Introduction to matlab lecture 4 of 4

>> pathname = 'C:UsersAyaDesktoppic';
>> for i = 1:10
imagename = strcat(pathname,int2str(i),'.jpg');
I = imread(imagename);
M(i) = mean(mean(mean(I)));% RGB image
end
>> save 'C:UsersAyaDesktopMEANS.mat' M;
>> MR=load ('C:UsersAyaDesktopMEANS.mat')
MR =
M: [93.6082 93.6082 93.6082 93.6082 93.6082]
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 Matlab plotting
› 2D plots
› 3D plots
 M-Files
› Scripts
› User defined functions
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 Matlab has a lot of function for plotting
data.
 The basic one will plot one vector vs.
another. The first one will be treated as the
abscissa (or x) vector and the second as
the ordinate (or y) vector.
 The vectors have to be the same length.
 >> plot (time, dist) % plotting versus time
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 Matlab will also plot a vector vs. its own
index. The index will be treated as the
abscissa vector.
 Given a vector “time” and a vector
“dist” we could say:
 >> plot (dist) % plotting versus index
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 Note: plot(g,h)  g and h must have same
length, direction
 Customize plot by editing in Figure Window
› Point and click edit mode with toolbar back
arrow
 Change axis property
 Change line style and color
 Change background color
 Add axis labels and title
 Insert legend and text
9

 There are commands in Matlab to
"annotate" a plot to put on axis labels,
titles, and legends.
 To put a label on the axes we would use:
› >> xlabel ('X-axis label')
› >> ylabel ('Y-axis label')
 To put a title on the plot, we would use:
› >> title ('Title of my plot')
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 Make a 3-D line plot
› Create 3 same-length vectors, e.g.,
 >> p = [0:0.1:10]; % range vector
 >> q = p./2; % same length range vector
 >> r = sin(p).*cos(q); % function vector
› Plot the 3-D curve –
 Example: >> plot3(p,q,r)
› Rotate the curve in 3-D using toolbar icon
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 Make a 3-D surface plot
› Create a matrix of function values
 Example: >> S = ((sin(p))') * (cos(q));
› Plot a surface of matrix values
 >> surf(S) % polygonal facets
 >> mesh(S) % wire mesh
› Rotate the plot in 3-D using toolbar icon
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 Example:
 Supposed we want to visualize a function
Z = 10e(–0.4a) sin (2ft) for f = 2
when a and t are varied from 0.1 to 7 and 0.1 to 2,
respectively
 >>> [t,a] = meshgrid(0.1:.01:2, 0.1:0.5:7);
>>> f=2;
>>> Z = 10.*exp(-a.*0.4).*sin(2*pi.*t.*f);
>>> surf(Z);
>>> figure(2);
>>> mesh(Z);
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 Example:
 >>> [x,y] = meshgrid(-
3:.1:3,-3:.1:3);
>>> z = 3*(1-x).^2.*exp(-
(x.^2) - (y+1).^2) ...
- 10*(x/5 - x.^3 -
y.^5).*exp(-x.^2-y.^2) ...
- 1/3*exp(-(x+1).^2 - y.^2);
>>> surf(z);
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Graph Functions (summary)
 Plot linear plot
 stem discrete plot
16

 grid add grid lines
 xlabel add X-axis label
 ylabel add Y-axis label
 title add graph title
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 subplot divide figure window
 figure create new figure window
 pause wait for user response
 hold on allows multiple plots on same axes
 clf clears the figure window
 axis([xmin,xmax,ymin,ymax]) controls axis properties
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 Example:
› plot(x,y,’r’) is a red line
› plot(x,y,’o’) plots circles rather than lines
› plot(x,y,’yp’) plots yellow pentagrams
 Specialized 1D graphics
› bar--bar chart
› pie--pie chart
› polar--polar coordintes
› semilogy, semilogx, loglog—plotting with log-
scales
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Drawing a bar graph
 >> clear, close all
>> clc
>> x = 0:pi/36:2*pi
>> y = cos(x)
>> bar(x,y,'b')
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Drawing a stair-stop plot
 >> clear, close all
>> clc
>> x = -10:0.5:10
>> y = x.^2 + 2.*x + 2
>> stairs(x,y,'b')
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Elements of Matlab as a programming
language:
› Expressions: Arithmetic, logical, etc.
› Flow Control blocks: Conditional and Iterations
› Scripts
› Functions
23

 When problems become complicated and require re–
evaluation, entering command at MATLAB prompt is not
practical
 M-files are text files containing Matlab programs. Can be
called form the command line or from other M-files
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M-files: Scripts
 Without input
arguments,
they do not
return any
value.
25

 To run the M-file, type in the name of the file
at the prompt e.g. >>> test1
 It will be executed provided that the saved
file is in the known path
 Type in matlabpath to check the list of
directories listed in the path
 Use path editor to add the path: File --> Set
path …
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M-files: Functions
 Function is a ‘black box’ that
communicates with workspace through
input and output variables.
27

M-files: Functions
 With parameters and returning values
 Only visible variables defined inside the
function or Parameters
 Usually one file for each function defined
 File must be saved to a known path with
filename the same as the function name and
with an extension ‘.m’
 Call function by its name and arguments
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 Write a script/function that
converts a Roman numeral to its
decimal equivalent.
 You should be able to handle
the following conversion table:
 It will be useful to get the
Roman number into your
program as a string

 Matrix operators in Matlab are much faster than loops. Fast
Matlab code uses * and avoids loops
 Use built-in functions as they are often heavily optimized
 Minimize division – x/2 takes longer than 0.5*x
 Do computations outside loop
 Pre-allocate arrays
for j=1:n;a(j)=<something>;end
Setting a=zeros(1,n) before the loop speeds things up
 Use subfunctions
file fname.m:
function O=fname(I)
function O2=fname2(I2)
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 http://www.cs.cornell.edu/Courses/cs401/2001fa
› Contains syllabus, lecture notes, examples, homework
 http://www.mathworks.com/matlabcentral/
32

 Advanced data objects (cell-arrays and
structs)
 Special tool boxes
 Simulink
 Graphical User Interface
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Introduction to matlab lecture 4 of 4

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Introduction to matlab lecture 4 of 4