CSCI1402: Lecture 1 Week 6
Dr. David A. Elizondo
Centre for Computational Intelligence
School of Computing
Office: Gateway 6.61
email: [email protected]
Selection



Sometimes we may want an instruction, or set of instructions,
to be executed only if a particular condition is true.
An example is a simple computerized guessing game that would
be suitable for small children. In the first version of the
game, the program asks the player for a letter between A and
Z. If the player chooses the same letter as the one stored in
the program, a message “**Right** is displayed on the screen.
The logic of this program is as follows:
 Get a letter from the player at the keyboard
 IF this letter is equal to the letter stored in the program
THEN display **Right**
Selection – if statement

The if conditional statement can execute selectively
a part of a program.
if (condition) statement;
Here condition is a Boolean expression (true or false).
If condition is true, then the statement is executed.
If condition is false, then the statement is bypassed.
Selection – if statement

Example:
if (10 < 11) System.out.println(“10 is less than 11”);
Since 10 < 11, the conditional expression is true, and
println() will execute.
if (10 < 9) System.out.println(“This won’t be
displayed”);
Since 10 is not < 9, the println will not be executed.
Selection – if statement

Complete form of the if statement:
if(condition) statement;
else statement;
The targets of the if and else are single statements
Selection – if statement

General form of the if statement:
if(condition) {
statement;
}
else {
statement;
}


The targets of both if and else are blocks of
statements
If the conditional expression is true, the target of the if will be
executed; otherwise, if it exists, the target of the else will be
executed.
Selection – if statement
if (number > = 0)
System.out.println(“+”);
else
System.out.println(“-”);
write out +
true
number >= 0
false
write out -
Selection – if statement
Example:
public class Guess {
public static void main(String args[])
throws java.io.IOException {
char ch, answer = ‘K’;
System.out.println(“I’m thinking of a letter between A and
Z.”);
System.out.println(“Can you guess it: ”);
ch = (char) System.in.read(); // read a character from the KB

}
}
if (ch == answer) System.out.println(“*** RIGHT ***“);
Selection – if statement
This program prompts the player and then reads a
character from the KB.
 Using an if statement, it then checks that
character against the answer, which is K in this
case.
 If K was entered, the message is displayed. When
you try this program K must be entered in
uppercase.

Selection – if statement
Enhancing the if statement:
public class Guess2 {
public static void main(String args[])
throws java.io.IOException {
char ch, answer = ‘K’;
System.out.println(“I’m thinking of a letter between A and
Z.”);
System.out.println(“Can you guess it: ”);
ch = (char) System.in.read(); // read a character from the KB
if (ch == answer) System.out.println(“*** RIGHT ***“);
else System.out.println(“... Sorry, you’re wrong.”);
}
}

Nested ifs


A nested if is an if statement that is the target of
another if or else.
Nested ifs are very common in programming.
IMPORTANT: an
else statement always
refers to the nearest if statement that is within
the same block as the else, and not already
associated with an else.
Nested ifs

Example:
if (i == 10) {
if(j < 20)
a = b;
if(k > 100)
c = d;
else
a = c; // this else refers to if (k < 100)
}
else
a = d; // this else refers to if(i == 10)
 The final else is not associated with if (j < 20), because it is not in
the same block.
Nested ifs
Enhancements to the guess program
public class Guess3 {
public static void main(String args[])
throws java.io.IOException {
char ch, answer = ‘K’;
System.out.println(“I’m thinking of a letter
between A and
Z.”);
System.out.println(“Can you guess it: ”);
ch = (char) System.in.read(); // read a character from the KB
if (ch == answer) System.out.println(“*** RIGHT ***“);
else {
System.out.print(“... Sorry, you’re wrong.”);
// a nested if
if (ch < answer) System.out.println(“too low”);
else System.out.println(“too high”);
}
}
}

Nested ifs

Sample run of enhanced guess program:
I’m thinking of a letter between A and Z.
Can you guess it: Z
...Sorry, you’re too high
The if-else-if Ladder

if-else-if ladder:
if (condition)
statement;
else if (condition)
statement;
else if (condition)
statement;
.
.
.
else
statement;
The if-else-if Ladder
The conditional expressions are evaluated from top
downward.
 As soon as a true condition is found, the statement
associated with it is executed, and the rest of the
ladder is bypassed.
 If none of the conditions is true, the final else
statement will be executed.
 The final else often acts as the default condition: if
all conditional tests fail, the last else statement is
performed.

The if-else-if Ladder
Example:
public class Ladder {
public static void main(String args[]) {
int x;
for(x=0;x<6;x++) {
if(x==1)
System.out.println(“x is one”);
else if(x==2)
System.out.println(“x is two”);
else if(x==3)
System.out.println(“x is three”);
else if(x==4)
System.out.println(“x is four”);
else
System.out.println(“x is not between 1 and 4”);

}
}
}
Default statement
The if-else-if Ladder

Program output:
x is not between 1 and 4
x is one
x is two
x is three
x is four
x is not between 1 and 4

Default else statement is executed only if none of
the preceding if statements succeeds.
Relational and Logical Operators



Relational operators: relationships than values can
have with one another.
Logical operators: ways in which true and false
values can be connected together.
Relational operators produce true and false results
Relational operators
Operator
Meaning
== (!!! Different than =)*
Equal to
!=
Not equal to
>
Greater than
<
Less than
>=
Greater than or equal
<=
Less than or equal
Logical Operators
Operator
Meaning
&
AND
|
OR
^
XOR
||
Short circuit OR
&&
Short circuit AND
!
NOT
Relational Operators Example
public class RelOps{
public static void main(String args []) {
int i, j;
i = 10;
j = 11;
if (i < j) System.out.println(“i < j”);
if (i <= j) System.out.println(“i <= j”);
if (i != j) System.out.println(“i != j”);
if (i == j ) System.out.println(“this won’t execute”);
if (i >= j) System.out.println(“this won’t execute”);
if (i > j) System.out.println(“this won’t execute”);
}
}
Relational Operators Example

Output of the program:
i<
i <=
i !=
j
j
j
Relational Operators Example

Output of the program:
i<
i <=
i !=
j
j
j
Logical Operators Example
public class LogOps{
public static void main(String args []) {
boolean b1, b2;
b1 = true;
b2 = false;
if (b1 & b2) System.out.println(“this won’t execute”);
if (!(b1 & b2)) System.out.println(“!(b1 & b2) is true”);
if (b1 | b2) System.out.println(“b1 | b2 is true”);
if (b1 ^ b2) System.out.println(“b1 ^ b2 is true”);
}
}
Logical Operators Example

Output of the program:
!(b1 & b2)
b1 | b2
b1 ^ b2
is true
is true
is true