Question

I was trying to do sub2ind on single-precision variables that I came across the following strange behavior. For example, when I try:

[a b] = ind2sub([50000 50000], sub2ind([50000 50000], single(1000), single(1000)))

I get:

a = 1001
b = 1000

Is this a bug or I am missing something? I know this can be because of an overflow somewhere in matlab’s code but it shouldn’t happen, right?

I get the same wrong behavior from 64-bit (glnxa64) R2012a, R2011a, R2010b, R2010a, but correct results from 32-bit (glnx86) R2010b.

Answer 1

The reason this happens is the following.

On line 35 of ind2sub.m is

vi = rem(ndx-1, k(i)) + 1;   

where ndx is the index that is passed in. Thus ndx is 49951000 via the call from sub2ind. Now, when you pass in a single precision value, it forces matlab to evaluate all math in single precision. Thus, compare the difference in what happens on l.35.

K>> 49951000-1
ans =
        49950999

K>> single(49951000)-1
ans =
        49951000

This subtraction of a small number from a large number is the issue. So no, this isn’t a bug, it’s a limitation to the precision of single-precision floating point. This might help some.

edit: As pointed out by Rasman, a nice way of showing this is with eps

eps(single(49951000))=4

so any value added or subtracted from single(49951000) that is on the range (-4,4) will result in 495100 being returned due to the accuracy of single-precision.