MSI 692: Special Topics in
Information Technology
Lecture 2
Sanjay Goel
University at Albany, SUNY
Fall 2004
Sanjay Goel, School of Business, University at Albany, SUNY
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Outline for the Class
Topics
•
•
•
•
•
Review
Statements and Control Flow
Boolean Expressions
Logical Operators
Logical Statements
– if, else if, nesting, switch
– Loops (while, for, break)
• Object-oriented Programming
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Statements and Control Flow
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Statements and Control Flow
Introduction
• We have learned how to write words in a language.
– We need to learn how to make sentences and paragraphs now.
• What do you need to do when you are doing a complex
problem?
– You have certain process to follow you have decision points
where you use your mind to make decisions.
• To build your own logic into a computer program you
need to write statements that capture your logic.
– How to introduce logic into the program?
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Statements and Control Flow
Statements
• Statements are roughly equivalent to sentences in
natural languages.
– A statement forms a complete unit of execution.
• It has many different types
– Variable Declaration Statement
– Expression Statement
• Assignment Statement
• Method Call Statement
– Control Flow Statement
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Statements and Control Flow
Variable declaration statements
• Variable declaration statements are used to identify
the type of the variable that is being declared
– start with a type and end with a semicolon
e.g.
int width, height, area;
String myString;
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Statements and Control Flow
Expressions
• An expression is a series of variables, operators, and method calls (constructed
according to the syntax of the language) that evaluates to a single value.
– Also defined as segments of code that perform computations and return values.
• Data type of the value returned by an expression depends on the elements
used in the expression.
• Compound expressions and statements can be constructed from various
smaller expressions as long as the data types required by one part of the
expression matches the data types of the other.
• If the order in which the operations in a compound expression need to be
performed is not explicitly indicated, the order is determined by the
precedence assigned to the operators
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Statements and Control Flow
Expression Statements
• Expression statements are formed by adding a semicolon at end of expression.
– These are used to compute and to assign values to variables and to help control the
execution flow of a program.
• The following types of expressions can be made into a statement by
terminating the expression with a semicolon (;):
–
–
–
–
Assignment expressions
Any use of ++ or –
Method calls
Object creation expressions
• Examples
–
–
–
–
aValue = 8933.234; //assignment statement
aValue++; //increment statement
System.out.println(aValue); //method call statement
Integer integerObject = new Integer(4); //object creation statement
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Statements and Control Flow
Assignment Statements
• Assignment statement can be any expression
involving the assignment operator.
– e.g. area = length * width;
• Method Call Statement invokes other functions
– Method call expression involves assignment when the
method returns a statement.
– e.g. System.out.println(“This is a test”);
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Statements and Control Flow
Blocks
• Block is a group of statements enclosed in braces.
– Blocks can occur within blocks and are called nested blocks
e.g.
{ float length = 2;
float width = 2;
float area = length * width;
System.out.println(“area = “ + area);
}
• Empty or Null Statement
– It is just a semicolon all by itself and results in no action.
e.g. Consider the following program – lines 2 & 3 are null statement
x = 2; 
Line 1
; 
Line 2
; 
Line 3
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Boolean & Logical Expressions
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Boolean Expressions
Introduction
• These are expressions which evaluate to true or false.
• The simplest Boolean expressions are true and false
• All conditional statements require Boolean expressions
to decide flow of logic
Sanjay Goel, School of Business, University at Albany, SUNY
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Boolean Expressions
Relational & Equality Operators
• Relational & Equality Operators
– used for comparing numeric values
<
>
<=
>=
==
!=
• These operators can be used between any two numeric values, e.g.
int a = 10;
int b = 20;
int c = 15
a > b  false
a < b  true
c < 10  false
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Logical Expressions
Operators
• These are used for combining multiple logical statements
• Three logical operators
– && - and
– ! - not
– || - or
• Examples: (See next page)
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Boolean & Logical Expressions
Exercises
•
•
•
Assume the following:
int i, j, k; boolean b; k = 10; j = 6; b = true;
Give the values of the following expressions, or state if illegal.
1. b
12.
!b & (j < 100)
2. !b
13.
k>j>3
3. !!!!!!b
14.
k > j && > 3
4. b || !b
15.
k > j && k > 3
5. b && !b
16.
b ||= false
6. b ^ true
17.
b |= false
7. b++
18.
k = (k>j)?k:j+1
8. b = 1
19.
(b && !b) | (!b & b)
9. b = 1 > 2
20.
1>2>3
10. true || 1234/26%3==1
11. true | 1234/26%3==1
21.
b = j > 0 && (i = k/j)!=0
Source: http://leepoint.net/notes-java/20language/20expressions/60booleanex.html
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Boolean Expressions
Exercises - Solutions
1
2
3
4
5
6
7
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10
11
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true b
false !b
true !!!!!!b
true b || !b
false b && !b
false b ^ true
illegalb++
illegalb = 1
false b = 1 > 2
true true || 1234/26%3==1
true true | 1234/26%3==1
false !b & (j < 100)
illegalk > j > 3
illegalk > j && > 3
15
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true k > j && k > 3
illegalb ||= false
true b |= false
10 k = (k>j)?k:j+1
false (b && !b) | (!b & b)
illegal1 > 2 > 3
true b = j > 0 && (i = k/j)!=0
Source: http://leepoint.net/notes-java/20language/20expressions/60booleanex.html
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Logical Statements
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Logical Statements
If Statement
• If statement is a conditional statement which proceeds
based on the evaluation of an expression, e.g.
if ( temp < 32)
System.out.println(“It is below freezing today”);
System.out.println(“Temperature = “ + temp);
• For multiple statements depending on a conditional we
use blocks, e.g.
if (temp < 32) {
System.out.println(“It is below freezing”);
System.out.println(“Keep all the windows closed”);
}
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Logical Statements
Sorting Algorithm
•Example to sort three numbers
/* Bubble Sort Algorithm. In this
* algorithm the smallest number
*/bubbles to the top
public static void main(String[] args) {
int a = Console.in.readInt();
int b = Console.in.readInt();
int c = Console.in.readInt();
int t; // Temporary variable
if (a > b) {
// Swap a and b
t = a;
a = b;
b = t;
}
Sanjay Goel, School of Business, University at Albany, SUNY
if (b > c) {
// swap b & c
t = b;
b = c;
c = t;
}
if (a > b) {
// Swap a & b
t = a;
a = b;
b = t;
}
System.out.println(“The sorted numbers
are: “ + a + “ “ + b + “ “ + c);
}
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Logical Statements
If else Statement
• If else statement is a bi-directional logic expression
• Syntax:
if (Boolean expression) {
Statement1
}
else {
Statement2
}
• Example:
if (temp < 32) {
System.out.println(“Close all the windows);
}
else {
System.out.println(“Open all the windows”);
}
Sanjay Goel, School of Business, University at Albany, SUNY
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Logical Statements
Nesting
• Nesting is allowed within if and if else statements
if (temp < 32) {
System.out.println(“Below Freezing”);
if (temp < 20)
System.out.println(“School is off ”);
else
System.out.println(“Go To School”);
}
else {
if (temp > 70)
System.out.println(“Go to the beach”);
else
System.out.println(“Go to school”);
}
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Logical Statements
Dangling Else
• else goes with the closest if statement
if (temp < 32)
if (temp < 20) {
System.out.println(“Stay indoors”);
}
else {
System.out.println(“Keep Warm”);
}
Here the else statement goes with the second if statement
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Logical Statements
Switch
• Switch statement conditionally executes statements based on an integer
expression
– it is a multidirectional logic statement that can replace multiple if statements
If
}
If
}
If
}
If
}
If
}
If
}
If
}
(day == 1) { System.out.println(“Sunday”);
(day == 2) {System.out.println(“Monday”);
(day == 3) {System.out.println(“Tuesday”);
(day == 4) {System.out.println(“Wednesday”);
(day == 5){System.out.println(“Thursday”);
(day == 6) {System.out.println(“Friday”);
(day == 7) {System.out.println(“Saturday”);
Sanjay Goel, School of Business, University at Albany, SUNY
switch (day) { case 1:
System.out.println(“Sunday”);
break;
case 2: System.out.println(“Monday”);
break;
case 3: System.out.println(“Tuesday”);
break;
case 4: System.out.println(“Wednesday”);
break;
case 5: System.out.println(“Thursday”);
break;
case 6: System.out.println(“Friday”);
break;
case 7: System.out.println(“Saturday”);
break;
default: System.out.println(“Illegal Value ” +
day);
}
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Logical Statements
While
• While statement is used to continually execute a block of
statements while a condition remains true.
• Syntax
– while (expression) { statement }
• Operation
– First, the while statement evaluates expression, which must return a boolean
value.
– If the expression returns true, then the while statement executes the
statement(s) associated with it.
– The while statement continues testing the expression and executing its
block until the expression returns false.
int i = 0;
while (i < 100) {
System.out.println(i);
i++;
}
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Logical Statements
do-while
• do-while is similar to while statement that instead of evaluating
the expression at the top of the loop, evaluates the expression at
the bottom.
– Statements associated with a do-while are executed at least once
• Syntax
– do { statement(s) } while (expression);
• Operation
– First the do statement executes the statement(s) associated with it.
– Then the while statement evaluates expression that must return a boolean.
– The do statement continues executing its block and the while statement
continues testing the expression until the expression returns false.
• Example
int i = 0;
do {
System.out.println(i);
i++;
} while (i < 100)
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Logical Statements
For
• for statement provides a compact way to iterate over a range of
values.
• Syntax
– for (initialization; termination; increment) { statement }
– The initialization is an expression that initializes the loop-it's executed
once at the beginning of the loop.
– The termination expression determines when to terminate the loop. This
expression is evaluated at the top of each iteration of the loop. When the
expression evaluates to false, the loop terminates.
– Finally, increment is an expression that gets invoked after each iteration
through the loop.
• e.g. Example
for (int i = 0; i < 10; i++) {
System.out.println(“My value is “+i);
}
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Logical Statements
Break
• Break causes an exit from the innermost enclosing loop.
– Break statements in loops should usually be avoided as they alter the flow
of control associated with loop statements
• from a clear "in at the top" to "out at the bottom" to something which is less
obvious.
– A break statement can sometimes be used "legitimately" to "break out" of
a continuous loop.
– However, in most cases there is an alternative strategy that will avoid the
use of a break.
• Continue: causes the current iteration of the program to stop and
the next to continue.
e.g.
for (int i=0; i<100; i++) {
System.out.println(“ Square of “ + i + “ = “ + i*i);
if (i*i > 10000) break;
}
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Logical Statements
Continue
• A continue statement returns to the beginning of the
innermost enclosing loop without completing the rest
of the statements in the body of the loop (for, while
and do-while).
– If you're in a for loop, the counter is incremented.
– Rarely used in practice
• e.g.
for (int i = 0; i < m.length; i++) {
if (m[i] % 2 == 0) continue; // process odd elements...
System.out.println(“Square of “ + i + “ = “ + i*i);
}
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Recap
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Recap
Statements
• Statements are roughly equivalent to sentences in natural
languages.
– A statement forms a complete unit of execution.
• It has many different types
– Variable Declaration Statement
– Expression Statement
• Assignment Statement
• Method Call Statement
– Control Flow Statement
• Variable declaration statements are used to identify the type of
the variable that is being declared
– These statements start with a type and end with a semicolon
Sanjay Goel, School of Business, University at Albany, SUNY
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Recap
Expression Statements
• Expression is a series of variables, operators, and method calls (constructed
according to the syntax of the language) that evaluates to a single value.
• Expression Statements are formed by adding a semicolon at end of
expression.
• Assignment statement
– Any expression involving the assignment operator.
– e.g. area = length * width;
• Method Call Statement
– Method call expression does not involve assignment.
– e.g. System.out.println(“This is a test”);
Sanjay Goel, School of Business, University at Albany, SUNY
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Recap
Blocks and Empty/Null Statements
• Block is a group of statements enclosed in braces. Blocks can
occur within blocks and are called nested blocks
e.g.{
float length = 2;
float width = 2;
float area = length * width;
System.out.println(“area = “ + area);
}
• Empty or Null Statement – It is just a semicolon all by itself
and results in no action.
e.g. x = 2;
;
;
Sanjay Goel, School of Business, University at Albany, SUNY
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Recap
Relational & Equality Operators
• Boolean Expressions
– Evaluate to true or false
– All conditional statements require Boolean expressions to decide flow of logic
• Relational & Equality Operators
– used for comparing numeric values
<
>
<=
>=
==
!=
• These operators can be used between any two numeric values, e.g.
int a = 10;
int b = 20;
int c = 15
a > b  false
a < b  true
c < 10  false
Sanjay Goel, School of Business, University at Albany, SUNY
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Recap
Logical Operators
• These are used for combining multiple logical
statements
• Three logical operators
– && - and
– ! - not
– || - or
Sanjay Goel, School of Business, University at Albany, SUNY
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Recap
Logical Statements
• if statement
– conditional statement which proceeds based on the evaluation of an
expression
– if (boolean-expression) then statement
• if-else statement
– bi-directional logic expression
– if (boolean expression) Statement11 else Statement2
• Nesting of if and if else statements
– Allows complex logic to be modeled
• Dangling else
– else goes with the closest if statement
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Recap
Switch Statement
• Switch is a multidirectional logic statement.
Switch(op) {
case a:
case b:
…
}
•
default:
statementa;
break;
statementb;
break;
statement;
Notes
1. Only one default statement in the switch
2. Case and Default cannot occur outside of the switch
3. break – exits the switch statement
4. Without break execution falls to next statement in the succeeding case
5. By not using break, you can combine multiple cases
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Recap
Loops or iterations
• while Statement
while (expression) statement
• do-while is similar to while where expression evaluated at bottom.
– do { statement(s) } while (expression);
• for Statement
for(init statement; termination expression; increment statement) {
block of statements
}
• break and continue
– break: causes an exit from the innermost enclosing loop.
– continue: causes the current iteration of the program to stop and the next
to continue.
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