MATLAB basics

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MATLAB basics

• Get to know Matlab
• Interact with Matlab
• Write and save a program
• Run and debug a program
• Loop and for loop structure
• Simple 2D plotting
• Get help in Matlab

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Uses of MATLAB

• MATLAB is a sophisticated mathematical
  computation tool. It has many capabilities,
  including
• Mathematical operations
• Computations with matrices
• Symbolic calculations
• Graphical capabilities, including many plotting
  features
• MATLAB can be used in many engineering
  applications.

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MATLAB Desktop
Command Window Workspace Window Current
Directory Window Command History Window Start
Button
clc clears the command window clear clears
the workspace
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Interacting with MATLAB
Scalar variables
matrices

• Type in the Command Window
• 22
• The solution was stored in the default variable
  ans.
• Then type
• clear
• Now define C_as
• C_as0.6
• The semi-colon () prevents the result from being
  printed to the screen
• Only letters, numbers and _ can be used.
• Case sensitive.

• Define 1D matrix (vector) x
• x10.24 or
• xones(4,1)
• The first index is the number of rows and the
  second index is the number of columns.
• The solution can also be viewed in workspace.
• Then define a 2D matrix
• y1,2,34,5,6
• We can change the value of any elements
• y(2,3)5
• The matrix dimension can also be changed
• y(4,4)7

5
Writing a Program

• A MATLAB program can be a collection of command
  lines and is in the form of an M-file
• An M-file is to MATLAB what a doc-file is to
  Microsoft Word.
• An M-file is written in text Editor.
• M-files can be used to
• write programs
• save and reopen a program
• Fix errors in a program

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Writing an M-File

• Create a new M-file
• Type edit into the Command Window
• or use the menu File
• Type the following code in the Editor
• epsilon10
• C_as0.5
• r0R/20R
• R5
• rhor/R
• To insert a comment line in an M-file, use the
  comment operator , then type in your comment.
• After you have typed the code, save it as
  concentration.m

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Commenting

• Every time you write a program, it should be
  well-commented.
• MATLAB
• will not run lines that begin with the comment
  operator
• shades comments in green
• Commenting can explain
• what the program does
• what the variables in the program represent
• any calculations in the program
• allow programmers to more easily understand your
  program
• make difficult operations easier to understand
• document when code was written and by whom
• define variables used
• help clarify your own thinking

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Changing the Directory

• To change your directory, click the Browse for
  Folder button next to where the Current
  Directory is shown.
• Navigate through this window to My Computer and
  then to where you want to save your M file and
  click OK.
• You should make sure that your Current Directory
  is where your file is, otherwise Matlab will not
  run it.

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Running an M-file

• Make sure that your Current Directory is where
  your file is. Type concentration into the Command
  Window and press enter, or use the Debug menu.
• If you typed the code as written on the previous
  slide you will get an error. What went wrong?
• Error checking, also called debugging, helps to
  verify a program works properly.
• The advantage of an M-file is that we can go make
  the change and run it again without having to
  type all the code again.

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Syntax Errors

• Syntax errors are errors in a MATLAB statement
  itself, such as spelling or punctuation errors.

sni(pi) ln(x) z 1,2,3,4,5
sin(pi) log(x) z 1,2,3,4,5
Built-in functions sin, cos, log are built-in
functions in MATLAB. There are more built-in
functions we will be using in this class, they
are besseli, besselj, besselk, bessely,
gamma For more information on these functions,
please use help.
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Run-Time Errors

• Run-time errors occur when illegal operations are
  attempted during program execution.
• One run-time error occurs when attempting to
  access an element in a matrix that exceeds the
  dimensions of that matrix.
• Type the following into the Command Window
• a rand(3,4)
• b a(7,1)
• There are not 7 rows in the matrix A, so an error
  message is generated.

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Logical Errors

• Logical errors occur when the program runs
  without displaying an error, but produces an
  unexpected result.
• Such errors can be very difficult to find. We can
  compare simple test cases with known correct
  results in an attempt to find where these errors
  occur.
• For example, the following code is meant to
  determine the volume of a sphere
• V(4/3)pir2
• Where is the error?

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The Concept of For Loops

• Loops are MATLAB constructs that allow a sequence
  of MATLAB statements to be executed more than
  once.
• For loops repeat a block of commands for known
  number of times before the loop is executed.

• For loop syntax
• for index index matrix
• command 1
• command 2
• .
• .
• end

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A Simple For Loop Example

• The commands in a for loop are executed for each
  value in the index matrix.
• for i15
• xfactorial(i)
• end
• What is the value of the variable x in the
  Workspace?
• After executing the loop, if we call for x, it
  has only one value the value of the index the
  final time through the loop.
• If all the values shall be saved, use the
  following code
• for i15
• y(i)factorial(i)
• end

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Creating Matrices With for Loops

• Now we will fill a vector, element by element
  using a for loop

for n15 fpringf('The value of n is now
d\n',n) vector_1(n)n
vector_2(n)n2 end

• vector_1 stores the value of n
• vector_1 1 2 3 4 5
• vector_2 stores the square of n
• vector_2 1 4 9 16 25

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Try This

• An example

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The Code
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Nested Loops

• One loop can be written inside another loop. The
  inner loop is called a nested loop.
• One application is a multiplication table

for i 13 for j 13 prod
ij fprintf('d d d \n', i, j,
prod) end end
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Plotting in 2D

• Use the plot() command

epsilon10 C_as0.5 r0R/20R R5 rhor/R Ca
_Casbesseli(0,rhoepsilon)/besseli(0,epsilon)
plot(rho,Ca_Cas)

• This will create a plot
• of rho vs. Ca_Cas, or dependent
• vs. independent.

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Changing the plot

• A grid can help to interpolate the value of a
  function or a set of data.
• grid on
• grid off
• Adding a title and labels gives more meaning to a
  plot.
• title('\rho vs. C_A/C_A_s')
• xlabel('\rho')
• ylabel('C_A/C_A_s')

\rho is used to input Greek letter ?. Use the
help feature to search for how to input other
special characters (under text properties).
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Plotting Multiple Curves on a Figure
epsilon10 C_as0.5 r0R/20R R5 rhor/R Ca
_Casbesseli(0,rhoepsilon)/besseli(0,epsilon) ep
silon21 Ca2_Casbesseli(0,rhoepsilon2)/besseli(
0,epsilon2) plot(rho,Ca_Cas,rho,Ca2_Cas)
or plot(rho,Ca_Cas) hold on plot(rho,Ca2_Cas)
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Changing plot style

• Plot the data with a red, dash-dot line with red
  stars for points
• Rescale axis
• Add in a title
• Add in x, y labels
• Add in legends
• Add in text

plot(rho,Ca_Cas,'r-.') hold on plot(rho,Ca2_Cas)
axis(-0.1,1.1,-0.1,1.1) title('\rho vs.
C_A/C_A_s') xlabel('\rho') ylabel('C_A/C_A_s') leg
end('\epsilon 10','\epsilon
1',2) text(0,0.2,'prepared for CH561')
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Style reference table
Line type Indicator Marker type Indicator Color Indicator
solid - point . blue b
dotted circle o green g
dash-dot -. x-mark x red r
dashed -- plus cyan c
star magenta m
square s yellow y
diamond d black k
triangle down v
triangle up
triangle left lt
triangle right gt
pentagram p
hexagram h
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Subplots

• The subplot() function allows for putting
  multiple graphs in one figure.
• subplot(m,n,p) divides graphing window into a
  grid of m rows and n columns, where p identifies
  the part of the window where the plot will be
  drawn. These positions are counted from left to
  right along each row.

p 1 p 2
p 3 p 4
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Examples of Subplots

• To graph sin(x) and cos(x) on the same figure in
  separate plots

subplot(1,2,1) plot(rho,Ca_Cas) title('C_A_1/C_A
_s') xlabel('\rho') ylabel('C_A_1/C_A_s') subpl
ot(1,2,2) plot(rho,Ca2_Cas,'r-.') title('C_A_2/
C_A_s') xlabel('\rho')
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Saving Figures

• There are several ways to save plots created in
  MATLAB
• Store the MATLAB code for generating the plot in
  an M-file
• Save the figure as a .fig file, which is MATLABs
  graphics format, or in any other standard graphic
  format.
• Copy the figure into another document EditgtgtCopy
  Figure.

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Logarithmic Plots(pg. 155 1 pg. 178 2)

• MATLAB has three kinds of logarithmic plots
• semilogx
• semilogy
• loglog
• These plots replace linear scales with
  logarithmic scales.
• Logarithmic scales are used when a variable
  ranges over many orders of magnitude.

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Getting help in Matlab

• Function help
• Topic help

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Your Turn!

• Need an example