EOSC 473/573 Matlab Tutorial R. Pawlowicz with changes by M. Halverson

Transcription

1 EOSC 473/573 Matlab Tutorial R. Pawlowicz with changes by M. Halverson February 12, 2008 Getting help 1. Local On-line help (a) text-based help: >> help <function name> (b) GUI-help >> helpwin (c) Browser-based HTML (and/or PDF) documentation: >> helpdesk (d) Don t know the function name? >> lookfor <keyword>. (e) Where is the function? >> which <function name> (f) What s the code look like? >> type <function name> (g) A bunch of neat demos - >> demo 2. Other resources (a) TMW Web site: user-contributed code bug reports and solutions (with search engine) technical support All manuals etc. (b) Matlab-for-oceanographers: All kinds of matlab code and toolboxes by and for oceanographers. (c) Newsgroups: news:comp.soft-sys.matlab Q-and-A by people around the world as well as programmers/mathematicians at The Mathworks (also available through (d) Books - lots of them. Mastering Matlab 7 by Hanselman and Littlefield is a good reference. 3. Alternatives OCTAVE is a free (GPL ed) Matlab-a-like ( the mathematical parts of Matlab code can often run under Octave with at most minor changes (and vice versa). PYTHON is a programming language that can be used in a matlablike way. 1

2 Where to begin 1. First thing is...what is a command line for? Simple commands, e.g. x=5 Command line >> Language constructs, e.g. for k=1:10, disp(k); end; (loops, if/else, built in functions such as plot) m files (scripts collections of commands and constructs and. functions code chunks with specified input/output parameters.) 2. Fundamental concept of data - MATRICES AND VECTORS (or TA- BLES and COLUMNS). Note that everything after a % is treated as a comment in Matlab. A ; at the end of a line (before comments) causes the result of the operation NOT to print out. >> x = [ ]; % This is a row* >> y = [3;5;2;1]; % This is a column >> z = [ ; ]; % This is a table with two rows and 4 columns >> z2 = rand(10,5); % This returns a 10x5 table of random numbers generated using >> % the rand function (*Note that square brackets [, ] are used to open/close the table.) 3. What can we do with a variable? >> xx=x+2 % A new variable >> x*3 % 3 times those values >> mean(x)^2 % Square of the mean >> mean(x.^2) % mean of the square. Note the. for elementwise operations >> % If you type x*x you get MATRIX MULTIPLICATION 4. Are all variables numbers? No...we also have strings, and the handy NaN which is often used as a flag for MISSING DATA. >> ss= Station Deep Hole ; % String Variable >> dd=[1 2 5 NaN 3 4 1]; % 4th data element missing 5. Can I change one (or a few) elements in a Vector without retyping the whole thing? YES: >> dd(2)=3 % Change only 2nd element >> dd([1 5])=[ ] % Change first and 5th elements >> z(:,[1 2]) % Print out first two columns >> dd([1:2:end]) % prints out all the odd elements >> % using a <start number>:<step>:<end number> convention 2

3 6. Are all variables just tables or strings? Nope - we can put together STRUCTURES - variables that contain subvariables: >>c.name = These numbers represent times ; >>c.vals = [ ]; Now the variable c contains both a string (as c.name) and a table (as c.vals). 7. Can I save these variables? Yes, the whole workspace can be saved using >> save workspacename % Save everything into workspacename.mat (binary file) >> save workspacename x % Saves only the variable x >> save x.dat x -ascii % saves it as an ASCII table. 8. And what about reloading data? >>load workspacename % Reads a binary file created with save >>load x.dat % Reads an ASCII table as input 9. Let s plot something... >>plot(dd) plots the elements in the vector dd against their INDEX. A more complicated plot would be >>plot(dd, color, r, marker, o, linestyle, -, linewi,3); which plots things as tick red circles, as well as joinging the points with a line. Handle Graphics The plot command is a relatively simple kind of plotting that dates back to the early days of Matlab. Starting in Version 4 they incorporated what are called HANDLE GRAPHICS. The idea is that everything you see on a screen is actually special kind of structure whose name is a HANDLE. These come in a hierarchy of parents and children, and also have associated properties: 1. How do I get the handle of the current figure/axes/object? >> gcf >> gca >> gco 3

4 FIGURE properties size, colour, location, etc. AXES 1 AXES 2 AXES 3 AXES 4... properties size, colour, location, grid lines, axes and grid labels LINES SURFACE TEXT IMAGE properties colour, linewidth, fontsize, clipping, etc. 2. How do I see the properties of the current figure/axes/object? >> get(gcf) >> get(gca) >> get(gco) 3. How do I change the properties of the current figure/axes/object? You add PROPERTY/VALUE pairs in a set command. The first parameter of the set call is the HANDLE, which can be the value returned by a call to (for example) gca: >> set(gca, color, r ) >> set(gca, xticklabel,[1:10], fontsize,20, color, b ) Note that you can make pretty much any kind of plot you want - there are many built-in functions and you can of course write code to your heart s content. For example, a bar graph would be >>bar(x,y); and a more complicated version might look like this (note we have added a LEGEND with labels): >> bar(rand(10,5), stacked ), colormap(cool) >> legend( A, B, C, D, E ); 4. How do I make more than one plot axes per figure? The easiest way is to use the subplot command, which defines an m-by-n grid of plots and makes pth one the current one: >> subplot(m,n,p); A more fancy way is to use the axes command: >> axes( position,[ ]); % vector is [x_left y_bottom width height] 4

5 5. Other useful graphing commands are: >> axis([xmin xmax ymin ymax]); % To force axis limits (see also xlim and ylim commands) >> title( A string for the plot title ); >> xlabel( something for the xlabel );- >> ylabel( a label for the vertical axis ); >> text(x,y, a string to placed at location (X,Y) ); >> grid; % Toggles a grid on/off Plotting CTD data Here we ll go through an example of how to load a CTD cast and plot vertical profiles. Additionally, we will invent some bottle data to plot on the CTD profile. Finally, we ll beautify the plot to make it publication quality. 1. First load the CTD cnv file with ctd_rd.m. >>ctd=ctd_rd( bam03045.cnv, IR ); %ctd will be a structure Note that this is also an example of a function call - the m-file ctd_rd.m is called with two input parameters - the filename (a string) and IR (a flag for files generated by internally recording CTDs), and it returns a structure ctd. 2. Now make up some bottle data (or use the real thing after Bamfield!)... >> btl_depth=[ ]; >> btl_sal=[ ]; 3. plot the CTD cast... >> figure; >> plot(ctd.sal00,ctd.prsm, k ); 4. if you have more casts to plot on the same axis... >> hold on >> plot(ctd2.sal00,ctd2.prsm, k, linewidth,2) where ctd2 is another ctd profile loaded into Matlab with ctd_rd. 5. over-plot the bottle data... >> plot(btl_sal,btl_depth, k:o, markerfacecolor, k ) 6. set axis properties and annotate... >> set(gca, ydir, reverse, ylim,[0 260], xlim,[24 34], fontsize,14); >> title({ctd.station datestr(ctd.mtime)}); >> ylabel( depth [m] ); >> xlabel( salinity ) 7. add a legend... >> legend( CTD salinity, bottle salinity ) 5

6 8. finally plot at T-S diagram for the cast... >> figure; >> tsdigrm([25 34],[7 11],0,[20:27]) >> line(ctd.sal00,ctd.t090c, color, k ) 9. Fancy methods to plot depth profiles of multiple variables (T, S, O 2, etc) on a single axis using the floataxis command can be seen in ctdplot_example.m and BAMPLOTS.m. 6