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Debugging in Matlab

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.

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Debugging in Matlab

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  1. Debugging in Matlab C. Reed 4/5/11

  2. 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).

  3. 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.)

  4. 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).

  5. 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;

  6. 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

  7. 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

  8. Debugger Example • type modes • type edit modes • use breakpoints to walk through code • try the keyboard command

  9. Handling Errors Yourself • try-catch: • http://www.mathworks.com/help/techdoc/ref/catch.html

  10. Decent Debugging Tutorials • http://www.youtube.com/watch?v=9iTj--Q6ZRE • https://engineering.purdue.edu/ECN/Support/KB/Docs/DebuggingMatlabMfile

  11. Additional Resources https://ceprofs.civil.tamu.edu/ssocolofsky/engr111a/Downloads/Lectures/Class%2012.1%2004c%20Version%20A.ppt http://jagger.berkeley.edu/~pack/e77/Debugging.ppt

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