Common Problems with Matlab

Sometimes Matlab does not give you the result which you would expect. There are some peculiarities you have to be aware of.

General

How can I see more than 5 digits of my results?: Matlab does all computations with 15 digits accuracy, but shows only 5 to keep the output more compact. Also, in vectors and matrices Matlab uses a common factor 10^m for all elements. If matrix elements have very different sizes, small elements will be displayed as 0.0000. Use format long g to see all computed digits, and to get scientific notation for each matrix element.
For fprintf use %.15g, e.g., fprintf('The answer is %.15g\n',x)
When I use ``e^x'' or ``ln(x)'' I get strange results or errors: The exponential function must always be written as exp(x), not e^x. The natural logarithm function must be written as log(x), not ln(x). The base 10 logarithm is log10(x), not log(x).

Graphs

How can I label a graph?

Use the title, xlabel, ylabel, legend commands, e.g.

x=0:.1:10; y1=sin(x); y2=cos(x); plot(x,y1,'-',x,y2,'--') title('Trigonometric functions') xlabel('x') ylabel('y') legend('sin(x)','cos(x)')

ezplot does not show all of my plot

ezplot tries to select the y-range in such a way that you only see the interesting part of your functions. Try using axis tight or axis([xmin xmax ymin ymax]) after the ezplot command..

My graph does not fill the whole page.

By default the paper is vertically oriented (``portrait''), and the plot is in a rectangle which is wider than high (ratio 4:3), so that there is a lot of empty space on the page. To obtain a larger plot with the page in horizontal orientation (``landscape''), use

orient landscape
print test1.ps    % save plot in file test1.ps

How can I put several graphs on one page?

Here is an example which puts one graph in the top half and one graph in the bottom half of the page:

subplot(2,1,1)    % divide page in 2 rows, 1 col, use 1st part
ezplot('sin(x)',[-3 3])  % plot command for first graph
subplot(2,1,2)    % use 2nd part of divided page
ezplot('x^2', [-3 3])    % plot command for second graph
orient tall       % make plot tall enough to fit whole page
print test1.ps    % save plot in file test1.ps

How can I change the plot style and color in ezplot ?

First save the file setcurve.m in your home directory.

To change the curve color to red, use the following command after the ezplot command:

setcurve('color','red')

To change the curve color to green, use a dashed instead of a solid line, and make the lines thicker, use the following command after the ezplot command:

setcurve('color','green','linestyle','--','linewidth',2)

(Type help plot to see available line styles and colors.)

m-files

I saved my commands in the file prob1.m, but when I type prob1 Matlab says it is undefined

You must tell Matlab where it should look for m-files:

If you are on a Unix machine you might save your m-file in the directory /homes/myname/homework1 . At the Matlab command prompt ">>" you should type addpath('/homes/myname/homework1') .

If you are on a Windows machine you might save your m-file in the directory C:\My Documents\homework . At the Matlab command prompt you should type addpath('C:\My Documents\homework') .
On WAM/Glue you should use addpath('H:\mydocuments\homework').

In the integrated environment you can also select in the "File" menu the item "Set Path...". Then use "Add Folder...", select a folder and hit "OK". Then click "Save", so that you do not have to do this again the next time you start Matlab.

How can I output numerical results together with describing text?

For scalar values use fprintf with %g for each value you want to insert in the text, e.g.,

fprintf('The volume is %g, the area is %g\n',volume,area)

If you want to see e.g. 15 digits use %.15g instead of %g in fprintf.
For vectors and matrices use disp, e.g.,

x = sqrt(1:6); disp('The square roots are'); disp(x)

When I use echo on and my m-file contains for ... end the commands in the loop get echoed many times

Replace ``end'' with ``

echo off; end; echo
      on

'' to supress echoing after the first loop iteration.

I generate several plots in my m-file. When I run the m-file I only see the last plot.

Method 1: put a pause statement after each plot command. Then Matlab will wait for you to press a key before moving on.

Method 2: put the statement figure(1) before the first plot command, figure(2) before the second plot command etc. . This will generate a separate window for each plot.

I use hold on with several plot commands in my m-file. I want to see what the graph looks like after each plotting command, but Matlab waits until the last plotting command before showing the graph.

Use drawnow after each plotting command. This forces Matlab to update the graph immediately, rather than waiting until the end of the m-file.

How can I write loops without ``goto''?

If the termination condition is tested at the beginning of the loop use

while condition
  statements
end

Otherwise use

while 1
  statements
  if condition
    break
  end
  statements
end

See the Matlab help for while, break.

Tobias von Petersdorff