MATLABBasics

MATLAB User Environment

Workspace/VariableInspector

Command History

Command Window

Getting help

There are several ways of getting help:

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

>> help command-name

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

>> helpdesk

System Environment

• Windows– MATLAB installed in c:\matlab6.5– Your code…anywhere convenient (e.g. h:\matlab)

• Linux (Environment network)– MATLAB installed in /apps/matlab– Your code in /home/username/matlab– Your environment configuration in ~/.matlab

startup.m

• The script $matlab_root\toolbox\local\matlabrc.m is always run at startup – it reads system environment variables etc, and initialises platform 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.m for setting paths, forcing preferred settings, etc.

%-------------------------------------% 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/ndbcaddpath 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 cw96 addpath 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/jun23addpath 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)

• addpath – adds directories to the search path. MATLAB will look in ALL directories on the path for:– Functions and scripts (.m files)

– MATLAB data files (.mat files)

• It will also look in the current directory

• The ‘set’ commands in the example startup.m file set some default graphics properties, overriding the defaults – will cover these later.

The WORKSPACE

• MATLAB maintains an active workspace, any variables (data) loaded or defined here are always available.

• Some commands to examine workspace, move around, etc:

>> who

Your variables are:

x y

who : lists variables in workspace

>> whos Name Size Bytes Class

x 3x1 24 double array y 3x2 48 double array

Grand total is 9 elements using 72 bytes

whos : lists names and basic properties of variables in the workspace

>> pwdans =

D:\

>> cd cw96\jun02>> dir. 30m_wtv.mat edson2km.mat jun02_30m_runs.mat .. 960602_sst.mat edson_2km_bulk.mat

pwd, cd, dir, ls : similar to operating system (but no option switches)

VARIABLES

• Everything (almost) is treated as a double-precision floating point array by default– Typed variables (integer, float, char,…) are

supported, but usually used only for specific applications. Not all operations are supported for all typed variables.

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

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

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

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

x = 1 2 3 4

When defining variables, a space or comma separates elements on a row.

A newline or semicolon forces a new row; these 2 statements are equivalent. NB. you can break definitions across multiple lines.

• 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 6 4 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)

• Element-by-element operation is forced by preceding operator with ‘.’

>> a=[1 2];>> b=[3 4];

>> a.*b??? Error using ==> timesMatrix dimensions must agree. Size and shape must match

>> a=[1 2]A =

1 2

>> b=[3 4];

>> a.*bans =

3 8

>> c=a+bc =

4 6

Matrix addition & subtraction operate element-by-element anyway. Dimensions of matrix must still match!

No trailing semicolon, immediate display of result

Element-by-element multiplication

>> A = [1:3;4:6;7:9]A = 1 2 3 4 5 6 7 8 9

>> mean(A)ans = 4 5 6

>> sum(A)ans = 12 15 18

Most common functions operate on columns by default

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

>> A = [1:3;4:6;7:9]A = 1 2 3 4 5 6 7 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

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

• 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 =

110100220200330300

LOGICAL INDEXING

• Instead of indexing arrays directly, a logical mask can be used – an array of same size, but consisting of 1s and 0s – usually derived as result 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

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.

SAVING DATA

• MATLAB uses its own platform independent file format for saving data – files have a .mat extension– The ‘save’ command saves variables from the

workspace to a named file (or matlab.mat if no filename given)• save filename – saves entire workspace to filename.mat• save var1 var2 … filename – saves named variables

to filename.mat

– By default save overwrites an existing file of the same name, the –append switch forces appending data to an existing file (but variables of same name will be overwritten!)• save var1 var2 filename -append

– Data is recovered with the ‘load’ command• load filename – loads entire .mat file• load filename var1 var2 …– loads named variables

• load filename –ascii – loads contents of an ascii flatfile in a variable ‘filename’.

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

• X=load(‘filename’,’-ascii’) – loads the ascii file into a variable ‘X’

• save var1 filename –ascii – saves a single variable to an ascii flat file (rectangular array of numbers)

• There have been changes to the internal format of .mat files between MATLAB v4 and v5 (major changes to allow arrays with 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 to an old file format with –v4 and –v6 switches

save filename –v6