Selection Statements in Programming
Selections
The program can decide which
statements to execute based on a
condition.
Selection statements use conditions that
are Boolean expressions.
A
Boolean expression
is an expression that
evaluates to a
Boolean value
:
true
or
false
.
Object Oriented Programming
1
Boolean data type
The
boolean
data type declares a
variable with the value either
true
or
false
(they are literals like number).
Object Oriented Programming
2
Ex.
boolean
lightsOn =
true
;
If (one-way) statement
An
if
statement is a construct that
enables a program to specify alternative
paths of execution.
if
(boolean-expression)
{statement(s);
}
Object Oriented Programming
3
Cont.
Object Oriented Programming
4
Two-way if else
An
if-else
statement decides the
execution path based on whether the
condition is
true or false.
if
(boolean-expression) {statement(s)-for-the-true-case;
}
else
{statement(s)-for-the-false-case;
}
Object Oriented Programming
5
Nested or Multi-way if
An
if
statement can be inside another
if
statement to form a nested
if
statement.
The
if (j > k)
statement is nested inside
the
if (i > k)
statement in the following.
Object Oriented Programming
6
Generating Random Numbers
You can use
Math.random()
to obtain a
random double value between
0.0
and
1.0
,
excluding
1.0
.
SubtractionQuiz.java
Object Oriented Programming
7
Computing Body Mass Index (BMI)
BMI = weigh in KG/ (height in meters)
2
ComputeAndInterpretBMI.java
Object Oriented Programming
8
Logical Operators
The logical operators
!
,
&&
,
||
, and
^
can
be used to create a compound Boolean
expression.
Object Oriented Programming
9
Object Oriented Programming
10
Cont.
Write a program that checks whether a
number is divisible by
2
and
3
, by
2
or
3
,
and by
2
or
3
but not both.
TestBooleanOperators.java
LeapYear.java
–
(
A leap year is divisible by 4 but not by 100
or divisible by 400.)
Lottery.java
Object Oriented Programming
11
Switch statement
A
switch
statement executes statements based
on the value of a variable or an
expression.
switch
(
switch
-expression) {case
value1: statement(s)1;
break
;
case
value2: statement(s)2;
break
;
case
valueN: statement(s)N;
break
;
default
: statement(s)-
for
-
default
;
}
Object Oriented Programming
12
The
switch
statement observes the following rules:
■
The
switch-expression
must yield a value of
char
,
byte
,
short
,
int
,
or
String
type and must always be enclosed in parentheses. (The
char
and
String
types will be introduced in the next chapter.)
■
The
value1
, . . ., and
valueN
must have the same data type as the
value of the
switchexpression
. Note that
value1
, . . ., and
valueN
are
constant expressions, meaning that they cannot contain variables, such
as
1 + x
.
■
When the value in a
case
statement matches the value of the
switch-expression
, the statements
starting from this case
are executed
until either a
break
statement or the end of the
switch
statement is
reached.
■
The
default
case, which is optional, can be used to perform actions
when none of the specified cases matches the
switch-expression
.
■
The keyword
break
is optional. The
break
statement immediately
ends the
switch
statement.
Conditional Expression
A conditional expression evaluates an
expression based on a condition.
Object Oriented Programming
13
Operator Precedence and
Associativity
Operator precedence and associativity
determine the order in which operators
are
evaluated
Object Oriented Programming
14
Object Oriented Programming
15
Decreasing order of precedence
All binary operators except assignment operators are
left associative
.
Mathematical functions
They can be categorized as trigonometric
methods, exponent methods,
and service
methods.
They all are used like Math.?
Object Oriented Programming
16
Cont.
Object Oriented Programming
17
Character data type
A character data type represents a single
character.
Computers use binary numbers internally.
A character is stored in a computer as a
sequence of 0s and 1s.
Mapping a character to its binary
representation is called
encoding
.
Object Oriented Programming
18
Cont.
Java supports
Unicode
, an encoding
scheme established by the Unicode
Consortium to support the interchange,
processing, and display of written texts.
Unicode was originally designed as a 16-
bit character encoding.
The Unicode
standard therefore has been
extended to allow up to 1,112,064
characters.
Object Oriented Programming
19
Cont.
Those characters
that go beyond the original
16-bit limit are called
supplementary
characters
. Java supports
the supplementary
characters.
A 16-bit Unicode takes two bytes, preceded
by
\u
, expressed in four hexadecimal digits
that run from
\u0000
to
\uFFFF.
Unicode includes ASCII code, with
\u0000
to
\u007F
corresponding
to the 128 ASCII
characters.
Object Oriented Programming
20
Escape Sequence for special
characters
Java uses a special notation to represent
special characters.
Object Oriented Programming
21
Casting between character and
numeric type
A
char
can be cast into any numeric type,
and vice versa.
Object Oriented Programming
22
Comparing and Testing Character
Two characters can be compared using
the relational operators just like
comparing two
numbers.
This is done by comparing the Unicodes
of the two characters
Object Oriented Programming
23
Cont.
Object Oriented Programming
24
String
A string is a sequence of characters.
To represent a string of characters, use
the data type called
String
. For example;
String message =
"Welcome to Java"
;
String
is a predefined class in the Java
library, just like the classes
System
and
Scanner
.
The
String
type is not a primitive type. It
is known as a
reference type
.
Object Oriented Programming
25
Cont.
Strings are objects in Java.
Instance method and static method
Object Oriented Programming
26
Reading a string from the console
Object Oriented Programming
27
Enter three words separated by spaces: Welcome to Java
Reading single character
Object Oriented Programming
28
char c = (char) System.in.read();
String Comparison
Object Oriented Programming
29
OrderTwoCities.java
Obtain substring
You can obtain a single character from a
string using the
charAt
method. You can
also obtain a substring from a string using
the
substring
method in the
String
class
Object Oriented Programming
30
Finding a Character or a Substring
in a String
The
String
class provides several
versions of
indexOf
and
lastIndexOf
methods to find a character or a
substring in a string.
Object Oriented Programming
31
Conversion between strings and
numbers
You can convert a numeric string into a
number. To convert a string into an
int
value,
use the
Integer.parseInt
method, as
follows.
int
intValue = Integer.parseInt(intString);
(“123”)
Double
doubleValue =
Double.parseDouble(doubleString);
(“65.35”)
If the string is not a numeric string, the
conversion would cause a runtime error.
Object Oriented Programming
32
Cont.
You can convert a number into a string,
simply use the string concatenating
operator as
follows:
String s = number +
""
;
Object Oriented Programming
33
HexDigit2Dec.java
Formatting console output
You can use the
System.out.printf
method to display formatted output on
the
console.
Object Oriented Programming
34
Cont.
Object Oriented Programming
35
System.out.printf(format, item1, item2, ..., item
k
)
Cont.
Object Oriented Programming
36
Cont.
By default, the output is right justified.
You can put the minus sign (
-
) in the
format specifier to specify that the item is
left justified in the output within the
specified field.
Object Oriented Programming
37
Loops
A loop can be used to tell a program to
execute statements repeatedly.
While
Do… while
For
Object Oriented Programming
38
While
SubtractionQuizLoop.java
Controlling loop with sentinel value
SentinelValue.java
Input/output redirection
java SentinelValue < input.txt
java ClassName > output.txt
Object Oriented Programming
39
Do…while
A
do-while
loop is the same as a
while
loop except that it executes the loop
body first and then checks the loop
continuation condition.
Object Oriented Programming
40
For loop
A
for
loop has a concise syntax for
writing loops.
Object Oriented Programming
41
Which loop to use?
You can use a
for
loop, a
while
loop, or a
do-while
loop, whichever is convenient.
Pre test loop (while and for)
Post test loop (do…while)
Object Oriented Programming
42
Break and Continue
The
break
and
continue
keywords
provide additional controls in a loop.
Object Oriented Programming
43
Methods
Methods can be used to define reusable
code and organize and simplify coding.
Object Oriented Programming
44
Defining method
A method definition consists of its
method name, parameters, return value
type, and body.
Object Oriented Programming
45
Modularizing Code
Modularizing makes the code easy to
maintain and debug and enables the code to
be
reused.
By encapsulating the code for obtaining the
gcd in a method, this program has several
advantages:
1. It isolates the problem for computing the gcd from the rest
of the code in the main method. Thus, the logic becomes clear
and the program is easier to read.
2. The errors on computing the gcd are confined in the
gcd
method, which narrows the
scope of debugging.
3. The
gcd
method now can be reused by other programs.
Object Oriented Programming
46
Overloading methods
Overloading methods enables you to
define the methods with the same name
as long as their signatures are different.
Overloaded methods must have different
parameter lists. You cannot overload
methods based on different modifiers or
return types.
TestMethodOverloading.java
Object Oriented Programming
47
Cont.
Can you invoke the
max
method with an
int
value and a
double
value, such as
max(2,
2.5)
?
If so, which of the
max
methods is invoked?
The answer to the first question is yes.
The answer to the second question is that the
max
method for finding the maximum of two
double
values is invoked.
The argument value
2
is automatically
converted into a
double
value and passed to
this method.
The Java compiler finds the method that best matches a method invocation. Since the method
max(int, int)
is a better matches for
max(3, 4)
than
max(double, double)
,
max(int, int)
is used to invoke
max(3, 4)
.
Object Oriented Programming
48
Scope of Variables
The scope of a variable is the part of the
program where the variable can be
referenced.
The scope of a method parameter covers
the entire method.
A variable declared in the initial-action part
of a
for
-loop header has its scope in the
entire loop.
However, a variable declared inside a
for
-
loop body has its scope limited in the loop
body from its declaration to the end o the
block that contains the variable
Object Oriented Programming
49
You can declare a local variable with the
same name in different blocks in a method,
but you cannot declare a local variable twice
in the same block or in nested blocks
Object Oriented Programming
50
Cont.
Object Oriented Programming
51
for
(
int
i =
0
; i <
10
; i++) {}
System.out.println(i);
The last statement would cause a syntax error, because variable
i
is not
defined outside
of the
for
loop.
Single Dimensional Array
elementType[] arrayRefVar
: The
elementType
can be any data type, and
all elements in the array will have the
same data type
double
[] myList
: For example, this code
declares a variable
myList
that
references an array of double elements
Object Oriented Programming
52
Cont.
Unlike declarations for primitive data type
variables, the declaration of an array
variable
does not allocate
any space in
memory for the array.
It creates only a storage location for the
reference to an array.
If a variable does not contain a reference
to an array, the value of the variable is
null
Object Oriented Programming
53
Cont.
You can create an array by using the
new
operator and assign its reference to the
variable with the following syntax:
arrayRefVar =
new
elementType[arraySize];
This statement does two things: (1) it
creates an array using
new
elementType[arraySize]
;
(2) it assigns the reference of the newly
created array to the variable
arrayRefVar
.
Object Oriented Programming
54
Cont.
Declaring an array variable, creating an
array, and assigning the reference of the
array to the variable can be combined in
one statement as:
elementType[] arrayRefVar =
new
elementType[arraySize];
OR
elementType arrayRefVar[] =
new
elementType[arraySize];
Object Oriented Programming
55
Cont.
Size can be obtained using
arrayRefVar.length
When an array is created, its elements are
assigned the default value of
0
for the
numeric primitive data types,
\u0000
(NULL)
for
char
types, and
false
for
boolean
types.
Each element in the array is represented
using the following syntax, known as an
indexed variable:
arrayRefVar[index];
Object Oriented Programming
56
Initializing Array
Java has a shorthand notation, known as the
array initializer
, which
combines the declaration, creation, and initialization of an array in
one statement using the following syntax:
elementType[] arrayRefVar = {value0, value1, ..., valuek};double
[] myList = {1.9
,
2.9
,
3.4
,
3.5
};
OR
double
[] myList =
new double
[
4
];
myList[
0
] =
1.9
;
myList[
1
] =
2.9
;
myList[
2
] =
3.4
;
myList[
3
] =
3.5
;
Splitting it would cause a syntax error. Thus, the next statement is
wrong:
double
[] myList;
myList = {1.9
,
2.9
,
3.4
,
3.5
};
Object Oriented Programming
57
Processing Array
Initializing arrays with input
values/Initializing arrays with random
values
Displaying arrays
Summing all elements
Finding the largest element
Finding the smallest index of the largest
element
Random shuffling
Object Oriented Programming
58
Foreach loop
Java supports convenient
for
loop, known
as a
foreach loop
, which enables you to
traverse the array sequentially without
using an index variable.
for
(
double
e: myList) {System.out.println(e); }
Object Oriented Programming
59
Copying Arrays
To copy the contents of one array into
another, you have to copy the array’s
individual elements into the other array.
list2 = list1; -
However, this statement
does not copy the contents of the array
referenced by
list1
to
list2
, but instead
merely copies the reference value from
list1
to
list2
.
Object Oriented Programming
60
Cont.
Object Oriented Programming
61
Another approach is to use the
arraycopy
method in the
java.lang.System
class to copy arrays
instead of using a loop
arraycopy(sourceArray, srcPos, targetArray, tarPos, length);
System.arraycopy(sourceArray,
0
, targetArray,
0
, sourceArray.length);
Passing array to the method
When passing an array to a method, the
reference of the array is passed to the
method.
Object Oriented Programming
62
Returning array from the method
When a method returns an array, the
reference of the array is returned.
You can pass arrays when invoking a
method. A method may also return an
array.
Object Oriented Programming
63
Counting occurrence of each letter
CountLettersInArray.java
Object Oriented Programming
64
Variable length arguments list
A variable number of arguments of the same type
can be passed to a method and
treated as an
array.
You can pass a variable number of arguments of
the same type to a method.
Declared as follows:
typeName... parameterName
Only one
variable-length parameter may be
specified in a method, and this parameter must be
the last parameter.
Any regular parameters must precede it.
VarArgsDemo.java
Object Oriented Programming
65
Searching array
Linear search and Binary search.
If an array is sorted, binary search is more
efficient than linear search for finding an
element in the array.
Linear search
Object Oriented Programming
66
Cont.
Binary search
Object Oriented Programming
67
Sorting array
Sorting, like searching, is a common task
in computer programming.
Many different algorithms have been
developed for sorting.
Bubble sort, insertion sort, selection sort,
quick sort, merge sort, radix sort etc.
SelectionSort.java
Object Oriented Programming
68
Array class
The
java.util.Arrays
class contains
useful methods for common array
operations such as sorting and searching.
Object Oriented Programming
69
Binary search
Sort
Command line arguments
The
main
method can receive string
arguments from the command line.
commandLineArgs.java
Object Oriented Programming
70
Multi dimensional array
An element in a two-dimensional array is
accessed through a row and column
index.
elementType[][] arrayRefVar;
OR
elementType arrayRefVar[][];
int[][] matrix = new int[3][3];
matrix.length gives entire array size
matrix[0].length gives size of 0
th
row
Object Oriented Programming
71
Cont.
Each row in a two-dimensional array is
itself an array.
Thus, the rows can have different lengths.
An array of this kind is known as a
ragged
array.
Object Oriented Programming
72
If you don’t know the values in a ragged
array in advance, but do know the sizes—
say, the same as before—you can create a
ragged array
using the following syntax
Object Oriented Programming
73
Passing two – d array to methods
When passing a two-dimensional array to
a method, the reference of the array is
passed to the method.
PassTwoDimensionalArray.java
Multi dimensional array
double
[][][] scores =
new double
[
6
][
5
][
2
];
Object Oriented Programming
74
Multi dimensional array
Object Oriented Programming
75
Selection statements in programming use Boolean expressions to control program flow. Learn about if statements, if-else statements, nested if statements, and logical operators. Explore how to generate random numbers, compute BMI, and work with the boolean data type.
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Presentation Transcript
Selections The statements condition. program can execute decide based which on to a Selection statements use conditions that are Boolean expressions. A Boolean expression is an expression that evaluates to a Boolean value:true or false. Object Oriented Programming 1
Boolean data type The boolean data type declares a variable with the value either true or false (they are literals like number). Ex. boolean lightsOn = true; Object Oriented Programming 2
If (one-way) statement An if statement is a construct that enables a program to specify alternative paths of execution. if (boolean-expression) { statement(s); } Object Oriented Programming 3
Cont. Object Oriented Programming 4
Two-way if else An execution path based on whether the condition is true or false. if (boolean-expression) { statement(s)-for-the-true-case; } else { statement(s)-for-the-false-case; } if-else statement decides the Object Oriented Programming 5
Nested or Multi-way if An if statement can be inside another if statement to form a nested if statement. The if (j > k) statement is nested inside the if (i > k) statement in the following. Object Oriented Programming 6
Generating Random Numbers You can use Math.random() to obtain a random double value between 0.0 and 1.0, excluding 1.0. SubtractionQuiz.java Object Oriented Programming 7
Computing Body Mass Index (BMI) BMI = weigh in KG/ (height in meters)2 ComputeAndInterpretBMI.java Object Oriented Programming 8
Logical Operators The logical operators !, &&, ||, and ^ can be used to create a compound Boolean expression. Object Oriented Programming 9
Cont. Write a program that checks whether a number is divisible by 2 and 3, by 2 or 3, and by 2 or 3 but not both. TestBooleanOperators.java LeapYear.java (A leap year is divisible by 4 but not by 100 or divisible by 400.) Lottery.java Object Oriented Programming 11
Switch statement A switch statement executes statements based on the value of a variable or an expression. switch (switch-expression) { case value1: statement(s)1; break; case value2: statement(s)2; break; case valueN: statement(s)N; break; default: statement(s)-for-default; } The keyword break is optional. The break statement immediately ends the switch statement. The switch statement observes the following rules: The switch-expression must yield a value of char, byte, short, int, or String type and must always be enclosed in parentheses. (The char and String types will be introduced in the next chapter.) The value1, . . ., and valueN must have the same data type as the value of the switchexpression. Note that value1, . . ., and valueN are constant expressions, meaning that they cannot contain variables, such as 1 + x. When the value in a case statement matches the value of the switch-expression, the statements starting from this case are executed until either a break statement or the end of the switch statement is reached. The default case, which is optional, can be used to perform actions when none of the specified cases matches the switch-expression. Object Oriented Programming 12
Conditional Expression A conditional expression evaluates an expression based on a condition. Object Oriented Programming 13
Operator Precedence and Associativity Operator precedence and associativity determine the order in which operators are evaluated Object Oriented Programming 14
Decreasing order of precedence All binary operators except assignment operators are left associative. Object Oriented Programming 15
Mathematical functions They can be categorized as trigonometric methods, exponent methods, and service methods. They all are used like Math.? Object Oriented Programming 16
Cont. Object Oriented Programming 17
Character data type A character data type represents a single character. Computers use binary numbers internally. A character is stored in a computer as a sequence of 0s and 1s. Mapping a character to its binary representation is called encoding. Object Oriented Programming 18
Cont. Java supports Unicode, an encoding scheme established by the Unicode Consortium to support the interchange, processing, and display of written texts. Unicode was originally designed as a 16- bit character encoding. The Unicode standard therefore has been extended to allow up to 1,112,064 characters. Object Oriented Programming 19
Cont. Those characters that go beyond the original 16-bit limit are characters. Java supports the supplementary characters. A 16-bit Unicode takes two bytes, preceded by \u, expressed in four hexadecimal digits that run from \u0000 to \uFFFF. Unicode includes ASCII code, with \u0000 to \u007F corresponding to the 128 ASCII characters. called supplementary Object Oriented Programming 20
Escape Sequence for special characters Java uses a special notation to represent special characters. Object Oriented Programming 21
Casting between character and numeric type A char can be cast into any numeric type, and vice versa. Object Oriented Programming 22
Comparing and Testing Character Two characters can be compared using the relational operators comparing two numbers. This is done by comparing the Unicodes of the two characters just like Object Oriented Programming 23
Cont. Object Oriented Programming 24
String A string is a sequence of characters. To represent a string of characters, use the data type called String. For example; String message = "Welcome to Java"; String is a predefined class in the Java library, just like the classes System and Scanner. The String type is not a primitive type. It is known as a reference type. Object Oriented Programming 25
Cont. Strings are objects in Java. Instance method and static method Object Oriented Programming 26
Reading a string from the console Enter three words separated by spaces: Welcome to Java Object Oriented Programming 27
Reading single character char c = (char) System.in.read(); Object Oriented Programming 28
String Comparison OrderTwoCities.java Object Oriented Programming 29
Obtain substring You can obtain a single character from a string using the charAt method. You can also obtain a substring from a string using the substring method in the String class Object Oriented Programming 30
Finding a Character or a Substring in a String The String class provides several versions of indexOf and lastIndexOf methods to find a character or a substring in a string. Object Oriented Programming 31
Conversion between strings and numbers You can convert a numeric string into a number. To convert a string into an int value, use the Integer.parseInt method, as follows. int intValue = Integer.parseInt(intString); ( 123 ) Double doubleValue Double.parseDouble(doubleString); ( 65.35 ) If the string is not a numeric string, the conversion would cause a runtime error. = Object Oriented Programming 32
Cont. You can convert a number into a string, simply use the string concatenating operator as follows: String s = number + ""; HexDigit2Dec.java Object Oriented Programming 33
Formatting console output You can use the System.out.printf method to display formatted output on the console. Object Oriented Programming 34
Cont. System.out.printf(format, item1, item2, ..., itemk) Object Oriented Programming 35
Cont. Object Oriented Programming 36
Cont. By default, the output is right justified. You can put the minus sign (-) in the format specifier to specify that the item is left justified in the output within the specified field. Object Oriented Programming 37
Loops A loop can be used to tell a program to execute statements repeatedly. While Do while For Object Oriented Programming 38
While SubtractionQuizLoop.java Controlling loop with sentinel value SentinelValue.java Input/output redirection java SentinelValue < input.txt java ClassName > output.txt Object Oriented Programming 39
Dowhile A do-while loop is the same as a while loop except that it executes the loop body first and then checks the loop continuation condition. Object Oriented Programming 40
For loop A for loop has a concise syntax for writing loops. Object Oriented Programming 41
Which loop to use? You can use a for loop, a while loop, or a do-while loop, whichever is convenient. Pre test loop (while and for) Post test loop (do while) Object Oriented Programming 42
Break and Continue The break and continue keywords provide additional controls in a loop. Object Oriented Programming 43
Methods Methods can be used to define reusable code and organize and simplify coding. Object Oriented Programming 44
Defining method A method definition consists of its method name, parameters, return value type, and body. Object Oriented Programming 45
Modularizing Code Modularizing makes the code easy to maintain and debug and enables the code to be reused. By encapsulating the code for obtaining the gcd in a method, this program has several advantages: 1. It isolates the problem for computing the gcd from the rest of the code in the main method. Thus, the logic becomes clear and the program is easier to read. 2. The errors on computing the gcd are confined in the gcd method, which narrows the scope of debugging. 3. The gcd method now can be reused by other programs. Object Oriented Programming 46
Overloading methods Overloading methods enables you to define the methods with the same name as long as their signatures are different. Overloaded methods must have different parameter lists. You cannot overload methods based on different modifiers or return types. TestMethodOverloading.java Object Oriented Programming 47
Cont. Can you invoke the max method with an int value and a double value, such as max(2,2.5)? If so, which of the max methods is invoked? The answer to the first question is yes. The answer to the second question is that the max method for finding the maximum of two double values is invoked. The argument value 2 is automatically converted into a double value and passed to this method. The Java compiler finds the method that best matches a method invocation. Since the method max(int, int) is a better matches for max(3, 4) than max(double, double), max(int, int) is used to invoke max(3, 4). Object Oriented Programming 48
Scope of Variables The scope of a variable is the part of the program where the variable can be referenced. The scope of a method parameter covers the entire method. A variable declared in the initial-action part of a for-loop header has its scope in the entire loop. However, a variable declared inside a for- loop body has its scope limited in the loop body from its declaration to the end o the block that contains the variable Object Oriented Programming 49
You can declare a local variable with the same name in different blocks in a method, but you cannot declare a local variable twice in the same block or in nested blocks Object Oriented Programming 50
