Anomalous histogram for small argument

Gordon Edwards (view profile)

on 6 Sep 2019
Latest activity Commented on by Gordon Edwards

on 7 Sep 2019

Bruno Luong (view profile)

Running the following code from the command line with c = 1e-15 or greater leads to a normal histogram
c = 1e-15; figure; histogram(c * rand(100,1))
Running the following code with c = 1e-16 or less leads to an incorrect histogram
c = 1e-16; figure; histogram(c * rand(100,1))
The problem appears also in apps when the histogram function is invoked.
Is this a bug?
Gordon

R2019a

Bruno Luong (view profile)

on 7 Sep 2019
Edited by Bruno Luong

Bruno Luong (view profile)

on 7 Sep 2019

Yes it is a BUG. I chase back and in the file
C:\Program Files\MATLAB\R2019a\toolbox\matlab\datafun\+matlab\+internal\+math\binpicker.m
line #20 (R2019a)
there is a test
if xrange > max(sqrt(eps(xscale)), realmin(class(xscale)))
To me the correct test should be (without sqrt)
if xrange > max(eps(xscale), realmin(class(xscale)))
Now if I change this line and rerun the code
c = 1e-16; figure; histogram(c * rand(100,1))
it produces correct plot
More seriously, the function binpicker is called by HISTCOUNT. So HISTCOUNT is also buggy. Not only plotting but calculation might be wrong.

Gordon Edwards

Gordon Edwards (view profile)

on 7 Sep 2019
Thanks for your reply.and I believe that you have spotted the error in function binpicker.Since eps(x) ~ 1e-16*x, and in my case xscale and xrange are nearly equal, the condition that
xrange > sqrt(eps(xscale))
leads to xrange >~ 1e-16 which is what I find.

Jackson Burns (view profile)

on 7 Sep 2019

Here's a similar question asked a few years ago which was never really solved either. This comment may be your best approach, but I would reccomend just bringing the values to unity with something like this:
c = 1E-16;
figure
mydata = c * rand(100,1);
histogram(mydata./max(mydata))
You could then modify the xticklabels to match whatever the original values were.