matlab (1): the basics

8/12/2019 Matlab (1): The Basics

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MATLABBasics


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MATLAB User Environment

Workspace/Variable

Inspector

Command History

Command Window


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Getting help

There are several ways of getting help:

Basic help on named commands/functions is echoed to the commandwindow by:

>> help command-name

A complete help system containing full text of manuals is started by:

>> helpdesk


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System Environment

Windows MATLAB installed in c:\matlab6.5

Your codeanywhere convenient (e.g. h:\matlab)

Linux (Environment network) MATLAB installed in /apps/matlab

Your code in /home/username/matlab

Your environment configuration in ~/.matlab


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startup.m

The script $matlab_root\toolbox\local\matlabrc.mis always run at startupit reads systemenvironment variables etc, and initialisesplatform dependent settings. If present it will run

a user defined initialisation script: startup.m Linux: /home/user/matlab/startup.m

Windows: $matlab_root\toolbox\local\startup.m

Use startup.mfor setting paths, forcingpreferred settings, etc.


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%-------------------------------------% Matlab startup file for IMB's laptop

%-------------------------------------

%-- add paths for my m-files --addpath d:/matlabaddpath d:/matlab/bulk2.5addpath d:/matlab/bulk2.6addpath d:/matlab/coastaladdpath d:/matlab/lidaraddpath d:/matlab/ndbc

addpath d:/matlab/pageaddpath d:/matlab/sectionsaddpath d:/matlab/sharemaddpath d:/matlab/waveletaddpath d:/matlab/LEMaddpath d:/matlab/GPSbookaddpath d:/matlab/FAAMaddpath d:/matlab/FAAM/windsaddpath d:/matlab/faam/bae

%-- add netCDF toolbox --addpath c:/matlab701/binaddpath c:/matlab701/bin/win32addpath d:/matlab/netcdfaddpath d:/matlab/netcdf/ncfilesaddpath d:/matlab/netcdf/nctypeaddpath d:/matlab/netcdf/ncutility

%-- add path for generic data --addpath d:/matlab/coastlines % coastline data

addpath d:/cw96/flight_data/jun02 % raw cw96addpath d:/cw96/flight_data/jun07 % aircraft dataaddpath d:/cw96/flight_data/jun11addpath d:/cw96/flight_data/jun12addpath d:/cw96/flight_data/jun17addpath d:/cw96/flight_data/jun19addpath d:/cw96/flight_data/jun21addpath d:/cw96/flight_data/jun23

addpath d:/cw96/flight_data/jun26addpath d:/cw96/flight_data/jun29addpath d:/cw96/flight_data/jul01

addpath d:/cw96/runs % run definitions for cw96 flights

%----------------------------------------------------------------------%-- set default figure options --set(0,'DefaultFigurePaperType','a4')

% this should be the default in EU anywayset(0,'DefaultFigurePaperUnits','inches')

% v6 defaults to cm for EU countriesset(0,'DefaultFigureRenderer','painters')

% v7 default OpenGL causes problems

Example startup.m file (for my laptop)


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addpathadds directories to

the search path. MATLAB willlook in ALL directories on thepath for:

Functions and scripts (.m files)

MATLAB data files (.mat files)

It will also look in the currentdirectory

The set commands in the

example startup.m file setsome default graphicsproperties, overriding thedefaultswill cover theselater.


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The WORKSPACE

MATLAB maintains an active workspace, anyvariables (data) loaded or defined here arealways available.

Some commands to examine workspace, movearound, etc:

>> who

Your variables are:

x y

who: lists variables in workspace


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VARIABLES

Everything (almost) is treated as a double-precision floating point arrayby default

Typedvariables (integer, float, char,) aresupported, but usually used only for specific

applications. Not all operations are supported for alltyped variables.

[IDL uses typed variables, but allows mixing oftypes...at least to some extent]


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>> x=[1 2 3]x =

1 2 3

>> x=[1,2,3]x =

1 2 3

>> x=[1234];

>> x=[1;2;3;4]

x =1234

When defining variables, a spaceorcommaseparates elements on arow.

A newlineor semicolonforces anew row; these 2 statements areequivalent.NB. you can break definitions acrossmultiple lines.


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1 & 2D arrays are treated as formal matrices Matrix algebra works by default:

>> a=[1 2];>> b=[3

4];

>> a*bans =

11

>> b*aans =

3 64 8

1x2 row oriented array (vector)(Trailing semicolon suppresses display of output)

2x1 column oriented array

Result of matrix multiplication depends on

order of terms (non-cummutative)


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Element-by-element operation is forced bypreceding operator with .

>> a=[1 2];>> b=[3

4];

>> a.*b??? Error using ==> times

Matrix dimensions must agree. Size and shape must match


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>> a=[1 2]A =

1 2

>> b=[3 4];

>> a.*bans =

3 8

>> c=a+b

c =4 6

Matrix addition & subtraction

operate element-by-elementanyway. Dimensions ofmatrix must still match!

No trailing semicolon,immediate display of result

Element-by-elementmultiplication


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>> A = [1:3;4:6;7:9]A =

1 2 3

4 5 67 8 9

>> mean(A)ans =

4 5 6

>> sum(A)ans =

12 15 18

Most common functions operate on

columns by default


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INDEXING ARRAYS

MATLAB indexes arrays:

1 to N [row,column]

[1,1 1,2 . 1,n

2,1 2,2 . 2,n

3,1 3,2 . 3,n. . .

m,1 m,2 . m,n]

IDL indexes arrays:

0 to N-1 [column,row]

[0,0 1,0 . n-1,0

0,1 1,1 . n-1,1

0,2 1,2 . n-1,2. . . .

0,m-1 1,m-1 . n-1,m-1]

n

m


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>> A = [1:3;4:6;7:9]A =

1 2 3

4 5 67 8 9

>> A(2,3)ans =

6

>> A(1:3,2)ans =

258

>> A(2,:)ans =

4 5 6

The colon indicates a range, a:b (a to b)

A colon on its own indicates ALL values


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THE COLON OPERATOR

Colon operator occurs in several forms To indicate a range (as above) To indicate a range with non-unit increment

>> N = 5:10:35

N =

5 15 25 35

>> P = [1:3; 30:-10:10]

P =

1 2 3

30 20 10


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To extract ALL the elements of an array(extracts everything to a single column vector)

>> A = [1:3; 10:10:30;

100:100:300]

A =1 2 3

10 20 30

100 200 300

>> A(:)ans =

1

101002202003

30300


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LOGICAL INDEXING

Instead of indexing arrays directly, a logicalmask can be usedan array of same size, butconsisting of 1s and 0susually derived asresult of a logical expression.

>> X = [1:10]X =

1 2 3 4 5 6 7 8 9 10

>> ii = X>6

ii =

0 0 0 0 0 0 1 1 1 1

>> X(ii)

ans =

7 8 9 10


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Basic Operators

+, -, *, / : basic numeric operators

\: left division (matrix division)

^: raise to power

: transpose (of matrix)flip along diagonal

fliplr(), flipud(): flip matrix about

vertical and horizontal axes.


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Data is recovered with the load command

load filenameloads entire .mat file load filenamevar1 var2 loads named variables

load filename asciiloads contents of an asciiflatfile in a variable filename.

The ascii file must contain a rectangular array of numbers sothat it loads into a single matrix.

X=load(filename,-ascii)loads the ascii file into

a variable X

save var1 filename asciisaves a single variableto an ascii flat file (rectangular array of numbers)


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There have been changes to the internal format of .mat files

between MATLAB v4 and v5 (major changes to allow arrayswith more than 2 dimensions, structures, cell arrays), and

again with v7 (minor change to use unicode instead of ascii).Later versions will load old format files. You can force save toan old file format withv4 andv6 switches

save filename v6

matlab (1): the basics

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