Matlab Training Session 4: Control, Flow and Functions

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Matlab Training Session 4Control, Flow and
Functions
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• Course Outline
• Weeks
• Introduction to Matlab and its Interface (Jan 13
  2009)
• Fundamentals (Operators)
• Fundamentals (Flow)
• Importing Data
• Functions and M-Files
• Plotting (2D and 3D)
• Statistical Tools in Matlab
• Analysis and Data Structures
• Course Website
• http//www.queensu.ca/neurosci/matlab.php

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• Week 4 Lecture Outline
• Fundamentals of Matlab 4
• A. Week 3 Review
• B. Functions in Matlab
• C. Mini Project

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Matlab Scripts

• Entering Multiple Lines Without Running Them
• Matlab can execute sequences of commands stored
  in files
• Files that contain matlab commands have the
  extension .m
• .m files are written and saved in the built in
  matlab m-file editor
• M-files are executed from the M-file editor or
  the command prompt
• M-files can call other M-files
• The location path of the M-file must be set in
  matlab

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Matlab Scripts

• Advantages of M-files
• Easy editing and saving of work
• Undo changes
• Readability/Portability - non executable comments
  can be added using the symbol to make make
  commands easier to understand
• Saving M-files is far more memory efficient than
  saving a workspace

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Week 2 Review

• Relational Operators
• Relational operators are used to compare two
  scaler values or matrices of equal dimensions
• Relational Operators
• lt less than
• lt less than or equal to
• gt Greater than
• gt Greater than or equal to
• equal
• not equal

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Condition Statements

• It is often necessary to only perform matlab
  operations when certain conditions are met
• Relational and Logical operators are used to
  define specific conditions
• Simple flow control in matlab is performed with
  the If, Else, Elseif and Switch statements

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Condition Statements

• If, Else, and Elseif
• An if statement evaluates a logical expression
  and evaluates a group of commands when the
  logical expression is true
• The list of conditional commands are terminated
  by the end statement
• If the logical expression is false, all the
  conditional commands are skipped
• Execution of the script resumes after the end
  statement
• Basic form
• if logical_expression
• commands
• end

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Condition Statements

• If, Else, and Elseif
• The elseif statement forces execution of the
  commands below the else statement if the original
  logical expression evaluates to false
• Only one list of commands can be executed
• Basic form
• if logical_expression
• commands 1
• elseif logical_expression_2
• commands 2
• elseif logical_expression_3
• commands 3
• end

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Loops

• Loops are an important component of flow control
  that enables matlab to repeat multiple statements
  in specific and controllable ways
• Simple repetition in matlab is controlled by two
  types of loops
• For loops
• While loops

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Loops

• For Loops
• The for loop executes a statement or group of
  statements a predetermined number of times
• Basic Form
• for index startincrementend
• statements
• end
• If increment is not specified, an increment
  of 1 is assumed by matlab

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Loops

• While Loops
• The while loop executes a statement or group of
  statements repeatedly as long as the controlling
  expression is true
• Basic Form
• while expression
• statements
• end

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Functions

• Building blocks of programming
• Allow code to be generic and reusable
• Design from top down but build from bottom up
• Take a set of inputs, perform a series of
  manipulations and return an output

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Functions in Matlab

• In Matlab, each function is a .m file
• It is good protocol to name your .m file the same
  as your function name, i.e. funcname.m
• function outargsfuncname(inargs)

Function
input
output
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Simple Example

• Find the cube of a number -gt (x3)
• Type the code below into an .m file and save it
  as cube.m
• Set the Matlab directory appropriately
• In Matlab window, type cube(3), is the result
  correct?
• Now you have a reusable function that can
  calculate the cube of any number

gtgt cube(3) Ans 125 gtgt cube 1.75 Ans 5.3594
function y cube(x) y xxx
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Add Some Help

• Find the cube of a number -gt (x3)
• Add commented text between the funciton
  declaration and the first line of code
• Now type help cube in Matlab window

gtgt help cube Put some text here
function y cube(x) Put some text here y
xxx
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Find the cube of two numbers

• Any numbers of inputs and outputs can be handled
  in a function
• For a simple example, extend the cube function to
  accept two inputs and find the cube of each

function y1, y2 cube(x1, x2) Put some text
here y1 x1x1x1 y2 x2x2x2
gtgt cube(2,3) Ans 8 ??? gtgt a b
cube(2,3) a 8 b 27
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nargin

• Matlab will accept a function call with any
  number of inputs and outputs
• nargin can be used to find out how many inputs
  the user has provided
• It is automatically passed with the function

function y1, y2 cube(x1, x2) if nargin
1 y1 x1x1x1 y2 nan elseif nargin
2 y1 x1x1x1 y2 x2x2x2 end
gtgt cube(2,3) Ans 8 gtgt a b cube(2,3) a 8 b
27
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return

• return terminates computation of the function and
  returns whatever is calculated thus far

function y1, y2 cube(x1, x2) if nargin
1 y1 x1x1x1 y2 nan return end y1
x1x1x1 y2 x2x2x2
gtgt cube(2,3) Ans 8 gtgt a b cube(2,3) a 8 b
27
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Find the Cube of Any Number of Numbers

• Any ideas on how to accomplish this?
• Lets further extend the function to cube a vector
  of numbers
• Before we thought of the variables as scalars,
  now rethink the power of vectors

gtgt cube(2) Ans 8 gtgt cube(2 3) Ans 8 27
function y cube(x) Put some text here y
x.3
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Mini-Project

• Lets combine what we have learned thus far in the
  course
• Loops, If Statements, Functions
• Basic Matlab Operators
• Should finish this in class today
• If you get through this then you are doing well

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Mini-Project

• Raising any number of numbers to the nth power
• Inputs
• A vector of numbers to be raised (N1Nm)
• A vector of powers (P1Pm)
• Outputs
• A vector of raised values (N1P1 NmPm)
• An error flag 1 if error in calculation, 0 if
  successful
• Caveats
• If only one input is provided, the function
  should square each entry, so output (N12Nm2)
  and error flag is 0
• If the length of N and P are not the same, this
  is an error, return anything in the output vector
  and a 1 in the error flag
• Make sure to comment and document the function

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Mini-Project

• Plan before you code
• Remember that xnxxxx x (n times)
• Hint remember the built in functions length
  and size will return the dimensions of any
  given variable
• Ask questions and take your time

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Solution 1(simple)
function y, e raise(x,n) if nargin 1
y x.2 e 0 return elseif nargin
2 if length(x)length(n) y nan
e 1 return end for
i1length(x) y(i) x(i)n(i) end
e 0 return end
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Solution 2
function y, e raise(x,n) y
ones(1,length(x)) if nargin 1 y e
raise(x,2ones(1,length(x))) return elseif
nargin 2 if length(x)length(n) y
NaN e 1 return end
for i1length(x) for j1n(i)
y(i) y(i)x(i) end end e
0 return end
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Getting Help

• Help and Documentation
• Digital
• Accessible Help from the Matlab Start Menu
• Updated online help from the Matlab Mathworks
  website
• http//www.mathworks.com/access/helpdesk/help/tech
  doc/matlab.html
• Matlab command prompt function lookup
• Built in Demos
• Websites
• Hard Copy
• Books, Guides, Reference
• The Student Edition of Matlab pub. Mathworks Inc.