Transcript (looping and repetition) do

Looping and Repetition
SE-1010
Dr. Mark L. Hornick
1
Looping and Repetition Statements
control a block of code to be
executed for:
a fixed number of times

1, 10, 10000…
until a certain condition is met



a value remains positive (or negative)
a value remains below (or above) some limit
…
SE-1010
Dr. Mark L. Hornick
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The two (three) main loop types:
while() and it’s cousin do…while()
 Sentinel-controlled loops, where the loop body is
executed repeatedly until a designated condition,
called a sentinel, is encountered.
for()

A count-controlled loop, where the loop body is
executed a fixed number of times.
SE-1010
Dr. Mark L. Hornick
3
The do-while() Statement
Format:
do {
<statements>
} while (<boolean expression>);
The <statements> part is known as the loop
body
The loop body is executed until the <boolean
expression> becomes false

And is always executed at least once!
Even if <boolean expression> is false
SE-1010
Dr. Mark L. Hornick
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The do-while() statement

Demo
SE-1010
Dr. Mark L. Hornick
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The while Statement
Format:
while ( <boolean expression> ){
<statements that repeat> // zero or
more
}
As long as the <boolean expression> is true,
the loop body is executed
SE-1010
Dr. Mark L. Hornick
6
The while() statement

Demo
SE-1010
Dr. Mark L. Hornick
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while vs. do-while
The while statement is a pre-test loop


because the test is done before the loop
body is entered and executed
Therefore, the loop body may not be
executed if the test is false to begin with.
The do-while statement is a post-test
loop.

With a post-test loop statement, the loop
body is executed at least once.
SE-1010
Dr. Mark L. Hornick
8
while vs. do-while
int x=0, y=1;
while(x==y) { // never true
//statements never execute
};
do {
//statement executes once
} while(x==y); // never true
SE-1010
Dr. Mark L. Hornick
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Common mistakes when using
loops:
Identify the error:
int product = 0;
while (product < 5000) {
product = product * 5;
}
SE-1010
Dr. Mark L. Hornick
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The infinite loop pitfall
Identify the error:
int product = 0;
while (product < 5000) {
product = product * 5;
}
Because product is initialized to 0, product
will never be larger than 5000 (0 = 0 * 5), so
the loop will never terminate.
SE-1010
Dr. Mark L. Hornick
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Identify the error:
int count = 1;
while (count != 10) {
count = count + 2;
}
SE-1010
Dr. Mark L. Hornick
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The Overflow pitfall
Identify the error:
byte count = 1;
while (count != 10) {
count = count + 2;
}
Here, count will never equal 10 (1,3,5,7,9,11…)
…so the while() loop runs forever
Q: How large does count get??
SE-1010
Dr. Mark L. Hornick
13
About Overflow errors…
An overflow error occurs when you attempt to
assign a value larger than the maximum
value the variable can hold.
In Java, an overflow does not cause program
termination. With types float and double, a
value that represents infinity is assigned to
the variable. With type int, the value “wraps
around” and becomes a negative value.
SE-1010
Dr. Mark L. Hornick
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Identify the error:
float count = 1.3f;
while (count != 1300.0) {
count = count + 1.3;
}
SE-1010
Dr. Mark L. Hornick
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The Floating-point roundoff error
Identify the error:
float count = 1.3F;
while (count != 1300.0) {
count = count + 1.3;
}
Real numbers should not be used in testing
or increment, because only an
approximation of real numbers can generally
be stored in a computer.
SE-1010
Dr. Mark L. Hornick
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Loop-and-a-Half Repetition
Control
Terminates a loop early, in the middle of the
loop body
int i=0;
do {
i=i+1;
if( i>5 ) break; // quit early
} while( i<100 );
<more statements>
SE-1010
Dr. Mark L. Hornick
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The for Statement
Example:
for (int i=0; i<10; i++)
{
<statements> // zero or more
}
int i=0 is executed at the start
i<10 is evaluated each time before <statements> are executed
i++ is executed each time after <statements> are executed
SE-1010
Dr. Mark L. Hornick
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Syntax of the for Statement
Format:
for (<init smt>; <boolean expr>; <increment smt>)
{
<statements> // zero or more
}
<init smt> is executed at the start; can be any valid Java statement
<boolean expr> is evaluated each time before <statements> are
executed; can be any valid expression
<increment smt> is executed each time after <statements> are
executed; can be any valid Java statement
SE-1010
Dr. Mark L. Hornick
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for Statement – typical use
The variable i in the example statement is
called a control variable. It keeps track of the
number of repetitions.

The control variable can be declared within the
for() statement:
for( int i=0; i<100; i+=20) {
// how many repeats?
}
The <increment smt> can be by any
amount, positive or negative.
SE-1010
Dr. Mark L. Hornick
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Nested-for statements are for
statements embedded within other for
statements
// multiplication table of n rows and m columns
int product;
for (int n = 0; n<=10, n++){
for (int m = 1; m<=10, m+=1){
product = n * m;
}
}
SE-1010
Dr. Mark L. Hornick
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The break statement can be used in a
for() body to cause early termination
for (int i=0; i<100; i++ ) {
if( i>5 ) break; // exit early
}
// get here on normal or early exit
SE-1010
Dr. Mark L. Hornick
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When a control variable is declared
within the for() loop, it is only visible
within the loop
// variable i is not visible here
for (int i=0; i<100; i++ ) {
// statements here
// variable i is visible here
}
// variable i is not visible here
SE-1010
Dr. Mark L. Hornick
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Declare the control variable outside
the for() loop if you want to use it
outside the loop
int i;
for (i=0; i<100; i++ ) {
// statements here
// variable i is visible here
}
// variable i is still visible here
// What value does it contain??
// How many loops were executed??
SE-1010
Dr. Mark L. Hornick
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