A loop's control vari-
able also could overflow. Write your loop conditions carefully to prevent this.
A Closer Look at the
for
Statement's Header
Figure 5.3 takes a closer look at the
for
statement in Fig. 5.2. The first line—including
the keyword
for
and everything in parentheses after
for
(line 10 in Fig. 5.2)—is some-
times called the
for
statement header
. The
for
header “does it all”—it specifies each item
needed for counter-controlled repetition with a control variable. If there's more than one
statement in the body of the
for
, braces are required to define the body of the loop.
for
keyword
Control variable
Required semicolon
Required semicolon
for
(
int
counter =
1
; counter <=
10
; counter++)
Loop-continuation
condition
Incrementing of
control variable
Initial value of
control variable
o
Fig. 5.3
|
for
statement header components.
General Format of a
for
Statement
The general format of the
for
statement is
for
(
initialization
;
loopContinuationCondition
;
increment
)
statement
where the
initialization
expression names the loop's control variable and
optionally
provides
its initial value,
loopContinuationCondition
determines whether the loop should continue ex-
ecuting and
increment
modifies the control variable's value, so that the loop-continuation
condition eventually becomes false. The two semicolons in the
for
header are required. If
the loop-continuation condition is initially
false
, the program does
not
execute the
for
statement's body. Instead, execution proceeds with the statement following the
for
.
Representing a
for
Statement with an Equivalent
while
Statement
The
for
statement often can be represented with an equivalent
while
statement as follows:
initialization
;
while
(
loopContinuationCondition
)
{
statement
increment
;
}
