1. Looping and Repetition
SE-1010 Dr. Mark L. Hornick

2. 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

3. 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

4. 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

5. The do-while() statement
Demo
SE-1010 Dr. Mark L. Hornick

6. 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

7. The while() statement
Demo
SE-1010 Dr. Mark L. Hornick

8. 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

9. 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

10. Common mistakes when using loops:
Identify the error:
int product = 0;
while (product < 5000) {
product = product * 5; }
SE-1010 Dr. Mark L. Hornick

11. 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

12. Identify the error: int count = 1; while (count != 10) {
count = count + 2; }
SE-1010 Dr. Mark L. Hornick

13. 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

14. 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

15. Identify the error: float count = 1.3f; while (count != 1300.0) {
count = count + 1.3; }
SE-1010 Dr. Mark L. Hornick

16. The Floating-point roundoff error
Identify the error:
float count = 1.3F;
while (count != ) {
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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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 }
SE-1010 Dr. Mark L. Hornick

24. 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