Debugging in Matlab
C. Reed
4/5/11
Bugs
Debugging is a natural part of
programming:
Three standard types of errors:
– Syntax errors: you simply have typed
an illegal expression, independent of
the values of the variables in the
expression.
– Run-time errors: logic errors that
result in an illegal expression for
specific values of the data (harder to
fix)
– Logic errors that result in the
program executing completely, but
the answer that they return is
incorrect (hardest to fix).
Syntax Error Example
An M-file contains:
x = 7;
y = 8;
if x = y
x = y$2
else
x = y(3
end
Running at the command prompt gives:
>> temp
??? Error: File: C:\Programs\MATLAB6p5\work\temp.m Line: 3
Column: 6
Assignment statements do not produce results. (Use = = to
test for equality.)
Syntax Error Analysis
• Matlab only reported the first error it found.
• It gives you a link to the position in the files where
the error occurred.
• It tells you what type of error (assignment
statements do not produce results).
• And it suggests a fix (use == to test for equality).
Runtime Errors
x = [1 2 3 4];
y = magic(2);
% subtraction will cause an error - x is 2-D.
xn = x – y;
??? Error using ==> minus
Matrix dimensions must agree.
Error in ==> temp at 4
z = x - y;
Runtime Error Analysis
• Matlab gives the line number that caused the
error (sometimes errors can propagate)
– Also gives the reason for the runtime error!
• Runtime errors can often times be found
manually – harder problems rely on the
debugger
Logic Errors
%print odd numbers from 1 to 10
for i = 1:10
if rem(i,2) == 0
disp(i);
end
end
Simple example, but can be very hard to track down
Good debugging techniques come in handy here
Debugger Example
• type modes
• type edit modes
• use breakpoints to walk through code
– try the keyboard command
Handling Errors Yourself
• try-catch:
– http://www.mathworks.com/help/techdoc/ref/cat
ch.html
Decent Debugging Tutorials
• http://www.youtube.com/watch?v=9iTj-Q6ZRE
• https://engineering.purdue.edu/ECN/Support
/KB/Docs/DebuggingMatlabMfile
Additional Resources
https://ceprofs.civil.tamu.edu/ssocolofsky/engr111a/Downloads/Lectures/Class%201
2.1%2004c%20Version%20A.ppt
http://jagger.berkeley.edu/~pack/e77/Debugging.ppt