Download Lecture 14: for loops, break, continue and Which loop to use? Feb

Lecture 14:
for loops, break, continue and
Which loop to use?
Feb 13 2015
Example:
Syntax:
or (initial-action; loopint i;
for
loops
continuation-condition;
for (i = 0; i < 100; i++) {
Syntax:
Example:
for (initial-action;
action-after-each-iteration)
{
System.out.println(
int i;
loopcontinuation-condition;
// loop body;
for (i=0; i < 100; i++) {
action-after-each-iteration)
"Welcome
to
Java!");
{
System.out.println
Statement(s);
("Welcome to Java");
// loop body;
}
}
}
Statement(s);
}
Initial-Action
Loop
Continuation
Condition?
i=0
false
(i < 100)?
true
Statement(s)
(loop body)
true
System.out.println(
"Welcome to Java");
Action-After-Each-Iteration
i++
(A)
(B)
false
8
Note on for loops
•  The initial-action in a for loop can be a list of zero or
more comma-separated expressions.
•  The action-after-each iteration in a for loop can be a list
of zero or more comma separated statements.
Therefore, the following two for loops are correct. They
are rarely used in practice, however.
for (int i = 1; i < 100; System.out.println(i++));
for (int i = 0, j = 0; (i + j < 10); i++, j++) {
// Do something
}
Caution 1Note
on for loops
If the
loop-continuation-condition
in ainfor
looploop
is omitted,
If the
loop-continuation-condition
a for
is
it isomitted,
implicitly
Thus the
statement
given
below in (a),
it istrue.
implicitly
true.
Thus the
statement
which
is below
an infinite
loop,
is correct.
Nevertheless,
given
in (a),
which
is an infinite
loop, is it is
better
to use the
equivalent itloop
in (b)totouse
avoid
“correct”.
Nevertheless,
is better
theconfusion:
equivalent loop in (b) to avoid confusion:
!"#$%$&$&$'$($
$$))$*"$+",-./012$
3$
$$
(a)
Equivalent
4/05-$%.#6-'$($
$$))$*"$+",-./012$
3$$
$
(b)
$$
20
Caution 2 on for loops
Adding a semicolon at the end of the for
clause before the loop body is a common
logic error:
for (int i=0; i<10; i++);
{
System.out.println("i is " + i);
}
Semicolons and loops
Similarly, the following loop is also wrong:
int i=0;
while (i < 10);{//Logic error
System.out.println("i is " + i);
i++;
}
In the case of the do-while loop, the semicolon is needed to
end the loop.
int i=0;
do {
System.out.println("i is " + i);
i++;
} while (i<10);//Correct
break and continue statements
The break statement:
When the break statement is executed inside a loopstatement, the loop-statement is terminated immediately
The execution of the program will continue with the
statement following the loop-statement
The continue statement
When the continue statement is executed inside a loopstatement, the program will skip over the remainder of the
loop-body to the end of the loop body, without exiting the
loop completely
break and continue statements
•  while loops
•  do-while loops
•  For loops
Which Loop to Use?
The three forms Which
of loop statements,
while,
do-while,
and for, are
loop
to
use?
expressively equivalent; that is, you can write a loop in any of these
Which
Use?
three forms. For
example, a Loop
while loopto
in (a)
in the following figure
canwhile,
always
be converted
into theand
following
forare
loop in
(b):
do-while,
for,
The
three
forms
of loop statements,
while,
do-while,
and
for, are
expressively
equivalent;
that
is,
write
expressively
equivalent;
that is, you
can
write
ayou
loopcan
in any
of these
:2&*1$%*"".+,"'(&'/)(&"'+,"'0&(&"'3$4$
!"#$%$-$*"".+,"'(&'/)(&"'+,"'0&(&"'-$3$4$
Equivalent
$$55$6"".$7"08$
$$55$6"".$7"08$
threea forms.
For
example,
a whilethree
loop in9$
(a) in theExamples:
following figure
9$
loop
in
any
of
these
forms.
$
$
(b)
always be(a)converted into the following
$can
$
$$ for loop in (b):
:2&*1$%*"".+,"'(&'/)(&"'+,"'0&(&"'3$4$ Equivalent
!"#$%$-$*"".+,"'(&'/)(&"'+,"'0&(&"'-$3$4$
A
for
loop
in
(a)
in
the
following
figure
can
generally
be converted into the
$$55$6"".$7"08$
$$55$6"".$7"08$
9$
9$
following
while
loop
in
(b)
except
in
certain
special
cases (see Review Question
$
$
(a)
(b)
$$ 3.19 for one of them):
$$
!"#$%&'&(&)*+),(&"'-$$
&'&(&)*+),(&"'-$$
$$$$$*"".+,"'(&'/)(&"'+,"'0&(&"'-$$
:2&*1$%*"".+,"'(&'/)(&"'+,"'0&(&"'3$4$$
A for loop in (a) in the following figure
can generally
be converted into the
Equivalent
$$$$$),(&"'+)!(1#+1),2+&(1#)(&"'3$4$
$$55$6"".$7"08-$
following
while loop in (b) except in certain special
cases (see Review Question
$$55$6"".$7"08-$
$$),(&"'+)!(1#+1),2+&(1#)(&"'-$
9$
9$
3.19
for one of them):
$
$
(a)
(b)
$
$$
!"#$%&'&(&)*+),(&"'-$$
&'&(&)*+),(&"'-$$
23
$$$$$*"".+,"'(&'/)(&"'+,"'0&(&"'-$$
:2&*1$%*"".+,"'(&'/)(&"'+,"'0&(&"'3$4$$
Equivalent
$$$$$),(&"'+)!(1#+1),2+&(1#)(&"'3$4$
$$55$6"".$7"08-$
$$55$6"".$7"08-$
$$),(&"'+)!(1#+1),2+&(1#)(&"'-$
9$
9$
$
$
(a)
(b)
Which loop to use?
Use the one that is most intuitive and comfortable for you.
•  In general, a for loop may be used if the number of
repetitions is known, as, for example, when you need to
print a message 100 times.
•  A while loop may be used if the number of repetitions
is not known, as in the case of reading the numbers
until the input is 0.
•  A do-while loop can be used to replace a while loop
if the loop body has to be executed before testing the
continuation condition.