What is your favourite MATLAB/Octave programming trick? [closed]

Answer 1

LaTeX mode for formulas in graphs: In one of the recent releases (R2006?) you add the additional arguments ,'Interpreter','latex' at the end of a function call and it will use LaTeX rendering. Here's an example:

t=(0:0.001:1);
plot(t,sin(2*pi*[t ; t+0.25]));
xlabel('t'); 
ylabel('$\hat{y}_k=sin 2\pi (t+{k \over 4})$','Interpreter','latex');
legend({'$\hat{y}_0$','$\hat{y}_1$'},'Interpreter','latex');

Not sure when they added it, but it works with R2006b in the text(), title(), xlabel(), ylabel(), zlabel(), and even legend() functions. Just make sure the syntax you are using is not ambiguous (so with legend() you need to specify the strings as a cell array).

Answer 2

Here's a bunch of nonobvious functions that are useful from time to time:

• mfilename (returns the name of the currently running MATLAB script)
• dbstack (gives you access to the names & line numbers of the matlab function stack)
• keyboard (stops execution and yields control to the debugging prompt; this is why there's a K in the debug prompt K>>
• dbstop error (automatically puts you in debug mode stopped at the line that triggers an error)

Answer 3

Contour plots with [c,h]=contour and clabel(c,h,'fontsize',fontsize). I usually use the fontsize parameter to reduce the font size so the numbers don't run into each other. This is great for viewing the value of 2-D functions without having to muck around with 3D graphs.

Answer 4

Using ismember() to merge data organized by text identfiers. Useful when you are analyzing differing periods when entries, in my case company symbols, come and go.

%Merge B into A based on Text identifiers
UniverseA = {'A','B','C','D'};
UniverseB = {'A','C','D'};

DataA = [20 40 60 80];
DataB = [30 50 70];

MergeData = NaN(length(UniverseA),2);

MergeData(:,1) = DataA;

[tf, loc] = ismember(UniverseA, UniverseB);

MergeData(tf,2) = DataB(loc(tf));

 MergeData =

20    30
40   NaN
60    50
80    70

Answer 5

Using nargin to set default values for optional arguments and using nargout to set optional output arguments. Quick example

function hLine=myplot(x,y,plotColor,markerType)
% set defaults for optional paramters
if nargin<4, markerType='none'; end
if nargin<3, plotColor='k'; end

hL = plot(x,y,'linetype','-', ...  
              'color',plotColor, ...
              'marker',markerType, ...
              'markerFaceColor',plotColor,'markerEdgeColor',plotColor);

% return handle of plot object if required
if nargout>0, hLine = hL; end

Answer 6

Be strict with specifying dimensions when using aggregation functions like min, max, mean, diff, sum, any, all,...

For instance the line:

reldiff = diff(a) ./ a(1:end-1)

might work well to compute relative differences of elements in a vector, however in case the vector degenerates to just one element the computation fails:

>> a=rand(1,7);
>> diff(a) ./ a(1:end-1)

ans =
   -0.5822   -0.9935  224.2015    0.2708   -0.3328    0.0458

>> a=1;
>> diff(a) ./ a(1:end-1)
??? Error using ==> rdivide
Matrix dimensions must agree.

If you specify the correct dimensions to your functions, this line returns an empty 1-by-0 matrix, which is correct:

>> diff(a, [], 2) ./ a(1, 1:end-1)

ans =

   Empty matrix: 1-by-0

>> 

The same goes for a min-function which usually computes minimums over columns on a matrix, until the matrix only consists of one row. - Then it will return the minimum over the row unless the dimension parameter states otherwise, and probably break your application.

I can almost guarantee you that consequently setting the dimensions of these aggregation functions will save you quite some debugging work later on.

At least that would have been the case for me. :)