Lecture 02 visualization and programming

6.094
Introduction to programming in MATLAB
Danilo Šćepanović
IAP 2010
Lecture 2: Visualization and Programming

Outline
(1) Functions
(2) Flow Control
(3) Line Plots
(4) Image/Surface Plots
(5) Vectorization

User-defined Functions
• Functions look exactly like scripts, but for ONE difference
Functions must have a function declaration
Help file
Function declaration
InputsOutputs
Courtesy of The MathWorks, Inc. Used with permission.

User-defined Functions
• Some comments about the function declaration
• No need for return: MATLAB 'returns' the variables whose
names match those in the function declaration
• Variable scope: Any variables created within the function
but not returned disappear after the function stops running
function [x, y, z] = funName(in1, in2)
Must have the reserved
word: function
Function name should
match MATLAB file
name
If more than one output,
must be in brackets
Inputs must be specified

Functions: overloading
• We're familiar with
» zeros
» size
» length
» sum
• Look at the help file for size by typing
» help size
• The help file describes several ways to invoke the function
D = SIZE(X)
[M,N] = SIZE(X)
[M1,M2,M3,...,MN] = SIZE(X)
M = SIZE(X,DIM)

Functions: overloading
• MATLAB functions are generally overloaded
Can take a variable number of inputs
Can return a variable number of outputs
• What would the following commands return:
» a=zeros(2,4,8); %n-dimensional matrices are OK
» D=size(a)
» [m,n]=size(a)
» [x,y,z]=size(a)
» m2=size(a,2)
• You can overload your own functions by having variable
input and output arguments (see varargin, nargin,
varargout, nargout)

Functions: Excercise
• Write a function with the following declaration:
function plotSin(f1)
• In the function, plot a sin wave with frequency f1, on the
range [0,2π]:
• To get good sampling, use 16 points per period.
( )1sin f x
0 1 2 3 4 5 6 7
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1

Relational Operators
• MATLAB uses mostly standard relational operators
equal ==
not equal ~=
greater than >
less than <
greater or equal >=
less or equal <=
• Logical operators elementwise short-circuit (scalars)
And & &&
Or | ||
Not ~
Xor xor
All true all
Any true any
• Boolean values: zero is false, nonzero is true
• See help . for a detailed list of operators

if/else/elseif
• Basic flow-control, common to all languages
• MATLAB syntax is somewhat unique
IF
if cond
commands
end
ELSE
if cond
commands1
else
commands2
end
ELSEIF
if cond1
commands1
elseif cond2
commands2
else
commands3
end
• No need for parentheses: command blocks are between
reserved words
Conditional statement:
evaluates to true or false

for
• for loops: use for a known number of iterations
• MATLAB syntax:
for n=1:100
commands
end
• The loop variable
Is defined as a vector
Is a scalar within the command block
Does not have to have consecutive values (but it's usually
cleaner if they're consecutive)
• The command block
Anything between the for line and the end
Loop variable
Command block

while
• The while is like a more general for loop:
Don't need to know number of iterations
• The command block will execute while the conditional
expression is true
• Beware of infinite loops!
WHILE
while cond
commands
end

Exercise: Conditionals
• Modify your plotSin(f1) function to take two inputs: plotSin(f1,f2)
• If the number of input arguments is 1, execute the plot command
you wrote before. Otherwise, display the line 'Two inputs were
given'
• Hint: the number of input arguments are in the built-in variable
nargin

Plot Options
• Can change the line color, marker style, and line style by
adding a string argument
» plot(x,y,’k.-’);
• Can plot without connecting the dots by omitting line style
argument
» plot(x,y,’.’)
• Look at help plot for a full list of colors, markers, and
linestyles
color marker line-style

Playing with the Plot
to select lines
and delete or
change
properties
to zoom in/out
to slide the plot
around
to see all plot
tools at once

Line and Marker Options
• Everything on a line can be customized
» plot(x,y,'--s','LineWidth',2,...
'Color', [1 0 0], ...
'MarkerEdgeColor','k',...
'MarkerFaceColor','g',...
'MarkerSize',10)
• See doc line_props for a full list of
properties that can be specified
-4 -3 -2 -1 0 1 2 3 4
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
You can set colors by using
a vector of [R G B] values
or a predefined color
character like 'g', 'k', etc.

Cartesian Plots
• We have already seen the plot function
» x=-pi:pi/100:pi;
» y=cos(4*x).*sin(10*x).*exp(-abs(x));
» plot(x,y,'k-');
• The same syntax applies for semilog and loglog plots
» semilogx(x,y,'k');
» semilogy(y,'r.-');
» loglog(x,y);
• For example:
» x=0:100;
» semilogy(x,exp(x),'k.-');
0 10 20 30 40 50 60 70 80 90 100
10
0
10
10
10
20
10
30
10
40
10
50

-1
-0.5
0
0.5
1
-1
-0.5
0
0.5
1
-10
-5
0
5
10
3D Line Plots
• We can plot in 3 dimensions just as easily as in 2
» time=0:0.001:4*pi;
» x=sin(time);
» y=cos(time);
» z=time;
» plot3(x,y,z,'k','LineWidth',2);
» zlabel('Time');
• Use tools on figure to rotate it
• Can set limits on all 3 axes
» xlim, ylim, zlim

Axis Modes
• Built-in axis modes
» axis square
makes the current axis look like a box
» axis tight
fits axes to data
» axis equal
makes x and y scales the same
» axis xy
puts the origin in the bottom left corner (default for plots)
» axis ij
puts the origin in the top left corner (default for
matrices/images)

Multiple Plots in one Figure
• To have multiple axes in one figure
» subplot(2,3,1)
makes a figure with 2 rows and three columns of axes, and
activates the first axis for plotting
each axis can have labels, a legend, and a title
» subplot(2,3,4:6)
activating a range of axes fuses them into one
• To close existing figures
» close([1 3])
closes figures 1 and 3
» close all
closes all figures (useful in scripts/functions)

Copy/Paste Figures
• Figures can be pasted into other apps (word, ppt, etc)
• Edit copy options figure copy template
Change font sizes, line properties; presets for word and ppt
• Edit copy figure to copy figure
• Paste into document of interest

Saving Figures
• Figures can be saved in many formats. The common ones
are:
.fig preserves all
information
.bmp uncompressed
image
.eps high-quality
scaleable format
.pdf compressed
image

Advanced Plotting: Exercise
• Modify the plot command in your plotSin function to use
squares as markers and a dashed red line of thickness 2
as the line. Set the marker face color to be black
(properties are LineWidth, MarkerFaceColor)
• If there are 2 inputs, open a new figure with 2 axes, one on
top of the other (not side by side), and activate the top one
(subplot)
0 1 2 3 4 5 6 7
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.2
0.4
0.6
0.8
1
plotSin(6) plotSin(1,2)

Visualizing matrices
• Any matrix can be visualized as an image
» mat=reshape(1:10000,100,100);
» imagesc(mat);
» colorbar
• imagesc automatically scales the values to span the entire
colormap
• Can set limits for the color axis (analogous to xlim, ylim)
» caxis([3000 7000])

Colormaps
• You can change the colormap:
» imagesc(mat)
default map is jet
» colormap(gray)
» colormap(cool)
» colormap(hot(256))
• See help hot for a list
• Can define custom colormap
» map=zeros(256,3);
» map(:,2)=(0:255)/255;
» colormap(map);

Surface Plots
• It is more common to visualize surfaces in 3D
• Example:
• surf puts vertices at specified points in space x,y,z, and
connects all the vertices to make a surface
• The vertices can be denoted by matrices X,Y,Z
• How can we make these matrices
loop (DUMB)
built-in function: meshgrid
( ) ( ) ( )
[ ] [ ]
f x,y sin x cos y
x , ; y ,π π π π
=
∈ − ∈ −
2 4 6 8 10 12 14 16 18 20
2
4
6
8
10
12
14
16
18
20 -3
-2
-1
0
1
2
3
2 4 6 8 10 12 14 16 18 20
2
4
6
8
10
12
14
16
18
20 -3
-2
-1
0
1
2
3

surf
• Make the x and y vectors
» x=-pi:0.1:pi;
» y=-pi:0.1:pi;
• Use meshgrid to make matrices (this is the same as loop)
» [X,Y]=meshgrid(x,y);
• To get function values,
evaluate the matrices
» Z =sin(X).*cos(Y);
• Plot the surface
» surf(X,Y,Z)
» surf(x,y,Z);

surf Options
• See help surf for more options
• There are three types of surface shading
» shading faceted
» shading flat
» shading interp
• You can change colormaps
» colormap(gray)

contour
• You can make surfaces two-dimensional by using contour
» contour(X,Y,Z,'LineWidth',2)
takes same arguments as surf
color indicates height
can modify linestyle properties
can set colormap
» hold on
» mesh(X,Y,Z)

Exercise: 3-D Plots
• Modify plotSin to do the following:
• If two inputs are given, evaluate the following function:
• y should be just like x, but using f2. (use meshgrid to get
the X and Y matrices)
• In the top axis of your subplot, display an image of the Z
matrix. Display the colorbar and use a hot colormap. Set
the axis to xy (imagesc, colormap, colorbar, axis)
• In the bottom axis of the subplot, plot the 3-D surface of Z
(surf)
( ) ( )1 2sin sinZ f x f y= +

Specialized Plotting Functions
• MATLAB has a lot of specialized plotting functions
• polar-to make polar plots
» polar(0:0.01:2*pi,cos((0:0.01:2*pi)*2))
• bar-to make bar graphs
» bar(1:10,rand(1,10));
• quiver-to add velocity vectors to a plot
» [X,Y]=meshgrid(1:10,1:10);
» quiver(X,Y,rand(10),rand(10));
• stairs-plot piecewise constant functions
» stairs(1:10,rand(1,10));
• fill-draws and fills a polygon with specified vertices
» fill([0 1 0.5],[0 0 1],'r');
• see help on these functions for syntax
• doc specgraph – for a complete list

Revisiting find
• find is a very important function
Returns indices of nonzero values
Can simplify code and help avoid loops
• Basic syntax: index=find(cond)
» x=rand(1,100);
» inds = find(x>0.4 & x<0.6);
• inds will contain the indices at which x has values between
0.4 and 0.6. This is what happens:
x>0.4 returns a vector with 1 where true and 0 where false
x<0.6 returns a similar vector
The & combines the two vectors using an and
The find returns the indices of the 1's

Example: Avoiding Loops
• Given x= sin(linspace(0,10*pi,100)), how many of the
entries are positive?
Using a loop and if/else
count=0;
for n=1:length(x)
if x(n)>0
count=count+1;
end
end
Being more clever
count=length(find(x>0));
length(x) Loop time Find time
100 0.01 0
10,000 0.1 0
100,000 0.22 0
1,000,000 1.5 0.04
• Avoid loops!
• Built-in functions will make it faster to write and execute

Efficient Code
• Avoid loops
This is referred to as vectorization
• Vectorized code is more efficient for MATLAB
• Use indexing and matrix operations to avoid loops
• For example, to sum up every two consecutive terms:
» a=rand(1,100);
» b=zeros(1,100);
» for n=1:100
» if n==1
» b(n)=a(n);
» else
» b(n)=a(n-1)+a(n);
» end
» end
Slow and complicated
» a=rand(1,100);
» b=[0 a(1:end-1)]+a;
Efficient and clean.
Can also do this using
conv

End of Lecture 2
(1) Functions
(2) Flow Control
(3) Line Plots
(4) Image/Surface Plots
(5) Vectorization
Vectorization makes
coding fun!

MIT OpenCourseWare
http://ocw.mit.edu
6.094 Introduction to MATLAB®
January (IAP) 2010
For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.

Lecture 02 visualization and programming

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