Lecture 6 - Decision Structures_And_Task1_continued

8/14/2019 Lecture 6 - Decision Structures_And_Task1_continued

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Lecture 6: Decision Structures


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Outline

In this lecture, we will discuss VB decision structures:The If statement

Allowing multiple choices with Else and ElseIf

Nested If statements

Comparison operators:= , , >= ,


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Decision Structures

So far, our programs have been rather linearMost have executed single-path, unbranched computation:

Get Input Apply a single Computation Output Result

However, most real algorithms involve decision making .

Example: Assign grades to students, based on their scores.The ability to make decisions is called branching .

Blocks of code for making decisions are called DecisionStructures .

VB .NET provides several flavors:The IfThen statementThe IfThenElse statementThe Select Case statement

Each of these provides a means to control program flowVia evaluation of one or more test conditions .

Lets take a look at the simplest: The If Statement


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The If Statement

The simplest VB .NET decision structure is the IfThen statement.This allows conditional execution of a set of statements

Based on satisfaction of a specific condition.

The VB syntax for the If statement is:

If (condition ) Then

Statements to execute when the condition is met End If

Here, If is a VB keyword:Telling the computer to test for satisfaction of a condition .

Note: The condition may be composite.

Example: Is x larger than 3?Then is another VB keyword:

Telling the computer to execute the Blocks statements:Example: Add 5 to x.

End If is a pair of VB keywords:

Which tells the computer that the IfThen block is over.


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Understanding the SyntaxIn words, the IfThen statement says:

If the condition is true, then the statement(s) following the Then will be executed

Example:In words: If a students score is higher than 60, they can pass the test.VB code:

If (grade >= 60) Thenevaluation = pass

End If

The IfThen flow of control is shown moreclearly by a Flow Chart :

If the condition is True:The statements in the block take effect.Example: Student passes.

Otherwise:The statements in the block are by-passed.Example: Student fails to pass.


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A Simple IfThen Program


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The Else StatementIn the last example, we used two IfThen blocks:

The first tests whether the student passedAnd executes a pass statement.

The second tests whether the student failedAnd executes a fail statement.

This is rather redundant.

The Else statement allows binary selection of 2 processes:The Then blocks statements are executed if the condition is met.Otherwise, a set of statements in an Else block are executed.

The syntax for the IfThenElse structure is:

If (condition ) ThenStatements_1 (execute when the condition is met)

Else

Statements_2 (execute when condition is NOT met) End If


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The Else Statement (cont.)The Flow of Control may be made more clear via a Flow Chart :

An IfThenElse structure thus acts like a pair of IfThen blocks:One for the If case;One for the If NOT case.


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The ElseIf StatementThis idea may be generalized

To the linear testing of any number of conditionsusing the ElseIf statement (note: no space between Else and If ).

The syntax for the If Then ElseIf conditional structure is:

If (condition 1) Then

Statements_1 (execute when condition1 is met) ElseIf (condition 2)

Statements_2 (execute when condition2 is met).

.

.

Else

Statements_n (execute when NONE of the above conditions are met)End If


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The Else Statement (cont.)The Flow of Control may be made more clear via a Flow Chart :

An IfThenElseIf structure thus acts like n IfThen statements:Sequentially testing n-1 explicit conditions

And 1 implicit condition (default failure).

Note the exclusivity: exactly one conditions block will execute

Thus: If two conditions test true, only the 1st

will execute!


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A Simple IfThen Else ProgramSo, lets re-write our previous program (with 2 If Then Blocks)

As a singleIfThenElse

statement.


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The Select Case StatementIn the special case where:

There are a large number of conditions (alternatives);A branch is selected based on the value of a single variable We may use a Select Case statement, instead of ElseIfThen .

The syntax for the Select Case conditional structure is:

Select Case (name_of_variable )Case condition_1 statement(s)_1Case condition_2 statement(s)_2

Case Else

statement(s)_nEnd Select

Here:name_of_variable is a variable to be evaluated;

Each condition (case assignment) is a possible value or range of values;Case Else

is the default failure (no case is true)End Select ends the Case Block.


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A Simple Select Case ProgramSo, lets re-write our previous program

Using a Select Case statement.


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Comparison OperatorsA comparison operator compares two operands

One on the right side and one on the left side of the operator.Thus, a comparison has the general form:

x op y

And returns a logical value based on whether the comparison is True or False.

Example: The Equality (= ) Operator compares operands,and

Returns True if they are equal;Returns False if they are not equal.

Example:

If (x = 5) Thenstatement_1

End If

Here, ( x=5 ) evaluates to True if x is equal to 5In which case, statement_1 will be executed.


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Comparison Operators (cont.)Comparison operators come in 6 flavors:

Each compares the two operands , and returns a logical value


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Logical OperatorsLogical operators operate on Boolean variables/expressions

And return a Boolean ( True or False ) result. The VB .NET Logical Operators include:

Truth tables (examples of use)


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Program: Grade Assigner Make a program to evaluate student scores, and assign a

grade, according to the conditions below:

Our input will be the student test score .Our output will be the student grade.

For fun, lets allow use of either :an IfElse statement, or A Select Case statement.


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Lecture 6 - Decision Structures_And_Task1_continued

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