Chapter 3
Flow of Control




Chapter 3
Branching
Loops
exit(n) method
Boolean data type and expressions
Java: an Introduction to Computer Science & Programming - Walter Savitch
1
What is “Flow of Control”?



Flow of Control is the execution order of instructions in a
program
All programs can be written with three control flow
elements:
1. Sequence - just go to the next instruction
2. Selection - a choice of at least two
– either go to the next instruction
– or jump to some other instruction
3. Repetition - a loop (repeat a block of code)
at the end of the loop
– either go back and repeat the block of code
– or continue with the next instruction after the block
Selection and Repetition
are called Branching since these are branch points in
Java: an Introduction to Computer Science & Programming - Walter Savitch
the flow of control
Chapter 3
2
Simple Selection - if Example
… // some code before this …
change = 0;
if(amountTendered > amountDue)
// change is due (begin body of the if statement)
change = amountTendered – amountDue;
// (end body of the if statement)
System.out.println(“Change to be given back “
+ change); // always runs



Chapter 3
Test / comparison / condition is inside ( ) ‘s
If the test evaluates (returns) to true, then the body is
executed (otherwise they do not)
Statements after the body of the if statement always
execute
Java: an Introduction to Computer Science & Programming - Walter Savitch
4
Java if statement syntax
Chapter 3

Syntax:
if (Boolean_Expression)
Action if true;//execute only if true
next action;//always executed

Note the indentation for readability (not compile or
execution correctness)
Java: an Introduction to Computer Science & Programming - Walter Savitch
5
Java Statement Blocks:
Compound Statements


Action if true can be either a single Java statement or
a set of statements enclosed in curly brackets (a
compound statement, or block)
All statements between
For example:
braces are controlled by if
if(change > 0) {
// determine coins to give back (begin body)
quarters = change / 25;
int amountLeft = change % 25;
dimes = amountLeft / 10;
…
}
// end if
System.out.println(“Returning change of “ + change
+ “\n
using “ + quarters + “quarters”
Chapter
Java: an Introduction to Computer Science & Programming - Walter Savitch
…3
6
Two-way Selection - if-else Example

Select either one of two options, depending on test value
… // price is determined; taxability is determined
if (taxable == ‘y’) {
tax = price * TAXRATE;
}
else {
tax = 0;
}
total = price + tax;

Curly braces are NOT necessary if only one
statement BUT I always use them
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
7
Two-way Selection: if-else Syntax

Chapter 3
Syntax:
if (Boolean_Expression)
{
Action1 //execute only if true
}
else
{
Action2//execute only if false
}
Action3//always executed
Java: an Introduction to Computer Science & Programming - Walter Savitch
8
OK – Let’s Solve a Problem

Chapter 3
Tickets for the Olympic Downhill Ski race are $100
each. If you buy 10 or more, then there is a service
charge of $100. If you buy fewer, the service charge
is $15 per ticket. Write a program to handle the I/O
and calculations – displaying total cost.
Java: an Introduction to Computer Science & Programming - Walter Savitch
9
Definition of Boolean Values





With if’s, we have seen that which branch is taken
depends on a test condition which evaluates to either
true or false
Variables (or expressions) that are either true or false
are called boolean variables (or expressions)
Hence the syntax was written as
if (Boolean_Expression)
The test for the if is a boolean expression – a part of a
statement that evaluates to (returns) either true or
false
boolean is a primitive data type in Java
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
10
Boolean Expressions



Chapter 3
Often two values are compared
For example:
Is amountTendered less than amountDue?
Is taxable equal to ‘y’ ?
Is numberOfTix greater than or equal to 10?
etc.
Values being compared can be any data type (or
class), but they should be the same data type (or
class)
Java: an Introduction to Computer Science & Programming - Walter Savitch
11
Java Comparison Operators
Math
Notation Name
=
equal to

not equal to
>
greater than
Java
Java
Notation Examples
==
balance == 0
answer = 'y'
!=
income  tax
answer != 'y'
>
income > outgo

greater than or equal to
>=
points >= 60
<
less than
<
pressure < max

less than or equal to
<=
income <=
outgo
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
12
What about a range?





Chapter 3
Sometimes we want to know whether a value is within a
range (e.g. is a test score between 80 and 90?)
Knowing mathematics, we’d like to write this as
if (80 < testScore < 90) { …
But alas … this is not how things work with computers
Instead
if ( ( testScore >80) && (testScore < 90) ) { …
AND (&&) is a logical operator. Something AND
somethingelse evaluates to true if both components
evaluate to true
Java: an Introduction to Computer Science & Programming - Walter Savitch
13
Compound Boolean Expressions




Chapter 3
Use && to AND two or more conditions
Use || to OR two or more conditions – if either component
is true then the whole expression is true
For example,
if ( (gpa < 0.0) || (gpa > 4.0) ) { …
In this example the parentheses are not required but are
added for clarity
» See text (and later slides) for Precedence rules
» Note the short-circuit, or lazy, evaluation rules in text
(and later in slides)
» Use a single & for AND and a single | for OR to avoid
short-circuit evaluation and force complete evaluation of
a boolean expression
Java: an Introduction to Computer Science & Programming - Walter Savitch
14
NOT !



Chapter 3
NOT essentially reverses
Indicated with !
if ( ! ( x > 0) ) { …
Usually you can do without NOT
Java: an Introduction to Computer Science & Programming - Walter Savitch
15
Java Comparison Methods for
String Class




Chapter 3
“==“ does not do what you may think for String objects
» When “==“ is used to test objects (such as String objects) it
tests to see if the storage addresses of the two objects are
the same
– are they stored in the same location?
– more will be said about this later
Use “.equals” method to test if the strings, themselves, are equal
String s1 = “What are you doing?”;
String s2;
s2 = SavitchIn.readLine();
//s1.equals(s2) returns true if the user enters What are
you doing?, false otherwise
.equals()is case sensitive
Use .equalsIgnoreCase()to ignore case
Java: an Introduction to Computer Science & Programming - Walter Savitch
16
Nested If’s


Chapter 3
An IF-Else statement can have any kind of statement(s)
inside the if or inside the else
This includes another IF
if (numCredits >= 12) {
if (resident == ‘y’) {
totalCost = …
}
else {
totalCost = …
}
else { …
Java: an Introduction to Computer Science & Programming - Walter Savitch
17
if-else if-else if-…-else
Example

Chapter 3
Can have if statements nested inside else too
if(score >= 90)
grade = ‘A’);
else if (score >= 80)
grade = ‘B’;
else if (score >= 70)
grade = ‘C’;
else if (score >= 60)
grade = ‘D’;
else
grade = ‘F’;
Java: an Introduction to Computer Science & Programming - Walter Savitch
18
Multibranch selection:
if-else if-else if-…-else


One way to handle situations with more than two possibilities
Syntax:
if(Boolean_Expression_1)
Action_1
else if(Boolean_Expression_2)
Action_2
.
.
.
else if(Boolean_Expression_n)
Action_n
else
Default_Action
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
19
switch Example

An alternative way of doing a multibranch selection
switch(seatLocationCode)
{
case 1:
System.out.println(“Orchestra”);
price = 40.00;
break;
case 2:
System.out.println(“Mezzanine”);
price = 30.00;
break;
case 3:
System.out.println(“Balcony”);
price = 15.00;
break;
default:
System.out.println(“Unknown seat code”);
break;
}
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
20
Multibranch selection Syntax:
switch

break may be omitted IF appropriate for your algorithm (it usually is
NOT appropriate)
switch(Controlling_Expression)
{
case Case_Label:
Can be any number of
statements
cases like this one.
…
break;
case Case_Label:
statements
…
break;
}
Chapter 3
default:
statements
…
break;
Default case is optional.
Java: an Introduction to Computer Science & Programming - Walter Savitch
21
Multibranch selection:
switch



Chapter 3
Controlling_Expression must be char, int,
short or byte
Controlling Expression and Case_Label
must be same type
When a break statement is encountered, control
goes to the first statement after the switch.
Java: an Introduction to Computer Science & Programming - Walter Savitch
22
Repetition: Loops



Chapter 3
Structure:
» Usually some initialization code
» body of loop
» loop termination condition
Several logical organizations
» counting loops
» sentinel-controlled loops
» infinite loops
» minimum of zero or minimum of one iteration
Several programming statement variations
» while
» do-while
» for
Java: an Introduction to Computer Science & Programming - Walter Savitch
23
while : a counting loop example

Chapter 3
A loop to sum 10 numbers entered by user
int next;
//Loop initialization
int count = 1;
int total =0;
while(count <= 10) //Loop termination
condition
{ //Body of loop
next = SavitchIn.readLineInt();
total = total + next;
count++; //Loop termination counter
}
Java: an Introduction to Computer Science & Programming - Walter Savitch
24
while:
a sentinel controlled loop example



Chapter 3
A loop to sum positive integers entered by the user
next is the sentinel
The loop terminates when the user enters a negative number
//Initialization
int next = 0;
int total = 0;
while(next >= 0) //Termination condition
{ //Body
total = total + next;
next = SavitchIn.readLineInt();
}
Java: an Introduction to Computer Science & Programming - Walter Savitch
25
while Loop




Chapter 3
Syntax:
while(Boolean_Expression)
{
//body of loop
First_Statement;
...
Last_Statement;
}
Something in body of loop
should eventually cause
Boolean_Expression to
be false.
Initialization statements usually precede the loop.
Boolean_Expression is the loop termination condition.
May be either counting or sentinel loop
» Good choice for sentinel loop
Java: an Introduction to Computer Science & Programming - Walter Savitch
26
while: A Minimum of Zero
Iterations

Because the first input value read and the test precedes the
loop, the body the while loop body may not execute at all
//Initialization
int next;
int total = 0;
next = SavitchIn.readLineInt();
while(next >= 0)//Termination condition
{ //Body
total = total + next;
next = SavitchIn.readLineInt();
}

Chapter 3
If the first number the user enters is negative the loop body
never executes
Java: an Introduction to Computer Science & Programming - Walter Savitch
27
OK – Let’s Solve another Problem

Chapter 3
Calculate the GPA for a student – ask continually if
there are more classes to consider. For each class,
ask for the grade (A,B,C,D, or F) and the number of
credits. Display the student’s GPA.
Java: an Introduction to Computer Science & Programming - Walter Savitch
28
Do code for GPA?
Pseudocode
totalPoints = 0
totalCredits = 0
While more courses
ask for and get grade
ask for and get # credit hours
Figure # points (A – 4, B – 3, C - 2, D – 1, F – 0)
current course points = points * credit hours
add current course points to totalPoints
add one to totalCredits
ENDWHILE
Gpa = totalPoints / totalCredits
Display GPA

Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
29
Another Task Involving Loops

Calculate yx (assume x is an integer)
…
// declare, ask for, and get y and x
…
int count = 1;
double total = y;
while (count < x) {
total = total * y;
count++;
}
// display result
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
30
Another Task Involving Loops

Ensuring valid inputs (e.g. making sure # hours worked is
between 1 and 60)
System.out.println(“How many hours?”);
double numHours = SavitchIn.readLineDouble();
while ((numHours < 1) || ( numHours > 60) {
System.out.println(“I’m sorry, you have entered an invalid number of hours”);
System.out.println(“Please re-enter a number between 1 and 60”);
numHours = SavitchIn.readLineDouble();
}
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
31
Another Task Involving Loops
Find the average score on a quiz
 Pseudocode
Initialize total to 0
Initialize counter to 0
Ask for and get how many numbers
While counter < number of numbers
ask for and get next score
add score to total
add one to counter
If counter > 0
set ave to total / counter
print the average

Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
32
do-while Example
int count = 1;
int number = 10;
do //Display integers 1 - 10 on one line
{
System.out.println(count + “ “ );
count++;
}while(count <= number);

Note System.out.print() is used and not
System.out.println() so the numbers will all be on one line
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
33
do-while Loop




Chapter 3
Syntax
do
Something in body of loop
{ //body of loop
should eventually cause
Boolean_Expression to
First_Statement;
be false.
...
Last_Statement;
} while(Boolean_Expression);
Note Semicolon
Initialization code may precede loop body
Loop test is after loop body so the body must execute at
least once (minimum of at least one iteration) May
be to
Compare
While!!
either counting or sentinel loop
» One Good choice for sentinel loop
Java: an Introduction to Computer Science & Programming - Walter Savitch
34
for Loop



Good choice for counting (counter-controlled) loop
Initialization, loop test, and loop counter change are part of the
syntax
Syntax:
for(Initialization; Boolean_Expression; Update_Action)
loop body;
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
35
for Example

Count up from 1 to 5
for(int count = 1; count <= 5; count++) {
System.out.println(count + " ");
}

(only need {}’s if more than one statement
in body)
Don’t put a semicolon after the ) – you’ll
end the loop right there – no body!!

Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
36
for Loop
for(Initialization; Boolean_Expression; Update_Action)
loop body;


Execution sequence:
1. Initialization - executes only once, before the loop
body is executed the first time
2. Boolean_Expression - the loop test
3. loop body - execute only if loop test is true
4. Update_Action - typically changes the loop counter
5. Boolean_Expression - Repeat the loop test (step 2), etc.
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
37
for Flowchart

Chapter 3
<draw on board>
Java: an Introduction to Computer Science & Programming - Walter Savitch
38
Any for Could be done with a while
Initialization;
while (Boolean_Expression) {
loop body;
Update_Action;
}
int count = 1;
while (count <= 5) {
System.out.println(count + " ");
count++;
}
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
39
for Loop is THE Loop to use for
Counter-Controlled Repetition


If you know when writing the program how many
times loop will need to go around
If the program will know how many times loop will
need to go around by the time we get to the loop
the first time
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
40
Example where “program knows…”

…
Calculate yx (assume x is an integer)
// declare, ask for, and get y and x
…
double total = y;
for (int count = 1; count < x; count++) {
total = total * y;
}
// display result
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
41
BTW
Increment can be down, something more than 1
for (int cnt = 10; cnt > 0; cnt--){
…
for (int cnt = 0; cnt < 10; cnt = cnt + 2) {
…
 Expressions are optional (e.g. no initialization (leaving
others off would be weird)
int cnt = 0;
for ( ; cnt < 10; cnt++) {
…

Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
42
BTW




Chapter 3
Use of control variable inside loop
Displaying – legal and often done
Use in calculation – legal and sometimes done
e.g. sq = cnt * cnt;
Changing – legal but generally not a good idea
Java: an Introduction to Computer Science & Programming - Walter Savitch
43
Let’s Do A Problem
Calculate n!
int total = 1;
for (int cnt = 1; cnt < n; cnt++) {
total = total * cnt;
}
System.out.println(n + “! equals: “ + total);

Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
44
Nested for

for loops, like while loops and other control
structures, can be nested inside each other
for (int tens = 1; tens <= 2; tens++) {
for (int ones = 0; ones < 10; ones++) {
System.out.println(“ “ + tens + ones);
}
}
<Walkthrough>
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
45
Some more about designing for loops

If loop design isn’t coming naturally …
» You can write pseudocode without a loop and
look for a repeating pattern
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
46
Some Practical Considerations
When Using Loops





The most common loop errors are unintended infinite loops
and off-by-one errors in counting loops
Sooner or later everyone writes an unintentional infinite
loop
» To get out of an unintended infinite loop enter ^C
(control-C)
Off-by-one – loop goes around one too many or one too
few times
Another problem – testing for == or != with double / float
Loops should tested thoroughly, especially at the
boundaries of the loop test, to check for off-by-one and
other possible errors
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
47
Tracing a Variable in a Loop

Tracing a variable: print out the variable each time through
the loop – gathering evidence to capture bug

A common technique to test loop counters and troubleshoot
off-by-one and other loop errors

Can insert temporary output statements to print values.
Modern systems provide a built-in tracing system that
allows you to trace a variable without having to change your
program (We will talk about soon)

Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
48
Java Flow Control Statements
Sequence
 the default
 Java automatically
executes the next
instruction unless you
use a branching
statement
Chapter 3
Branching: Selection
 if
 if-else
 if-else if-else if- … - else
 switch
Branching: Repetition
 while
 do-while
 for
Java: an Introduction to Computer Science & Programming - Walter Savitch
49
break Statement





Chapter 3
As with in switch, break takes you out of loop
Any kind of loop – while, for, do-while
Example in display 3.13 on p176
Makes loop harder to understand
Use sparingly
Java: an Introduction to Computer Science & Programming - Walter Savitch
50
The exit Method




If you have a program situation where it is pointless to
continue execution you can terminate the program with the
exit(n) method
n is often used to identify if the program ended normally or
abnormally
n is conventionally 0 for normal termination and non-zero
for abnormal termination
We did this in our GUI programs
System.exit(0);

In that case because the program was really done – to end
GUI controls
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
51
The Type boolean




A primitive type
Can have expressions, values, constants, and variables just
as with any other primitive type
Only two values: true and false
Can use a boolean variable as the condition in an if
statement
if (classIsFull)
System.out.println(“Please Choose Another Class.”);
else
…
// register

Using a boolean variable as the condition can make an if
statement easier to read by avoiding a complicated
expression.
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
53
Truth Tables for boolean Operators
&& (and)
Value of A Value of B
|| (or)
A && B
A || B
true
true
true
true
true
true
true
false
false
true
false
true
false
true
false
false
true
true
false
false
false
false
false
false
! (not)
Chapter 3
Value of A Value of B
Value of A
!A
true
false
false
true
Java: an Introduction to Computer Science & Programming - Walter Savitch
55
Precedence Rules Revisited
Highest Precedence
 ( )’s override all precedence rules








First: the unary operators: +, -, ++, --, and ! (right to
left)
Second: the binary arithmetic operators: *, /, %
Third: the binary arithmetic operators: +, Fourth: the boolean operators: <, >, =<, >=
Fifth: the boolean operators: ==, !=
Sixth: the boolean operator &
Seventh: the boolean operator |
Eighth: the boolean operator &&
Ninth: the boolean operator ||

Lowest Precedence
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
57
Revisiting Short-Circuit Evaluation


Short-circuit evaluation—only evaluating as much of a boolean
expression as necessary.
Example:
if ((assign > 0) && ((total/assign) > 80))
System.out.println(“Good work”);
else
System.out.println(“Work harder.”);



If assign > 0 is false, then the complete expression cannot be true
because AND is only true if both operands are true.
Java will not evaluate the second part of the expression.
Here Short-circuit evaluation prevents a divide-by-zero exception
when assign is 0.
Chapter 3
Java: an Introduction to Computer Science & Programming - Walter Savitch
58