Difference Subtracting Double Precision from Single Precision

Can you explain the results for the lines below

a=99999999; b=single(a); b-a

     1

a=20000001; b=single(a); b-a

     -1

Answer 1

[Matt Fig]

Yes, have a look at

E = eps(b)

When you convert to single you are going to get the nearest single value to a within E. To see what you are dealing with, look at this:

a = (999999999-32):(999999999+34); b = single(a);
fprintf('%10.0f    %10.0f \n',[a;double(b)])

Now when you do

c = b-a

MATLAB does the equivalent of this:

c2 = single(double(b)-a)

Also have a look at this:

FAQ 6.1