8/8/2019 Algo Selection WEEK4

1/16

AlgorithmAlgorithm::

Selection Control StructuresSelection Control Structures

ISB10103

8/8/2019 Algo Selection WEEK4

2/16

ObjectivesObjectives

In this chapter you will be able to:

Elaborate on the uses of simple selection, multipleselection, and nested selection in algorithms

Develop algorithms using variations of theselection control structure

8/8/2019 Algo Selection WEEK4

3/16

8/8/2019 Algo Selection WEEK4

4/16

Simple selection occurs when a choice is made between

two alternate paths, depending on the result of a

condition being true or false

The structure is represented in pseudocode using thekeywords IF, THEN, ELSE, and ENDIF

Only one of the THEN or ELSE paths will be followed,

depending on the result of the condition in the IF clause

1 Simple Selection (Simple IF1 Simple Selection (Simple IF

Statement)Statement)

8/8/2019 Algo Selection WEEK4

5/16

2 Simple Selection with Null2 Simple Selection with NullFalse Branch (Null ELSEFalse Branch (Null ELSE

Statement)Statement)

The null ELSE structure is a variation of thesimple IF structure

It is used when a task is performed only whena particular condition is true

If the condition is false, then no processing

will take place and the IF statement will bebypassed

8/8/2019 Algo Selection WEEK4

6/16

3 Combined Selection3 Combined Selection

(Combined IF Statement)(Combined IF Statement) A combined IF statement is one that contains multiple

conditions, each connected with the logical operatorsAND or OR

If the connector AND is used to combine the conditions,then both conditions must be true for the combinedcondition to be true

If the connector OR is used to combine any twoconditions, then only one of the conditions needs to betrue for the combined condition to be considered true

8/8/2019 Algo Selection WEEK4

7/16

The NOTOperatorThe NOTOperator

The NOT operator can be used for the logical negationof a condition, as follows:

IF NOT (grade = F) THEN

Display Congratulations! Youve passed

ENDIF

Note that the AND and OR operators can also be usedwith the NOT operator, but great care must be takenand parentheses should be used to avoidambiguity/uncertainty.

8/8/2019 Algo Selection WEEK4

8/16

4 Nested Selection (Nested4 Nested Selection (Nested

IF Statement)IF Statement) Nested selection occurs when the word IF appears more

than once within an IF statement

Nested IF statements can be classified as

i. Linear

ii. Non-linear

8/8/2019 Algo Selection WEEK4

9/16

i. Linear Nested IFi. Linear Nested IF

StatementsStatements The linear nested IF statement is used when a

field is being tested for various values and a

different action is to be taken for each value

This form of nested IF is called linear, because

each ELSE immediately follows the IF

condition to which it corresponds

8/8/2019 Algo Selection WEEK4

10/16

ii. Nonii. Non--Linear Nested IFLinear Nested IF

StatementsStatements A non-linear nested IF occurs when a number

of different conditions need to be satisfied

before a particular action can occur

It is termed non-linear because the ELSE

statement may be separated from the IF

statement with which it is paired

8/8/2019 Algo Selection WEEK4

11/16

Algorithms Using SelectionAlgorithms Using Selection

Let us look at some programming examples

that use the selection control structure

In each example, a solution algorithm will bedeveloped using pseudocode and flowchart

8/8/2019 Algo Selection WEEK4

12/16

Example 1: ProblemExample 1: Problem

Calculate the area of a triangle.

If the area is greater than 100m2,

display This is a BIG triangle.

Otherwise, display

This is a SMALL triangle.

8/8/2019 Algo Selection WEEK4

13/16

Example 1 : SolutionExample 1 : SolutionAlgorithmAlgorithm

Area_of_triangle

Prompt for base, height

Get base, height

area 0.5 x base x height

Ifarea > 100 Then

Display This is a BIG triangleElse

Display This is a small triangle

End If

End

Stop

Display This is aBIG triangle

Start

Get base, height

area 0.5 x base x height

If area > 100?

False

True

Prompt for base, height

Display Thisis a small

triangle

Pseudocode

Flowchart

8/8/2019 Algo Selection WEEK4

14/16

Example 2 : ProblemExample 2 : Problem

Get a value in miles and determine the charges. Convertmiles into kilometers (km).

If value of km is less than 10km, display Local. Thecharge is RM25.

If distance is between 10 and 100km (inclusive), displayOutsider with a charge of RM50.

Otherwise charge RM100, display Alien.

(Formula : 1 mile = 2.6 km)

8/8/2019 Algo Selection WEEK4

15/16

Example 2: SolutionExample 2: SolutionAlgorithmAlgorithm

Area_of_originPrompt for milesGet miles

km miles x 2.6Ifkm < 10 Then

charge 25

Display LocalElse

If km >=10 AND km

8/8/2019 Algo Selection WEEK4

16/16

ExerciseExercise

What is the difference between Sequential andSelection statements?

List all SIX Relational Operators.

List all THREE Logical Operators. Briefly explain the purpose of each keyword

used in selection statements