How to recognize overflow bugs in Matlab?

Question

I spent part of yesterday and today tracking down a bug in some Matlab code. I had thought my problem was indexing (with many structures that I didn't define and am still getting used to), but it turned out to be an overflow bug. I missed this for a very specific reason:

>> uint8(2) - uint8(1)

ans =

    1

>> uint8(2) - uint8(2)

ans =

    0

>> uint8(2) - uint8(3)

ans =

    0

I would have expected the last one to be something like -1 (or 255). In the middle of a big vector, the erroneous 0s were difficult to detect, but a 255 would have stood out easily.

Any tips on how to detect these problems easily in the future? (Ideally, I'd like to turn off the overflow checking to make it work like C.) Changing to double works, of course, but if I don't realize it's a uint8 to begin with, that doesn't help.

Answer 1

You can start by turning on integer warnings:

intwarning('on')

This will give you a warning when integer arithmetic overflows.

Beware though, as outlined here, this does slow down integer arithmetic so only use this during debug.

Answer 2

Starting with release R2010b and later, the function INTWARNING has been removed, along with these warning messages for integer math and conversion:

• MATLAB:intConvertNaN
• MATLAB:intConvertNonIntVal
• MATLAB:intConvertOverflow
• MATLAB:intMathOverflow

So using INTWARNING is no longer a viable option for determining when integer overflows occur. An alternative is to use the CLASS function to test the class of your data and recast it accordingly before performing the operation. Here's an example:

if strcmp(class(data),'uint8')  %# Check if data is a uint8
  data = double(data);          %# Convert data to a double
end

You could also use the ISA function as well:

if ~isa(data,'single')  %# Check if data is not a single
  data = single(data);  %# Convert data to a single
end