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Decision Structures
Lecture 3
MIT 12043, Fundamentals of Programming
By: S. Sabraz Nawaz
3-2
Chapter Topics
o Relational Operators
o The if Statement
o The if-else Statement
o The if-else-if Statement
o Nested if statements
o The switch Statement
MIT 12043, By S. Sabraz Nawaz
Relational Operators
• Relational operators in Java are used to compare two variables.
• Java provides six relational operators:
• The result of any relational operator is “true” or “false”
Equal ==Greater than >Less than <Greater than or equal to >=
Less than or equal to <=
Unequal !=
MIT 12043, By S. Sabraz Nawaz 3
Boolean Expressions
int a = 3; b = 4;
a == b will evaluate to false
a != b will evaluate to true
a > b will evaluate to false
a < b will evaluate to true
a >= b will evaluate to false
a <= b will evaluate to true
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Sequence Structure
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Decision Structure
• Programs often need more than one path of execution
• Many algorithms require a program to execute some
statements only under certain circumstances. This can be
accomplished with a decision structure
• In a decision structure’s simplest form, a specific action is
taken only when a condition exists
• If the condition does not exist, the action is not performed
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Decision Structure…
• Simple decision structure logic
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Decision Structure…
• Three-action decision structure logic
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Selection Statements
• The control statement that allows alternative actions
based upon conditions that are evaluated at run time are
generally called selection statements
• One way to code a decision structure in Java is with the
if statement
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3-10
The if Statement
• The if statement is used to create a decision structure, which
allows a program to have more than one path of execution
o The if statement causes one or more statements to execute only when a boolean
expression is true.
• The if statement decides whether a section executes or not
if (boolean expression is true)
execute this statement;
MIT 12043, By S. Sabraz Nawaz
3-11
The if Statement…
• A block if statement may be modeled as:
if (coldOutside)
{
wearCoat;
wearHat;
wearGloves;
}
Note the use of curly
braces to block several
statements together.
MIT 12043, By S. Sabraz Nawaz
3-12
if Statements and Boolean Expressions
if (x > y)System.out.println("X is greater than Y");
if(x == y)System.out.println("X is equal to Y");
if(x != y)
{System.out.println("X is not equal to Y");
x = y;
System.out.println("However, now it is.");
}
MIT 12043, By S. Sabraz Nawaz
3-13
Programming Style and if Statements
• An if statement can span more than one line; however, it is still
one statement.
if (average > 85)
grade = ′A′;
is functionally equivalent to
if(average > 95) grade = ′A′;
MIT 12043, By S. Sabraz Nawaz
3-14
Block if Statements
• Conditionally executed statements can be grouped into a block by using curly braces {} to enclose them.
• If curly braces are used to group conditionally executed statements, the if statement is ended by the closing curly brace.if (expression)
{
statement1;
statement2;
}Curly brace ends the statement.
MIT 12043, By S. Sabraz Nawaz
3-15
Block if Statements
• Remember that when the curly braces are not used, then only the next statement after the if condition will be executed conditionally
if (expression)
statement1;
statement2;
statement3;
Only this statement is conditionally executed.
MIT 12043, By S. Sabraz Nawaz
Example 01
• Write a Java program that takes a person’s age as input and
decides whether that person is eligible to vote
• If the person’s age is above 18, he or she is eligible to vote
• The program should display a message whether he or she is
eligible to vote
MIT 12043, By S. Sabraz Nawaz 16
Example 01…
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Example 02
• Write a Java program that takes a person’s age and his or her
name as inputs and decides whether that person is eligible to
vote
• If the person’s age is above 18, he or she is eligible to vote
• The program should display a message with the Name of the
person and his or eligibility to vote
MIT 12043, By S. Sabraz Nawaz 18
Example 02
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3-20
if-else Statements
• The if-else statement adds the ability to conditionally
execute code when the if condition is false.
if (expression)
statementOrBlockIfTrue;
else
statementOrBlockIfFalse;
• It works the same way as the if statement except that, when the
condition is false, the statement within the else clause executes
MIT 12043, By S. Sabraz Nawaz
3-21
if-else Statement Flowcharts
Wear a coat.
YesIs it cold
outside?
Wear t-Shirt
No
MIT 12043, By S. Sabraz Nawaz
Example 03
• Write a Java program that takes a person’s age and his or her
name as inputs and decides whether that person is eligible to
vote or not
• If the person’s age is above 18, he or she is eligible to vote
• The program should display a message with the Name of the
person and his or eligibility to vote
MIT 12043, By S. Sabraz Nawaz 22
Example 03…
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Exercise
• A company’s payment structure to its employees is as
followso If the Basic salary exceeds 30,000/=, the Dearness Allowance 40% the BS,
otherwise 50% of the BS
o If the BS exceeds 25,000/=, the House Rent Allowance is 20% of the BS,
otherwise 3,000 of the BS
o Income Tax is calculated at the rate of 12% if the Total Salary exceeds
50,000/=
• Write a Java program that takes BS as input, calculate
DA, HRA, IT, Total Salary, Net Salary, and display
them all on screen
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Solution
25
Solution…
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3-27
Nested if Statements
• To test more than one condition, an if statement can
be nested inside another if statement
• If an if statement appears inside another if
statement (single or block) it is called a nested if
statement
• The nested if is executed only if the outer if
statement results in a true condition
MIT 12043, By S. Sabraz Nawaz
3-28
Alignment and Nested if Statements
if (expression)
{
if (expression)
{
statement;
}
else
{
statement;
}
}
else
{
statement;
}
This if and else
go together.This if and
else
go together.
MIT 12043, By S. Sabraz Nawaz
Example 04
• For example, consider a banking program that
determines whether a bank customer qualifies for a
special, low interest rate on a loan
• To qualify, two conditions must exist:
1. The customer’s salary must be at least Rs.30,000
2. The customer must have held his or her current job for at
least 02 years
MIT 12043, By S. Sabraz Nawaz 29
Example 04…
MIT 12043, By S. Sabraz Nawaz 30
if-else-if Statements
• The if-else-if statement tests a series of conditions
• It is often simpler to test a series of conditions with the if-else-
if statement than with a set of nested if-else statements
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3-32
if-else-if Syntax
if (expression_1)
{
statement;
statement;
etc.
}
else if (expression_2)
{
statement;
statement;
etc.
}
Insert as many else if clauses as necessary
else
{
statement;
statement;
etc.
}
If expression_1 is true these statements are
executed, and the rest of the structure is ignored.
Otherwise, if expression_2 is true these statements are
executed, and the rest of the structure is ignored.
These statements are executed if none of the
expressions above are true.
MIT 12043, By S. Sabraz Nawaz
Example 05
• Your lecturer has asked you to write a program that will allow
a student to enter a test score and then display the grade for
that score
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34
Logical Operators
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Logical Operators
• Logical operators connect two or more relational
expressions into one or reverse the logic of an expression.
• Java provides two binary logical operators,
• && and ||
• used to combine two boolean expressions into a single expression
• It also provides the unary ! Operatoro reverses the truth of a boolean expression
MIT 12043, By S. Sabraz Nawaz 36
MIT 12043, By S. Sabraz Nawaz 37
Logical Operators…
Operator Meaning Effect
&& AND Connects two boolean expressions into one. Both expression must be true for the overall expression to be true
|| OR Connects two boolean expressions into one. One or both expressions must be true for the overall expression to be true. It is only necessary for one to be true, and it does not matter which one.
! NOT Reverses the truth of a boolean expression. If it is applied to an expression that is true, the operator returns false. If it is applied to an expression that is false, the operator returns true
Expression Meaning
x > y && a < b Is x greater than y AND is a less than b?
x == y || x == z Is x equal to y OR is x equal to z?
!(x > y) Is the expression x > y NOT true?
MIT 12043, By S. Sabraz Nawaz 38
Logical Operators…
Truth table for the && operator
Expression Value of the Expression
true && false false
false && true false
false && false false
true && true true
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Truth table for the ||operator
Expression Value of the Expression
true || false true
false || true true
false || false false
true || true true
MIT 12043, By S. Sabraz Nawaz 40
Truth table for the ! operator
Expression Value of the Expression
!true false
!false true
MIT 12043, By S. Sabraz Nawaz 41
Example 06 – Improved Ex 05
42
43
The % Operator
• The above modulus operator returns the remainder
• Eg.o 3 % 2 1 (remainder is one)
o 15 % 4 3 (remainder is three)
• Odd or Even number finder
MIT 12043, By S. Sabraz Nawaz 44
Comparing String Objects
• You cannot use relational operators to compare String objects
• To compare, you should use the String class’s equals method.
• The general form of the method is as follows:
StringReference1.equals(StringReference2)
o StringReference1 is a variable that references a String object, and StringReference2 is another variable that references a String object
• The method returns true if the two strings are equal, or false if they are not equal. Here is an example:
if (name1.equals(name2))
• The expression in the if statement will return true if they are the same, or false if they are not the same
MIT 12043, By S. Sabraz Nawaz 45
Example 07
• Note: the equal method is case sensitive; if you want to ignore the case
case, use string. equalsIgnoreCase() method
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Classroom Exercise
A car rental company has three types of cars; Premio, Alien, andCivic. The rental scheme is as follows.
Write a Java program to input Vehicle Type, Number of DaysRented, and Kilometres run to calculate the Total Rent Payableand display on the screen.
MIT 12043, By S. Sabraz Nawaz 47
Type Premio Alien Civic
Fixed Rent per day 1,500/- 2,000/- 2,500/-
Rate per KM 20/- 30/- 40/-
Free Allowance per day 80/- 60/- 50/-
• For decisions involving many possible paths and values we use the switch statement
• The switch statement can have a number of possible execution paths
• At the end of each sequence we insert a breakstatement that means ‘go to the end of the switch’
• A switch works with the byte, short, char, andint primitive data types. It works with the String class as well.
The switch Statement
48
The switch Statement
switch (variable)
{
case value1:
statement;
statement;
break;
case value2:
statement;
statement;
break;
...
...
default:
statement;
break;
}
Don’t forget breaks!
switch
case
default
break
Reserved words
49
• You can have any number of case statements within a switch.
Each case is followed by the value to be compared to and a
colon
• The value for a case must be the same data type as the variable
in the switch, and it must be a constant or a literal (character)
• When the variable being switched on is equal to a case, the
statements following that case will execute until a break
statement is reached
Rules apply to a switch statement
50
• When a break statement is reached, the switch terminates, and the flow of control jumps to the next line following the switch statement
• Not every case needs to contain a break. If no break appears, the flow of control will fall through to subsequent cases until a break is reached
• A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case
Rules apply to a switch statement
51
• The same case can have two or more labels. For
example:
switch (num)
{
case 1:
case 2:System.out.println (“Stand up and sing a song");
break;
case 3:
...
}
switch - Example
52
53
Month Finder
54
Switch with Multiple case labels
Next Chapter: Looping
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