CS100M Lab Exercise 12
1 “Debugging” code
As you begin to write longer and more complicated programs, you need to get organized about testing and
“debugging” your code. To debug a program is to rid the program of bugs, i.e., errors. You need to come up
with a set of tests to make sure that all the individual functions and the final program are correct.
Testing. The general principle is to exercise every line of code. For example, your code has this structure:
function out = foo(a, b, c, d)
if a<0 % && blah blah blah, some other conditions
% some code here
elseif b>a % && other conditions
% some code here
elseif c>0 % && other conditions
for k= c:2:d
% k-loop. Some code here
end
r = rand(1,d);
end
Then you want to make test cases that cover all branches of the if-statement to make sure that every branch of
the program works. Furthermore, you should have test cases where d<c to make sure that skipping the k-loop
doesn’t cause any problem. Test the general case and the boundary cases.Ifrand or some other randomization
is involved, be sure to run tests multiple times. Sometimes it is possible to set the random variable to some fixed
values for the purpose of testing the other parts of the program. (Remember to remove the fixed values used for
testing afterwards.) Choosing test cases is very important! Take the time necessary to test thoroughly.
Debugging. If you identify problems during testing, then you have to debug. Read any Matlab-produced
error messages carefully. If your program runs without Matlab-generated error messages but the result isn’t
correct, then your test case has identified errors—good job! Do (some of ) the following to debug your code:
1. Display intermediate values. This is really easy to doin Matlab: just remove a few semi-colons! Sometimes
you may need to actually add a print statement.
2. Check variable types and dimensions. Look at the Workspace pane; is the variable of the type and dimen-
sions you expect?
3. What if there is a non-specific error message and you’re not sure where the problem is?
Add print statements to indicate which parts of the program has been “successfully” executed. An incomplete
program is shown below; look at the print statements:
docTitles = {}; termList = {}; tdm = [];
fid = fopen(dataFile, ’r’);
docTitles = split(fgetl(fid));
disp(’... MADE IT THROUGH DocTitles ...’)
r= 0; maxLen=0;
while ~feof(fid)
items = split(fgetl(fid)); r= r+1;
termCellArray{r} = items{1}; termLength(r)= length(items{1});
if termLength(r)>maxLen
maxLen= termLength(r);
end
disp(’... MADE IT THROUGH THE TERM ...’)
for c=2:length(items)
tdm(r,c-1)= str2double(items{c});
end
end
This is very similar to printing intermediate values in (1) above, but is more specific to finding the location
of where the program terminates abnormally.
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