Adding Hist2D empty_clone causes ROOT warning (different bin limits)

[Luke Kreczko]

Dear all,

I see strange behaviour when adding the original histogram to an empty clone:
```python
from rootpy.plotting import Hist2D
h1 = Hist2D( 2, 0, 1, 2, 0, 1 )
h2 = h1.empty_clone()
h2.Add( h1 )
```
```
WARNING:ROOT.TH2F.Add] Attempt to add histograms with different bin limits
```
But the final histogram looks fine. Do you know what is going on?

Cheers,

Luke

[Noel Dawe]

Hi Luke,

h2 ends up being initialized like this inside empty_clone (with explicit bin edges):

>>> h2 = Hist2D([0.0, 0.5, 1.0], [0.0, 0.5, 1.0])

And then ROOT stores the edges in the internal TArrayD.

h1 was initialized with fixed-width bins and ROOT doesn't bother keep the array of bin edges.

Then in TH1::CheckConsistency / TH1::CheckBinLimits the DifferentBinLimits exception is thrown when the lengths of the TArrays don't match:

>>> h1.xaxis.GetXbins().fN

0

>>> h2.xaxis.GetXbins().fN

3

So like you saw:

>>> h2.Add(h1)

WARNING:ROOT.TH2F.Add] Attempt to add histograms with different bin limits

True

Interestingly this is not symmetric....

>>> h1.Add(h2)

True

No warning there.

Overall, I'd say that this is a ROOT issue. Their consistency check should be able to handle this properly.

We can avoid this in rootpy by either suppressing that warning, or detecting the fixed-width bins and initializing the histogram the simple way. I'd rather that the ROOT guys fix it on their end though.

Noel

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[Noel Dawe]

You can also avoid the warning by initializing h1 by specifying the bin edges:

>>> from rootpy.plotting import Hist

>>> from rootpy.plotting import Hist2D

>>> h1 = Hist2D([0, .5, 1], [0, .5, 1])

>>> h2 = h1.empty_clone()

>>> h1.Add(h2)

True

>>> h2.Add(h1)

True

[Luke Kreczko]

Hi Noel,

Thanks a lot for your replies. I think I understand now.

In our case we read h1 from a file, so the initialisation is not in our hand. I will see what I can do.

But as you mention, it is just a warning and the bins are all right.

Thanks for helping me understand.

Cheers,

Luke

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[Noel Dawe]

Hi Luke,

No problem.

If you know what you are doing is fine, and you just want to ignore these warnings, you can use rootpy's logger to suppress them:

from rootpy.plotting import Hist2D

from rootpy import log

import logging

log['/ROOT.TH2F.Add'].setLevel(logging.FATAL)

h1 = Hist2D(2, 0, 1, 2, 0, 1)

h2 = h1.empty_clone()

h2.Add(h1)

Noel

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