Class 10.1 04c Version A

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Fall 2004 ENGR 111A - 10.1 1

MatLab – Palm Chapter 4, Part 2The if and switch structure

Class 10.1

Sections: 4.4 and 4.6



Fall 2004 ENGR 111A - 10.1 2

RAT 10.1

Take out a piece of paper, write your name, team #,today’s date and RAT 10.1 .

As an INDIVIDUAL, you have 2-minutes to determine thevalue of y in the MatLab code shown below.

x = 10;

if x >= 12

y = sqrt(x);

else

y = x^2;

end

Pass your answer to the center aisle

Answer: y = 100



Fall 2004 ENGR 111A - 10.1 3

Learning Objectives

Students should be able to:

Use conditional statements to developlogical program flow. if, elseif, and else commands

switch structure

Develop flow charts in standard notation.



Fall 2004 ENGR 111A - 10.1 4

4.4 Conditional Statements

The MatLab conditional statements enableus to write programs that make decisions

Understanding the IF-THEN-ELSE logic isfundamental to all software development.

Make sure that you understand: if statement on p. 201

else statement on p. 202/203

elseif statement on p. 205



Fall 2004 ENGR 111A - 10.1 5

LOGICAL CONTROLPROGRAMMING CONSTUCTS

A conditional (Boolean) statement is anexpression which tests the validity of aspecified condition

e.g., z = I ==J

z = I > J

These are used in selection structures(conditional statements) to control theflow of a program.



Fall 2004 ENGR 111A - 10.1 6

LOGICAL CONTROLPROGRAMMING CONSTRUCTS

Syntax of the if statement:

if logical expression

statements

end

Proper indentation is MANDATORY oryou will receive NO CREDIT! Note you can right click and choose smart

indent in the m-file editor.

See page 187 for a flow chart


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Flowchartrepresentation of the

if statement.

Figure 4.1 –2


Conditionalstatement

SequentialStatement(s)




MATLAB starts at the beginning of the if

sequence

It proceeds one condition to the next

When it finds a true statement, theappropriate section of the code isexecuted

THE SEQUENCE IS THEN TERMINATED!!!

The last section of code is closed using thekeyword end



EXAMPLE

x = some given value

if x >= 0y = sqrt (x)

end



EXAMPLE

x = 10;

y = 20;

if x >= 0 & y >= 0

z = sqrt(x) + sqrt(y);

w = log(x) – 3*log(y);

end



LOGICAL PROGRAMMINGCONTRUCTS Nested “if ” statements:

if logical expression 1

statement group 1if logical expression 2

statement group 2

end

end

Note the indentions – an absolute must

NestedStatement



Typical flow Chart fornested if…end Logic



Fall 2004 ENGR 111A - 10.1 13

LOGICAL PROGRAMMINGCONTRUCTSTHE else STATEMENT:

If two mutually exclusive actions can occur as a resultof a decision, use the else statement.

if logical expression

statement group 1

else

statement group 2

end

See page 204 for a flow chart of a typical if-else structure.



Fall 2004 ENGR 111A - 10.1 14

Flowchart of the

else structure.

Figure 4.4 –2

LOGICAL PROGRAMMINGCONTRUCTS

Write thesewords



Fall 2004 ENGR 111A - 10.1 15

In-class Exercise (5 minutes)

Suppose y = x1/2 for x >= 0

and y = ex – 1 for x < 0

Write a program (.m script file) tocalculate y assuming that x already hasa scalar value.

Test your program for x = 3 and x = -2.



Fall 2004 ENGR 111A - 10.1 16

SOLUTION (Script File)

% Solution to In-Class Exercise

if x >= 0

y = sqrt (x);else

y = exp (x) -1;

end Did you indentproperly?!



Fall 2004 ENGR 111A - 10.1 17

LOGICAL PROGRAMMINGCONSTRUCTS

The elseif statement:

When three actions can occur as a resultof a decision, the else and elseif statements are used along with the if statement.

Remember: ONLY ONE ACTION WILL ACTUALY OCCUR!!!



Fall 2004 ENGR 111A - 10.1 18


if logical expression 1

statement group1

elseif logical expression 2

statement group 2

else

statement group 3

end



Fall 2004 ENGR 111A - 10.1 19

if part

elseif check

else is here

Note: else isNOT a

conditionalstatement



Fall 2004 ENGR 111A - 10.1 20

EXAMPLE

Given:

y = ln x for x > 10

y = x1/2

for x >= 0 and x <= 10y = ex – 1 for x < 0

Compute y if x has been assigned a scalarvalue.



Fall 2004 ENGR 111A - 10.1 21

SOLUTION (Script file)

% Solution to example

if x > 10

y = log (x)elseif x >= 0

y = sqrt (x)

elsey = exp (x) -1

end

Does the orderthat I check

things matter? YES!



Fall 2004 ENGR 111A - 10.1 22


As a TEAM , take three minutes tocomplete the following exercise.

Write the syntax for the if-elseif-else-endconstruct if there are more than three

alternatives.



Fall 2004 ENGR 111A - 10.1 23

SOLUTION

if logical expression1

Statements group1

elseif logical expression2

Statements group2elseif logical expression3

Statements group3

elseif logical expression4

Statements group4

…

else

Statement if all other cases are false

end



Fall 2004 ENGR 111A - 10.1 24

In-class Assignment 10.1-1

As an INDIVIDUAL , you have 10 minutes .

Write an .m script file that converts a numericaltest score to a letter grade.

(90 –100) – A

(80 – 89) – B

(70 – 79) – C

(60 – 69) – DLess than 60 – F

Test your program for the grades of 95 and 72.



Fall 2004 ENGR 111A - 10.1 25

SOLUTION (Script file)% Program grades.m

grade = 72;

if grade >=90

letter = 'A'

elseif grade >= 80letter = 'B'

elseif grade >= 70

letter = 'C'

elseif grade >= 60letter = 'D'

else

letter = 'F'

end



Fall 2004 ENGR 111A - 10.1 26

4.6 The switch Structure

THE “switch” STATEMENT:

Provides a convenient way to execute

conditional code when there are manycases to choose from.

This construct can replace series of if-else-end statements



Fall 2004 ENGR 111A - 10.1 27


SYNTAX:

switch expression (scalar or string)

case value1

statement group 1case value2

statement group 2

…

otherwisestatement group n

end



Fall 2004 ENGR 111A - 10.1 28

EXAMPLE for “switch”

Suppose you are given a variable anglein degrees to represent the followingdirections: Northeast, Southeast,

Southwest, and Northwest.

Use the switch statement to display the

desired direction given the angle.



Fall 2004 ENGR 111A - 10.1 29

SOLUTIONswitch angle

case 45

disp('Northeast')

case 135

disp('Southeast')case 225

disp('Southwest')

case 315

disp('Northwest')otherwise

disp('Direction Unknown')

end

Decision variable name

Value to test

Default case



Fall 2004 ENGR 111A - 10.1 30

EXAMPLE #2 for “switch”

You input a numerical value of aquantity in one set of units (e.g.,centimeters) and you desire an output

in another set of units (e.g., inches,feet, or meters, etc…).

Write a program using the switch-case

construction that transforms a length incentimeters, inches, feet, meters, etc…to length in centimeters



Fall 2004 ENGR 111A - 10.1 31

SOLUTIONswitch unitscase {'inch','in'} % ‘units’ contains type of

y = x*2.54; % input, output is in cm

case {'feet','ft'}

y = x*2.54*12;

case {'meter','m'}y = x*100;

case {'centimeter','cm'}

y = x;

case {'millimeter','mm'}

y = x/10;otherwise

disp(['Unknown Units: ' units])

y = NaN;

end



F ll 2004 ENGR 111A 10 1 32

Assignment 10.1

Individual assignment.

Due: Nov. 9, 2004

Palm’s MatLab: Chaper 4; #16, 19a,and 35.

Read Section 4.5 in the Palm MATLABbook

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