# Dealing with `divide 0` or `Inf` in GNU Radio C++ code

Let’s think about the following auto/cross correlator:
http://i.imgur.com/Fy5hdVj.png
I’ve placed `divide` because I need to normalize the correlation result.

If we place `null source`, or `constant source with value of zero` right
before the `pad source`, the simplified form of the correlator becomes 0
divided by 0.

I’ve placed `time sink` right after the `pad sink` then, I can see the
line
hit the infinity. With a simplified flow graph:
http://i.imgur.com/Msx9EQj.png

Back to the my correlator, how can I handle such `infinity` value coming
into the next block?

My guess is that if a source block is a type of float, then `infinity`
might have a maximum value of float. So, I think I can reject `infinity`
value with `if (in[i] > threshold)`, where `threshold` is big enough,
for
correlator it is fine to have a value just larger than one.

Do you think what I guess is right? Or can some boost numeric types, or
PMT
help somehow to deal with it?

In pages std::numeric_limits<T>::infinity - cppreference.com
and
INFINITY - cppreference.com,
non floating point numeric types can’t handle infinity.
Is it also applied to GNU Radio?

Regards,
Jeon.

Jeon,

You can add an appropriate constant before the divide and compensate for
it afterward. The exact math will depend on the values coming out of the
correlators.

Jeff

(That should be “moving averages”, not “correlators”). Also, subtracting
instead of dividing might do what you want.

Jeff

Hi Jeff,

you could use a strcture like this
MA_divisor => Divide: % => multiply => pad sink.
|-> threshold detector -^

i.e. take the divisor path in parallel into threshold detector and use
the output (in {0,1}) in a multiplier for masking the dividing by zero
cases.

By adjusting the threshold to 0+eps (a small number of your choice),
this also solves the problem of huge output if the divisor is almost
0.

Best regards

Stephan Ludwig

Robert Bosch GmbH
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-----Ursprngliche Nachricht-----
Im Auftrag von Jeff L.
Gesendet: Mittwoch, 19. August 2015 13:31
An: [email protected]
Betreff: Re: [Discuss-gnuradio] Dealing with `divide 0` or `Inf` in GNU

(That should be “moving averages”, not “correlators”). Also, subtracting
instead of dividing might do what you want.

Jeff

On 08/19/2015 07:25 AM, Jeff L. wrote:

http://i.imgur.com/Fy5hdVj.png
Back to the my correlator, how can I handle such `infinity` value