How to change the precision of a value in a pmt pairs-CodePudding

I am trying to realize a block in gnuradio. It must send a message to the output and command the frequency of a signal source block. The message is a pmt pairs which contains the frequency value in the cdr field. When I put it into the pairs, its precision decreases a lot. For example, with this code

import numpy as np
import pmt

freqval = np.float64(123494235.48994166)
msg = pmt.cons(pmt.intern("freq"), pmt.to_pmt(freqval))

print("freqval : ", freqval)
print("msg : ", msg)

freqval = np.float64(123.49423548994166)
msg = pmt.cons(pmt.intern("freq"), pmt.to_pmt(freqval))

print("freqval : ", freqval)
print("msg : ", msg)

i get

freqval :  123494235.48994166
msg :  (freq . 1.23494e 08)
freqval :  123.49423548994166
msg :  (freq . 123.494)

while I want the value in the msg to be the same as the one contained in freqval. I tried to replace to_pmt() whith from_float() and from_double(), but nothing changed, and making the value as string, but gnuradio raises the warning

gr::log :WARN: sig_source0 - frequency value needs to be a number

Is there a way to change the precision of the pmt pairs value?

CodePudding user response：

This is just the precision of the "pretty print string representation". When you convert your PMT back to a python float or a C double, you should get sufficient precision.

> value = 1/3
# one third has very many fractional digits – it 
# can't be exactly represented in base 10
# but it can't even be exactly stored in a float,
# because 1/3 can also not be exactly represented
# in binary.
> print(value)
0.3333333333333333
# note how most of these digits are just
# "made up (i.e. the closest approximation to 1/3
# in binary of given length)"; python floating points
# don't have 50 significant digits!
> print(f"{value:.50f}"
0.33333333333333331482961625624739099293947219848633
> a = pmt.pmt_to_python.python_to_pmt(value)
> b = pmt.pmt_to_python.pmt_to_python(a)
> print(b-value)
0.0
> print(f"{b-value:.50f}")
0.00000000000000000000000000000000000000000000000000

This is basically true for most floating point formats: The way a number is printed is not equal to the number itself!

PMT knows this and makes a decision to not try and represent many digits when printing – that's really more of a convenience function. When you need to work with the value of a PMT, you'd always convert that PMT to a native type again.