Introduction to MATLAB

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Introduction to MATLAB
Lecture 1 the MATLAB environment, variables, and
flow control

• Damon Tomlin
• February 19, 2008

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Why Program?

• Fast
• Error free calculations
• Computers dont get bored

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Whats a Program?

• Set of instructions
• Takes an input, creates an output

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The MATLAB Environment

• The workspace
• The command line
• Create new variables
• Evaluate comparison
• Execute functions
• Check your variables
• Alerts you to errors

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Variables

• Store values
• Different types of data
• Scalars (9, 3.14, 0, -1000000)
• Vectors 1 2 3 4 5 6
• Matrices 1 2 3 4 5 6
• Strings Hello World!
• Helpful name XCoord, Filename

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Scripts

• Scripts are programs .m
• Scripts use the workspace
• Scripts work with variables youve already created

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Functions

• Functions are programs .m
• Functions create their own workspace
• Functions take an input argument
• Functions can produce an output
• Variable inputs
• MySum sum(a)
• MySum sum(a,2)
• New RotatePoint(orig, point, angle)

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Operations

• Mathematical statements
• Add (), subtract (-), multiply(), divide(/)
• Built-in functions exp(), log(), sqrt(), sin()
• Order of operations 3(exp(log(2)) 2)

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Expressions

• Assignments
• MyMatrix 1 2 3 4
• MyName Damon
• Comparisons
• (MyAddress 1514)
• (MyAge gt 20)

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Manipulating Variables Indexing

• An index is the location of a piece of data
  within a variable (like an address)
• Indexing lets you manipulate a particular part of
  a variable
• Vector(8)
• Matrix(2,2)

5 1 2 3 4 8 9 5 1 2 3 4
5 1 2 3 4 8 9 5 1 2 3 4
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Manipulating Variables Indexing

• Indexing can be a little complex . . .
• Vector(i)
• y Vector(length(Vector))
• x length(Vector)
• Y Vector(x)
• Multiple Dimensions Y(32,40,12,30)
• Searching variables find
• find(Vector 3)

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Manipulating Variables Concatenating

• Putting variables together
• Z x y
• Z x y
• The size of the variable is important!
• Often combined with indexing
• Z(2,) y

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Manipulating VariablesChanging Values

• Creating a matrix
• MyMatrix 1 2 3 4
• When altering values, make sure the size of the
  output equals the size of the input
• MyMatrix(1,2) 5
• MyMatrix(,1) 1 1
• Exception scalars
• MyMatrix(,) 0

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Manipulating VariablesMATLAB and matrices

• The empty matrix
• X(8)
• Null results
• Transpose X
• Matrix vs. element operations
• XY (rows in X columns in Y)
• X.Y (X and Y are the same size)
• Inf, NaN, pi, and i

5 1 2 3 4 8 9
5
1 2 3 4
5 9 5 1 1 2 2 3 3 4 4 8
5 1 2 3 4 8 9 5 1 2 3 4
p 8
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Logical Operations

• Comparison statements
• (x 5)
• (x gt 3)
• (x 6)
• Comparison statements return 1s and 0s
• find is a function returning the indices of 1s
• Where does this get us?

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Logical Operations

• Conditional statements tell a program what to do
  when certain events occur
• What if a file isnt found?
• What if there are stimuli after the scans are
  over?
• What if my array has duplicated elements?
• Contingencies can be stated as
• IF condition 1 exists, perform instruction 1,
  OTHERWISE perform instruction 2

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The if Statement

• if (rem(x,2) 1)
• disp(Odd)
• end

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The if Statement

• if (rem(x,2) 1)
• disp(Odd)
• else
• disp(Even)
• end

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The if Statement

• if (rem(x,2) 1)
• disp(Odd)
• elseif (x 0)
• disp(Zero)
• else
• disp(Even)
• end

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Combining Conditions

• Sometimes execution depends on a coincidence of
  conditions
• and if (rem(x,2) 0) (x gt 10)
• or if (rem(x,2) 0) (x gt 10)
• Combinations
• if ((rem(x,2) 0) (x gt 10)) ((rem(x,2)
  1) (x lt 10))

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Optimizing Conditionals

• Sometimes we dont need to calculate every
  comparison
• and as soon as one condition is false, the
  whole combination will return false
• or as soon as one condition is true, the
  whole combination will return true
• Short circuit and and or (newer
  versions only)

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Loops

• We often need to do a calculation multiple times
• Registering multiple image files
• Plotting multiple data sets for review
• Iterating a simulation through time

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for Loops

• Perform instructions a set number of times
• Have a counter variable
• Good when the number of times needed to loop is
  easily calculable
• Known of images

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for Loops

• for counter 014
• X(counter1) 10counter
• end
• Answer X 1 10 100 1000 10000

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while Loops

• Perform instructions until a condition is met
• Do not necessarily have a counter variable
• Good when the number of times needed to loop is
  not easily calculable
• Iterating a simulation as long as a criterion is
  met

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while Loops

• X 3
• while (X(length(X)) lt 1000)
• X(length(X)1) 3X(length(X))
• end
• Answer X 3 9 27 81 243 729 2187

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Comparing for andwhile Loops

• for counter 014
• X(counter1) 10counter
• end
• counter 0
• while (counter lt 4)
• X(counter1) 10counter
• counter counter 1
• end
• Initialization
• Updating counters
• Altering counters

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switch statements -- a special conditional

• Good for discrete numbers of possiblities (esp.
  strings)
• Equivalent to a series of if/elseif/elses
• switch(X)
• case 1
• disp(X equals 1)
• case 2
• disp(X equals 2)
• otherwise
• disp(Not in the list)
• end

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Summary

• MATLAB uses programs to manipulate data through
  mathematical operations and functions
• Data are stored in variables that can be indexed
• Programs can manipulate data in different ways
  depending upon testable conditions
• Programs can do repetitive calculations to save
  us time

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Class Materials

• www.hnl.bcm.edu
• Courses/scm
• Lectures, homeworks