1. Computer Science
Selection Structures

2. Relational & Logical Operators
In the previous unit we learned about arithmetic operators. They allowed us to generate new values in a program
In this unit you will be introduced to relational and logical operators
Relational operators enable you to compare values and from that comparison make decisions
Logical operators also known as Boolean operators decide whether an expression is true or false and enable you to combine conditions

3. Relational Operators
Relational operators are also called comparative operators because they compare things
They symbols used in programming for these operators are:
= = equal to (comparison)
!= not equal to
< less than
> greater than
<= less than or equal to
>= greater than or equal to

4. Comparing Numbers If you have two numbers: Let A=10 and B=5 Then:
A > B is true
A < B is false
A >= B is true
A <= B is false
A != B is true
A == B is false

5. Comparison vs Assignment
Comparison Assignment
= = =
What is the difference?
The Assignment Operator (=)
Changes the value of the variable on the right side to the left side
Example: Given A=3 and B=5 then for A=B, A becomes 5
The Comparison Operator (= =)
Compares the right side to the left sides and asks if they are equal or if its true
Example: Example: A=3 and B=5 then for A==B, then the equation is FALSE

6. Logical Operators AND OR NOT
Logical operators are used to connect simple conditions into a more complex condition called a compound conditions.
The logical operators are:
AND OR
NOT

7. If (X > 5) AND (X < 10) Then …
The AND Operator
Joins two conditions
Both conditions must be true for the statement to be true.
It is false if even one of the conditions is false.
Example: The statement:
If (X > 5) AND (X < 10) Then …
is true only if X is 6, 7, 8, or 9.

8. If (X > 5) OR (X < 10) Then …
The OR Operator
Joins two conditions
One condition must be true for the statement to be true.
It is false only if both conditions are false.
Example: The statement:
If (X > 5) OR (X < 10) Then …
is true for all numbers

9. The NOT Operator AND and OR affect 2 simple conditions.
NOT negates the condition. It makes the condition the opposite of what is given.
A condition with the NOT operator is true only if the condition is false.
Example: The statement:
NOT ( A < B)
is true only if B is greater than or equal to A.
Example: The statement
If ( X > 100) AND NOT ( X == Y) Then…
is true only if X is greater than 100 but not equal to the value of Y.

10. NOT A > 3 OR B < 3 AND A – B <= 0
Combining Operators
Example:
Let A = 3 and let B = 5
Is the following expression true or false?
NOT A > 3 OR B < 3 AND A – B <= 0
(NOT(false)) OR ((false) AND (true))
(true) OR (false)
true

11. Truth Tables X Y
X OR Y
X AND Y
NOT X
true
false

12. Hierarchy of Operations
Type
Operator
Order Performed
Arithmetic operations
are performed first, in order shown
( ) ^
* / %
+ -
1st parentheses
2nd exponentiation
3rd: multiplication, division, modulus
4th: addition, subtraction
Relational operations
are performed second
== != < <= > >=
All relational operators have equal precedence
Logical operations
are performed last, in the order shown
NOT
AND (&&)
OR (||)
1st: NOT
2nd: AND
3rd: OR

13. A note on ASCII code
Recall all data, including characters are stored in the computer’s memory in binary form.
A programming language uses a method or scheme to associate each character with a number so that it can be converted into binary
One such scheme is called ASCII code
American Standard Code for Information Interchange
-- pronounced “askey.”
In ASCII each character is associated with a number from 0 to 127
For a complete list see text:
Prelude to Programming (Venit) on page 131

14. Why mention ASCII?
These schemes affect how operators handle strings in a programming language
If you look at the ASCII table you will notice the following is true:
“a” > “A”
“1” <= “5”
“2” >= “2”
The program compares ASCII numbers, not the characters you write
Character
ASCII # a 97 A 65 1 49 2 50 5 53

15. Making Choices
From the flow chart activity we saw how we can create algorithms that make choices
The way a program can handle these choices are known as Selection Structures or If-Else statements
If a condition is true then a statement is executed, else it is not.
Therefore the program selects to do one thing or the other.

16. Types of Selection Structures
Single-alternative (If-Then)
A single block of statements to be executed or skipped
Dual-alternative (If-Then-Else)
Two blocks of statements, one of which is to be executed, while the other one is to be skipped
Multiple-alternative (If-Then-Else-If or Case)
More than two blocks of statements, only one of which is to be executed and the rest skipped

17. Single Alternative – Simple If
The single alternative is the most basic structure. It uses one if statement
If condition is true Then statement is executed otherwise the statement is skipped
Pseudocode Example
If Age >= 18
Set Eligibility = “Yes”
Do other things…
End If
Condition
Statement T F

18. Dual Alternative – two branch
The dual alternative uses two statements
If conditions is true then the first statement is executed else the second statement is executed
Pseudocode Example
If Age >= 18
Set Eligibility = “Yes”
Else
Set Eligibility = “No”
End If
Condition
Statement1 T F
Statement2

19. Example in Java
class IfDemo { public static void main (String[] args) System.out.println("Please give one integer"); int first = In.getInt(); System.out.println("and a second"); int second = In.getInt(); if (first == second) System.out.println("The values are equal"); else System.out.println("The values are not equal"); }

20. Multi Alternative – Multi branch
The multi alternative uses more than two statements
If condition is true then the first statement is executed else the second statement is executed else the third statement is executed and so on..
Pseudocode Example
if (score >= 90)
grade = ‘A’;
else if (score >= 80)
grade = ‘B’;
else if (score >= 70)
grade = ‘C’; ...
else if (score >= 60)
grade = ‘D’;
else
grade = ‘F’;

21. Flowchart for a Multiple Alternative

22. Nested IF statements
A nested statement is a statement within another statement
You can nest if statements to determine if both of two Boolean expressions are true, or if either of the expressions is true.
Cond1
Stmt1 T F
Stmt2
Cond2
StmtN
CondN
StmtN+1
. . .

23. Multiple Alternative – Switch/Case
The switch statement is similar to an if /else, if /else statement.
It compares that value to two or more other values to determine which code to execute.
Pseudocode Example
Switch Choice
Case 1:
Set Operation = “Add”
Case 2:
Set Operation = “Subtract”
Case 3:
Set Operation = “Multiply”
End Case

24. Example in Java class AvgTest { public static void main(String[] args)
System.out.println("Please enter the test number.");
int testNum = In.getInt();
System.out.println("Please enter the score.");
int score = In.getInt();
switch (testNum)
case 1: System.out.println(score * 0.3);
break;
case 2:System.out.println(score * 0.1);
case 3:System.out.println(score * 0.2); }
System.out.print("is the weighted score of test " + testNum
+ " with a grade of " + score);