1. MatLab – Palm Chapter 4, Part 2 The if and switch structure
Class 10.1
Sections: 4.4 and 4.6
Fall 2004
ENGR 111A

2. RAT 10.1
Take out a piece of paper, write your name, team #, today’s date and RAT
As an INDIVIDUAL, you have 2-minutes to determine the value 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

3. Learning Objectives Students should be able to:
Use conditional statements to develop logical program flow.
if, elseif, and else commands
switch structure
Develop flow charts in standard notation.
Fall 2004
ENGR 111A

4. 4.4 Conditional Statements
The MatLab conditional statements enable us to write programs that make decisions
Understanding the IF-THEN-ELSE logic is fundamental 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

5. LOGICAL CONTROL PROGRAMMING CONSTUCTS
A conditional (Boolean) statement is an expression which tests the validity of a specified condition
e.g., z = I ==J
z = I > J
These are used in selection structures (conditional statements) to control the flow of a program.
Fall 2004
ENGR 111A

6. LOGICAL CONTROL PROGRAMMING CONSTRUCTS
Syntax of the if statement:
if logical expression
statements
end
Proper indentation is MANDATORY or you 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
Fall 2004
ENGR 111A

7. LOGICAL CONTROL PROGRAMMING CONSTRUCTS
Conditional
statement
Flowchart representation of the if statement.
Figure 4.1–2
Sequential
Statement(s)
Fall 2004
ENGR 111A

8. LOGICAL CONTROL PROGRAMMING CONSTRUCTS
MATLAB starts at the beginning of the if sequence
It proceeds one condition to the next
When it finds a true statement, the appropriate section of the code is executed
THE SEQUENCE IS THEN TERMINATED!!!
The last section of code is closed using the keyword end
Fall 2004
ENGR 111A

9. EXAMPLE x = some given value if x >= 0 y = sqrt (x) end Fall 2004
ENGR 111A

10. EXAMPLE x = 10; y = 20; if x >= 0 & y >= 0
z = sqrt(x) + sqrt(y);
w = log(x) – 3*log(y);
end
Fall 2004
ENGR 111A

11. LOGICAL PROGRAMMING CONTRUCTS
Nested “if” statements:
if logical expression 1
statement group 1
if logical expression 2
statement group 2
end
Note the indentions – an absolute must
Nested
Statement
Fall 2004
ENGR 111A

12. Typical flow Chart for nested if…end Logic
Fall 2004
ENGR 111A

13. LOGICAL PROGRAMMING CONTRUCTS
THE else STATEMENT:
If two mutually exclusive actions can occur as a result of 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

14. LOGICAL PROGRAMMING CONTRUCTS
Write these
words
Flowchart of the else structure.
Figure 4.4–2
Fall 2004
ENGR 111A

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) to calculate y assuming that x already has a scalar value.
Test your program for x = 3 and x = -2.
Fall 2004
ENGR 111A

16. SOLUTION (Script File)
% Solution to In-Class Exercise
if x >= 0
y = sqrt (x);
else
y = exp (x) -1;
end
Did you indent
properly?!
Fall 2004
ENGR 111A

17. LOGICAL PROGRAMMING CONSTRUCTS
The elseif statement:
When three actions can occur as a result of 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

18. LOGICAL PROGRAMMING CONSTRUCTS
if logical expression 1
statement group1
elseif logical expression 2
statement group 2
else
statement group 3
end
Fall 2004
ENGR 111A

19. if part elseif check else is here Note: else is NOT a conditional
statement
Fall 2004
ENGR 111A

20. EXAMPLE Given: y = ln x for x > 10
y = x1/2 for x >= 0 and x <= 10
y = ex – 1 for x < 0
Compute y if x has been assigned a scalar value.
Fall 2004
ENGR 111A

21. SOLUTION (Script file)
% Solution to example
if x > 10
y = log (x)
elseif x >= 0
y = sqrt (x)
else
y = exp (x) -1
end
Does the order
that I check
things matter?
YES!
Fall 2004
ENGR 111A

22. LOGICAL PROGRAMMING CONSTRUCTS
As a TEAM, take three minutes to complete the following exercise.
Write the syntax for the if-elseif-else-end construct if there are more than three alternatives.
Fall 2004
ENGR 111A

23. SOLUTION if logical expression1 Statements group1
elseif logical expression2
Statements group2
elseif logical expression3
Statements group3
elseif logical expression4
Statements group4 …
else
Statement if all other cases are false
end
Fall 2004
ENGR 111A

24. In-class Assignment 10.1-1 As an INDIVIDUAL, you have 10 minutes.
Write an .m script file that converts a numerical test score to a letter grade.
(90 –100) – A
(80 – 89) – B
(70 – 79) – C
(60 – 69) – D
Less than 60 – F
Test your program for the grades of 95 and 72.
Fall 2004
ENGR 111A

25. SOLUTION (Script file)
% Program grades.m
grade = 72;
if grade >=90
letter = 'A'
elseif grade >= 80
letter = 'B'
elseif grade >= 70
letter = 'C'
elseif grade >= 60
letter = 'D'
else
letter = 'F'
end
Fall 2004
ENGR 111A

26. 4.6 The switch Structure THE “switch” STATEMENT:
Provides a convenient way to execute conditional code when there are many cases to choose from.
This construct can replace series of if-else-end statements
Fall 2004
ENGR 111A

27. LOGICAL PROGRAMMING CONSTRUCTS
SYNTAX:
switch expression (scalar or string)
case value1
statement group 1
case value2
statement group 2 …
otherwise
statement group n
end
Fall 2004
ENGR 111A

28. EXAMPLE for “switch”
Suppose you are given a variable angle in degrees to represent the following directions: Northeast, Southeast, Southwest, and Northwest.
Use the switch statement to display the desired direction given the angle.
Fall 2004
ENGR 111A

29. SOLUTION switch angle case 45 Decision variable name disp('Northeast')
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

30. EXAMPLE #2 for “switch”
You input a numerical value of a quantity 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 in centimeters, inches, feet, meters, etc… to length in centimeters
Fall 2004
ENGR 111A

31. SOLUTION switch units case {'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
Fall 2004
ENGR 111A

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 MATLAB book
Fall 2004
ENGR 111A