if/else statement - Temple University

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Decision Making - CIS 1068
Program Design and Abstraction
Zhen Jiang
CIS Dept.
Temple University
1008 Wachman Hall, Main Campus
Email: [email protected]
4/13/2015
1
Table of Contents
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Taste of decision-making
If-else statement
Relational (Boolean) Expression
Complex Expression and Truth Table
Development Process
Multi-selection
Other If statements
Factoring
Other materials
Summary
2
Taste of decision making
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Rolling a dice.
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Sample execution (click on this link to try)
Each button in the above sample has
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No
Yes
Win/Lost?
Double the
money
Bankrupt
Restart
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If-else Statement
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if/else statement: A control structure that executes one block
of statements if a certain condition (checked by a Boolean
expression) is true, and a second block of statements if it is false.
We refer to each block as a branch.
General syntax:
if (<Boolean Expression>) {
<statement(s)> ;
} else {
<statement(s)> ;
}
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Example:
if (gpa >= 3.0) {
System.out.println("Welcome to Temple!");
} else {
System.out.println("Try applying to Penn.");
}
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No
Boolean
Expression
Action 2
else controlled
Yes
Action 1
If controlled
Action 3
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Figure 3.1 The Action
of the if-else
Statement, p142
http://www.cis.templ
e.edu/~jiang/BankBa
lance.pdf
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Relational (Boolean) Expression
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Relational expressions (p148) have
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numeric arguments and
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Boolean values.
A simple expression uses one of the following six
relational operators to check the relation of two values :
Operator
Meaning
Example
Value
==
equals
1 + 1 == 2
true
!=
does not equal
3.2 != 2.5
true
<
less than
10 < 5
false
>
greater than
10 > 5
true
<=
less than or equal to
126 <= 100
false
>=
greater than or equal to
5.0 >= 5.0
true
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Lesson 3 (p148)
!!! error: “(a=2)” instead of “(a==2)”
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Relational operators have lower precedence than math
operators.
5 * 7
5 * 7
35
35
true
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>=
>=
>=
>=
3 + 5 * (7 - 1)
3 + 5 * 6
3 + 30
33
Relational operators cannot be chained (unlike math
operators)
2 <= x <= 10
error!
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The boolean type has two possible values:
true and false
boolean variables are declared and initialized
just like other primitive data types:
boolean iAmSoSmrt = false;
System.out.print(iAmSoSmrt);
boolean minor = (age < 21);
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//just like int i = 2;
//print out ‘false’;
//just like int x = y*2;
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Complex Expression
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A complex Boolean expression is built out of simple
expressions connected with logical operators.
Logical operators have (p153)
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boolean arguments and
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boolean values
Operator
&&
Description
and
Example
(9 != 6) && (2 < 3)
Result
true
||
or
(2 == 3) || (-1 < 5)
!
not
!(7 > 0)
true
false
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Sub-expression S1 Sub-expression S2
S1 && S2
S1 || S2
!S1
True (T)
T
T
T
F (False)
T
F
F
T
F
F
T
F
T
T
F
F
F
F
T
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What is the result of each of the following expressions?
int x = 42;
int y = 17;
int z = 25;
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y < x && y <= z
x % 2 == y % 2 || x % 2 == z % 2
x <= y + z && x >= y + z
!(x < y && x < z)
(x + y) % 2 == 0 || !((z - y) % 2 == 0)
Answers: true, false, true, true, false
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You can compare char values with relational operators.
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'a' < 'b' and 'Q' != 'q'
Caution: You should NOT use these operators on a String!
Example:
// print the alphabet
if(c>='a' && c<='z' && c>='A' && c<='Z') {
//ASCII
System.out.print(c);
}
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Objects (such as String) should be compared for equality by
calling a method named equals (P155).
Example:
Scanner console = new Scanner(System.in);
System.out.print("What is your name? ");
String name = console.next();
if (name.equals("Barney")) {
System.out.println("I love you, you love me,");
System.out.println("We're a happy family!");
}
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There are more methods of a String object that can be
used in <test> conditions.
Method
Description
equals(str)
whether this string contains exactly the
same characters as the other string
equalsIgnoreCase(str)
whether this string contains the same
characters as the other, ignoring uppervs. lowercase differences
startsWith(str)
whether this string’s beginning matches
the argument
endsWith(str)
whether this string’s end matches the
argument
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Hypothetical examples, assuming the existence of various
String variables:
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if (title.endsWith("M.D.")) {
System.out.println("What's your number?");
}
if (fullName.startsWith("Giorgio")) {
System.out.println("When are you retiring?");
}
if (lastName.equalsIgnoreCase("lumBerg")) {
System.out.println("I need your TPS reports!");
}
if (name.toLowerCase().indexOf("sr.") >= 0) {
System.out.println("You must be old!");
}
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Exercises
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Boolean expression
21
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Development of a correct decision
program
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Grade.java
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Development Process
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Identify two exclusive options
Implement each handling in different action parts
Identify the situation (values) for option selection
Make a boolean expression so that all situation
values for option part 1 will lead to its evaluation
true.
Verify whether all situation values for option part
2 will lead to the evaluation of this expression in
test false, otherwise, revise the above expression!
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Exercise of simple expression
development
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Correct use of relational operator
Development of a complete expression
25
Multiple Selection
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Concepts
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Nested if, p158
Multibranch If-else, p160
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Idea of multiple selection
If
case 1
Else
if
case 2
else
…
//end of case 2 if
//End of case 1 if
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Example 1
Pass/fail => Excellent/pass/fail
More cases, p164
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http://www.cis.temple.edu/~jiang/Grader.pdf
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Example 2
15% tip of a meal, with the minimum
$1, but cannot exceed the amount of
the meal itself.
Case 1? Case 2? Case 3?
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Other if statements
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if statement: A control structure that executes a block
of statements only if a certain condition is true.
General syntax:
if (<test>) {
<statement(s)> ;
}
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Example (with grade inflation):
if (gpa >= 2.0) {
System.out.println("You get an A!");
}
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Chained if/else statement (also called mulit-branch if
statement): A chain of if/else that can select between many
different outcomes based on several tests.
General syntax:
if (<test>) {
<statement(s)> ;
} else if (<test>) {
<statement(s)> ;
} else {
<statement(s)> ;
}

Example:
if (number > 0) {
System.out.println("Positive");
} else if (number < 0) {
System.out.println("Negative");
} else {
System.out.println("Zero");
}
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if (<test>) {
<statement(s)>;
} else if (<test>) {
<statement(s)>;
} else {
<statement(s)>;
}
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if (<test>) {
<statement(s)>;
}
if (<test>) {
<statement(s)>;
}
if (<test>) {
<statement(s)>;
}
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A chained if/else can end with an if or an else.
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If it ends with else, one of the branches must be taken.
If it ends with if, the program might not execute any branch.
if (<test>) {
<statement(s)>;
} else if (<test>) {
<statement(s)>;
} else {
<statement(s)>;
}
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if (<test>) {
<statement(s)>;
} else if (<test>) {
<statement(s)>;
} else if (<test>) {
<statement(s)>;
}
35
if (<test>) {
<statement(s)>;
} else if (<test>) {
<statement(s)>;
} else {
<statement(s)>;
}
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if (<test>) {
<statement(s)>;
} else if (<test>) {
<statement(s)>;
} else if (<test>) {
<statement(s)>;
}
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Scope
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scope: The portion of a program where a given variable exists.
 A variable's scope is from its declaration to the end of the { }
braces in which it was declared.
public class ScopeExample {
public static void main(String[] args) {
int x = 3;
int y = 7;
if(x > 0 && y > 0) {
int sumPositives = x + y;
} else {
sumPositives = 0; // illegal: sumPositives is out of scope
}
// illegal: sumPositives is out of scope
System.out.println("sum = " + sumPositives);
}
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Why not just have the scope of a variable be the whole program?
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String message;
if (gpa >= 3.0) {
message = "Welcome to Temple!";
}
if (gpa >= 2.0) {
message = "Have you considered applying to Penn?";
}
if (gpa < 2.0) {
message = "I hear Harvard still needs students...";
}
System.out.println(message);
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The compiler will complain that "variable message might not
have been initialized". Why?
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The solution:
String message;
if (gpa >= 3.0) {
message = "Welcome to Temple!";
} else if (gpa >= 2.0) {
message = "Have you considered applying to Penn?";
} else { // gpa < 2.0
message = "I hear Harvard still needs students...";
}
System.out.println(message);
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Factoring if/else
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factoring: extracting common/redundant code
Factoring if/else code reduces the size of the if and else
statements
Factoring tips:
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If the start of each branch is the same, move it before the
if/else.
If the end of each branch is the same, move it after the if/else.
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if (money < 500) {
System.out.println("You have, $" + money + " left.");
System.out.print("Caution! Bet carefully.");
System.out.print("How much do you want to bet? ");
bet = console.nextInt();
} else if (money < 1000) {
System.out.println("You have, $" + money + " left.");
System.out.print("Consider betting moderately.");
System.out.print("How much do you want to bet? ");
bet = console.nextInt();
} else {
System.out.println("You have, $" + money + " left.");
System.out.print("You may bet liberally.");
System.out.print("How much do you want to bet? ");
bet = console.nextInt();
}
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System.out.println("You have, $" + money + " left.");
if (money < 500) {
System.out.print("Caution! Bet carefully.");
} else if (money < 1000) {
System.out.print("Consider betting moderately.");
} else {
System.out.print("You may bet liberally.");
}
System.out.print("How much do you want to bet? ");
bet = console.nextInt();
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Multiple Selection Exercise
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Other materials (reading
materials)
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Conditional Operator (P168)
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( Boolean Expression)? <value1> :<value 2>;
Precedence Rules (P172)
Input & Output of Boolean Variables
(P174)
Switch (P176-180)
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Switch([variable])
Case [constant value]:
Break (or no break), p180
46
Summary
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BankBalance.java, p142
Grader.java, p164
Exercises, slides 21, 25, 45
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Boolean Expression and its evaluation (P148-157, P86)
Precedence Rule (P172)
Input & output of boolean variables (P174)
If-else statement (P144) and If statement (P145)
Development of If-else or If statement (5 phase development)
Action factoring
Development of multiple choice selection
Nested if (P158) and Multi-branch if statement (P160)
Conditional operator (P168)
Scope
Switch (P176)
48
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