1. Chapter 5: Control Structure
Part 1: Selection Control Structure

2. Lesson Outcome At the end of this chapter, students should be able to:
Use control structure in problem solving
Apply types of selection control structure on the given problems
Understand how computer executes an algorithm using selection control structure
Understand how to trace an algorithm
Write an algorithm using selection control structure to solve a problem

3. Condition
A condition or testing is a comparison between at least two values.
It can also test variables that can have either TRUE or FALSE (YES or NO) as the answer.
To make a condition or comparison between two values or variables, the following relational operator can be used:

4. Condition (cont.)
Relational operator
Meaning
>
Greater than
<
Less than ==
Equals to, is ≤
Less or equals ≥
Greater or equals ≠
Not equal
There are a few special words that can be used in a condition like even, odd, negative, positive or divisible.

5. If Statement
A selection control structure is used when we have a choice to carry out actions based on certain conditions.
This control structure is implemented in an algorithm using the If statement and case statement.
Various of selection control structure are:
One way selection
Two way selection
Multiple selections

6. One Way Selection Is used when we have only one choice.
If the condition is true then the task is performed. If the condition is false, then no processing will take place.
The format is as follows:
If condition
statement(s)
EndIf

7. One Way Selection: Example
Example of pseudocode using one way selection:
If (a greater than b)
calculate p
calculate q
Display “p = ”, p
Display “q = ”, q
EndIF

8. One Way Selection: Example (cont.)
Example of flowchart using one way selection:
When the value of variable a is greater than the value of variable b then the condition is true. The computer will executes three statements. Otherwise the computer does nothing.

9. Tracing an Algorithm Using One Way Selection
Given the following pseudocode:
Test the above pseudocode with the following data:
20 17
41 63
Start
Display “Enter 2 numbers: ”
Read no1, no2
Initialize sum with 0
If (no1 is divisible by 2)
add no1 to sum
EndIF
If (no2 is divisible by 2)
add no2 to sum
Display “The total is ”, sum
End

10. Tracing an Algorithm Using One Way Selection (cont.)
We can draw the events in computer memory as follows when the data is 20 and 17 :
Expected screen:
no1
no2
Sum 20 17

11. Problem Solving using One Way Selection
Task: Ask the user to enter a number. Determine whether the number is an even number. If yes, display the number and a message to mention that the number is even number.
Problem definition:
Information
Display number and the message that mention the number is even number when it is an even number. Otherwise do nothing.
Data
One number
Process
Check whether the number is even or not. If yes, display the number and the message that mention the number is even number. Otherwise do nothing.

12. Problem Solving using One Way Selection (cont.)
Expected screen:
Expected Screen 1 when the user enters even number and expected Screen 2 when the user enter NOT an even number. 19
Screen Screen 2

13. Problem Solving using One Way Selection (cont.)
Pseudocode 1:
Flowchart 1
Pseudocode 2: Flowchart 2
Start
Read number
If (number is even)
Display number, “ is an even number”
EndIf
End
Start
Read number
If (number is divisible by 2)
Display number, “ is an even number”
EndIf
End

14. Problem Solving using One Way Selection (cont.)
Pseudocode 3:
Flowchart 3
Start
Read number
If (number % 2 == 0)
Display number, “ is an even number”
EndIf
End
Exercise 5A

15. Problem Solving using One Way Selection (cont.)
Flowchart for Pseudocode 1

16. Problem Solving using One Way Selection (cont.)
Flowchart for Pseudocode 2

17. Problem Solving using One Way Selection (cont.)
Flowchart for Pseudocode 3

18. Exercise 5A
Task: Ask the user to enter two numbers. If the first number can be divided with the second number, display both numbers and the result.
Problem definition:
Expected screen:
Detailed pseudocode:
Detailed flowchart
Information
Data
Process

19. Two Ways Selection
In this type of selection, the computer has two choices to choose from.
However, it can only execute one choice at one time, either to execute the first choice and ignore the second choice or process only the second choice and ignore the first choice.
The format for two ways solution is:
If (condition)
Statement1(s)
Else
Statement2(s)
EndIf

20. Two Ways Selection (cont.)
The following flowchart shows the use of two ways selection.
When the value of variable a is equal to value of variable b, then the condition is true. The computer only executes four statements. Otherwise, the computer only executes three statements.

21. Tracing an Algorithm using Two Ways Selection
Given the following pseudocode:
Trace the above pseudocode with the following data: 23, 12, 505
Draw a flowchart
Start
Display “Enter a number: ”
Read no1
If (no1 % 2 == 0)
Display
Else
Display “******”
Display newline
Display “&&&&&&”
EndIF
Display “The number entered = ”, no1
End

22. Tracing an Algorithm using Two Ways Selection (cont.)
The location of variable no1 in the computer memory can be visualized as follows when the data is 23 :
The expected screen is shown below:
no1 23

23. Tracing an Algorithm using Two Ways Selection (cont.)
FALSE
TRUE
Flowchart 1

24. Problem Solving Using Two Ways Selection
Task: Display the biggest number between two numbers.
Problem definition:
Information
The biggest
Data
Two numbers
Process
If the first number is greater than the second number, then the biggest is the first number. Otherwise, the bigger is the second number.

25. Problem Solving Using Two Ways Selection (cont.)
Expected screen:
A basic algorithm
Start
Read 2 numbers
Find the biggest
Display the biggest
End

26. Problem Solving Using Two Ways Selection (cont.)
A detailed algorithm:
Start
Display “Enter 2 numbers: ”
Read no1, no2
If (no1 > no2)
max  no1
Else
max  no2
EndIF
Display “The biggest = ”, max
End

27. Problem Solving Using Two Ways Selection (cont.)
The flowchart:

28. Exercise 5B Task: Calculate the difference between two numbers.
Problem definition:
Expected screen:
Detailed pseudocode:
Detailed flowchart
Information
Data
Process

29. Multiple Ways Selection
In solving a problem, sometimes we face a situation that has more than two choices.
We can solve this problem using nested if statement, where there is if statement inside another if statement.

30. Multiple Ways Selection (cont.)
The following formats refer to nested if statement:
Format 1:
If (condition1) ::
If (condition2)
EndIF
Else
Statement(s)
Inner If statement is in between If … Else

31. Multiple Ways Selection (cont.)
Format 2:
If (condition1)
Statement(s)
Else
If (condition2) ::
EndIF
Inner If statement is in between Else … EndIF

32. Multiple Ways Selection (cont.)
Format 3:
If (condition1)
If (condition2) ::
EndIF
Else
If (condition3)
Inner If statement is in between If … Else and also in between Else … EndIF

33. Multiple Ways Selection (cont.)
Examples of multiple ways selection are given below.
Example 1:
If (a > b)
Display “aaa”
Else
If (a > 100)
Display “bbb”
EndIF
Pseudocode
Flowchart

34. Multiple Ways Selection (cont.)
Example 2:
If (a > b)
If (a == c)
Display “bbb”
EndIF
Pseudocode
Flowchart

35. Multiple Ways Selection (cont.)
Example 3:
If (a > b)
If (a == 10)
Display “aaa”
EndIF
Else
If (b < c)
Display “bbb”
Display “ccc”
Pseudocode Flowchart

36. Tracing for Multiple Ways Selection
Example: Given the following pseudocode, trace the pseudocode using the following test data:
i) ii) iii) 7 7
Start
Display “Enter values a and b : ”
Read a, b
If (a ≥ b)
Display “2.1a”
If (b > 10)
Display “2.1.1a”
Else
Display “2.1.1b”
EndIF
Display “2.1b”
End

37. Tracing for Multiple Ways Selection (cont.)
Answer: i) Data are
The location in the computer memory as follows:
An expected screen: a b
100 54

38. Problem Solving using Multiple Ways Selection
Example: Ask the user to enter a number. Determine whether the number is positive, negative or zero.
Problem definition:
Required information
Type of a number (positive, negative or zero)
Data to be entered
A number

39. Problem Solving using Multiple Ways Selection (cont.)
Pseudocode:
Start
Display “Enter a number : ”
Read number
If (number == 0)
type  “zero”
Else
//number is either negative or positive
If (number > 0)
type  “positive”
type  “negative”
EndIF
Display “The number is ”, type
End

40. Problem Solving using Multiple Ways Selection (cont.)
Flowchart:

41. Exercise 5C
Analyze the following problems and write pseudocode followed by flowchart.
Display the following menu:
Ask the user to enter a choice and perform the operation that corresponds with the choice.
Choice
Operation +
Calculate addition of 3 numbers -
Substract 1st number with the 2nd number *
Multiply 3 numbers /
Divide 1st number with the 2nd number

42. Logical Operator AND and OR
We can combine more than one comparisons if it suitable.
To combine two or more comparisons, we use a logical operator AND or OR in if statement.

43. Logical Operator AND and OR (cont.)
The syntax to use logical operators is as follows:
Using AND logical operator:
Using OR logical operator:
If (condition1 AND condition2 AND …)
//block1
Else
//block2
EndIF
If (condition1 OR condition2 OR …)
//block1
Else
//block2
EndIF

44. Logical Operator AND and OR (cont.)
The following table are called truth table. It is used to determine the value when the testing implements the logical operator AND or OR.
Below is a truth table for logical operator AND:
Condition1
Condition2
Condition1 AND Condition2
Executable block
False
block2
True
block1

45. Logical Operator AND and OR (cont.)
Below is a truth table for logical operator OR:
Condition1
Condition2
Condition1 OR Condition2
Executable block
False
block2
True
block1

46. Logical Operator AND and OR (cont.)
Example 1: Ask the user to enter a letter. Determine whether the letter is a consonant or vowel.
Problem definition:
Required information
Consonant or vowel
Data to be entered
A letter

47. Logical Operator AND and OR (cont.)
Pseudocode:
Start
Read letter
If (letter == ‘a’ OR letter == ‘A’ OR
letter == ‘e’ OR letter == ‘E’ OR
letter == ‘i’ OR letter == ‘I’ OR
letter == ‘o’ OR letter == ‘O’ OR
letter == ‘u’ OR letter == ‘U’ OR )
Display “vowel”
Else
Display “consonant”
EndIF
End

48. Logical Operator AND and OR (cont.)
Example 2: You are given three numbers. Determine and display the biggest number.
Problem definition:
Required information
The biggest
Data to be entered
3 numbers

49. Logical Operator AND and OR (cont.)
Pseudocode:
Start
Read no1, no2, no3
If (no1 > no2 AND no1 > no3)
biggest = no1
Else
If(no2 > no3)
biggest = no2
biggest = no3
EndIF
Display “The biggest number = ”, biggest
End

50. Any Question