Flumes Short Introduction to MATLAB
Peter Nordin
July 30, 2014
This guide is meant for mechanical-engineering students with little or no prior MATLAB
experience. It introduces some useful MATLAB commands. This guide may be improved
sometimes so check http://www.iei.liu.se/flumes/guides?l=en for the latest version.
Contents
1 Introduction
1.1 What is MATLAB? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 GNU Octave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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2 Commands
2.1 Basic Commands . . . . . . . . . . . . . .
2.2 Creating Scalars, Vectors and Matrices . .
2.3 Accessing Vector and Matrix Elements . .
2.4 Accessing Struct Fields . . . . . . . . . . .
2.5 Plotting . . . . . . . . . . . . . . . . . . .
2.5.1 Line and Marker Style and Colors
2.5.2 Plot Handles and Properties . . . .
2.5.3 A Plot Example . . . . . . . . . .
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3 Scripts and Functions
3.1 Scripts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1
Introduction
1.1
What is MATLAB?
From the MathWorks webpage1 :
R
is a high-level language and interactive environment for numerical compu“MATLAB
tation, visualization, and programming. Using MATLAB, you can analyze data, develop
algorithms, and create models and applications. The language, tools, and built-in math
functions enable you to explore multiple approaches and reach a solution faster than with
spreadsheets or traditional programming languages, such as C/C++ or JavaTM .”
MATLAB is free to use if you are a student at Linköping University. You can find it
installed on the computers in “all” of our computer labs and you can also download it and
install it on your home computer using the university license (while you are a student).
1.2
GNU Octave
An open-source (and free) alternative to MATLAB is GNU Octave that offers similar capabilities (at least the basic ones). Its syntax is mostly compatible to MATLAB and most
examples from this guide will work in Octave too (there may be minor differences).
From the webpage2 :
“GNU Octave is a high-level interpreted language, primarily intended for numerical computations. It provides capabilities for the numerical solution of linear and nonlinear problems,
and for performing other numerical experiments. It also provides extensive graphics capabilities for data visualization and manipulation. Octave is normally used through its interactive
command line interface, but it can also be used to write non-interactive programs. The Octave
language is quite similar to Matlab so that most programs are easily portable.”
2
Commands
This section gives a few examples of MATLAB commands and syntax that might be useful.
2.1
Basic Commands
help
help command
doc command
format
clear
clear varName
close all
load ’data.mat’
save
1
2
List help topics
Show text help for particular command
Open manual page for command
Change display format of values, see help for options
Remove all variables
Remove one (or multiple) variables
Close all plot windows
Load data from .mat file
Save data to .mat
http://www.mathworks.se/products/matlab/ (2014-01-03)
http://www.gnu.org/software/octave/ (2014-01-03)
1
2.2
Creating Scalars, Vectors and Matrices
a = []
a=5
a = [1 2 3 4 5 6]
a = [1 2 3; 4 5 6]
a = 0:0.1:1
a = linspace(start,stop,nSteps)
t = linspace(Tstart,Tend,F0*Tend+1)
a = sin(2*pi*f*t)
a
a
a
a
=
=
=
=
2.3
zeros(nRows, nCols)
ones(nRows, nCols)
eye(nRowsCols)
[a, newData]
Accessing Vector and Matrix Elements
a = V(1)
a = A(2,3)
a = A(2,end)
a = A(:,1)
a = A(2,:)
a = A(1:799)
c = C1
l = length(V)
s = size(M)
2.4
Create empty variable
Assign/create a variable with value
Create a vector (1x6 Matrix)
Create a 2x3 Matrix
Create a vector from zero to one with steps of 0.1
An other way to create a linear space vector
Create a time vector at frequency F0
Create a data vector according to given function (t
is a time vector)
Create a matrix or vector of zeros
Create a matrix or vector of ones
Create a unit matrix
Append newData (scalar or vector) to a
Access element 1 in vector V
Access element at row=2 column=3
Access element at row=2 last column
Access elements at all rows in column 1
Access elements at row=2 all columns
Extract elements 1 to 799 from A
Access an element of a Cell-array
Get the length of a vector
Get the size of a matrix or vector (nRows,nCols)
Accessing Struct Fields
le.data = y
le.samp
le.samp.t
In struct named le, add variable y as a filed named data
From struct named le, access sub-struct samp and show contents
From struct named le, access sub-struct samp and finally data vector t
2
2.5
Plotting
plot(d)
Plot vector d (element number will appear on xaxis)
Plot vector t on x-axis and d on y-axis
Plot t,d with red color
Plot t,d with red color and marker *
Plot t,d with red color, marker * and dashed lines
Allows you to set additional plot properties
Set the axis limits
Set the x-axis label
Set the y-axis label
Set the plot title
Set the data legend
Create or open a figure, n: figure number
Clear current figure
Hold plots on consecutive plot commands to same
figure
Turns off hold
Can be used to save the current figure to file (or
printer)
Can be used as print but with less options
Print a png at 150 dpi
plot(t, d)
plot(t, d, ’r’)
plot(t, d, ’r*’)
plot(t, d, ’r*--’)
plot(t, d, ’PropName’, value)
axis([xmin xmax tmin ymax])
xlabel(’Text’)
ylabel(’Text’)
title(’Text’)
legend(’Curve1’, ’Curve2’)
figure(n)
clf
hold on
hold off
print
saveas
print(’-dpng’, ’-r150’, ’pic.png’)
2.5.1
Line and Marker Style and Colors
colors
bgrcmykw
line types
- : -. -marker types
. ox+*sd
See help plot for all colors lines and markers and more.
2.5.2
Plot Handles and Properties
h = plot(t,d)
get(h)
set(h, ’PropName’, value)
gca, gco, gcf
Get a handle (h) to the plot object
List all properties of a plot object, or get a particular one
Lets you change a property after the plot has been drawn
Returns a handle to the current Axis, GraphicsObject or
Figure. Works with get() and set() commands.
Use gca properties if you want to change axis appearance such as label font size.
2.5.3
A Plot Example
figure (1)
clf
h o l d on
p l o t ( t , y0 , ’ b ’ )
p l o t ( t , y1 , ’ k−− ’ )
hold o f f
%
%
%
%
%
%
Open f i g u r e 1
Clear the f i g u r e
Begin h o l d i n g p l o t data
P l o t t , y with b l u e c o l o r
p l o t t , y1 , with a dashed b l a c k l i n e
Stop h o l d i n g p l o t data
3
% Now s e t t h e s i z e o f c u r r e n t f i g u r e and e x p o r t f i g u r e a t 150 d p i
% g c f means c u r r e n t f i g u r e
s e t ( g c f , ’ PaperUnits ’ , ’ c e n t i m e t e r s ’ )
s e t ( g c f , ’ P a p e r S i z e ’ , [ 1 2 , 1 0 ] , ’ P a p e r P o s i t i o n ’ , [ 0 0 12 1 0 ] )
p r i n t ( ’−dpng ’ , ’−r 1 5 0 ’ , ’ p i c t u r e . png ’ )
3
Scripts and Functions
3.1
Scripts
You can write programs or scripts in .m files. A script is a file with multiple consecutive
commands. An example is the plot example in section 2.5.3. The following characters have
special meaning in scripts:
%
Write a comment after the % sign
%%
Start a New Matlab section (can be executed with ctrl+enter)
3.2
Functions
A function is a self contained group of commands that can be called multiple times. You can
reuse the code that is written in one place only. Functions usually take arguments and return
results. In MATLAB you can hover a function with your mouse and press F1, to see its help.
You can also write you own functions. This is an example of a reusable MATLAB function
f u n c t i o n [ r e t 1 , r e t 2 ] = doSomethingCool ( arg1 , arg2 , a r g 3 )
% This i s t h e h e l p t e x t
% doSomthingCool d o e s something c o o l and h e r e we d e s c r i b e what
% Syntax : [ r e t 1 , r e t 2 ] = doSomethingCool ( arg1 , arg2 , a r g 3 )
% Arguments :
%
a r g 1 = D e s c r i p t i o n o f argument 1
%
a r g 2 = D e s c r i p t i o n o f argument 2
%
a r g 3 = D e s c r i p t i o n o f argument 3
% Returns :
%
ret1 = Description of return value 1
%
ret2 = Description of return value 2
>> Your code g o e s h e r e <<
% I t s h o u l d do something c o o l and c r e a t e t h e r e t 1 and r e t 2
% based on i n p u t t h e i n p u t arguments
You can call the function like this:
[ x y ] = doSomethingCool ( 1 , 2 , ’ monkey ’ ) ;
The x and y variables will get the ret1 and ret2 values. Inside the function arg1 will be 1,
arg2 will be 2 and arg3 will be the text string monkey
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