"math domain error" in mapping about qiskit-terra HOT 7 CLOSED

This is due to numerical error in the argument of acos in yzy_to_zyz. For example, math.sin(math.pi/2) * math.cos(math.pi/4) / math.sin(math.pi/4) = 1.0000000000000002 > 1. One solution is to round the argument.

from qiskit-terra.

One option is to round, but that might add errors after several computations. I suggest to move to symbolic computation instead of flouting point math to avoid this issue:

> import sympy
> sympy.sin(sympy.pi/2) * sympy.cos(sympy.pi/4) / sympy.sin(sympy.pi/4)
< 1

Comments?

from qiskit-terra.

I think @awcross1 was giving an example where it fails, but it general the inputs are not symbolic they are floats.

from qiskit-terra.

The particular case in yzy_to_zyz, can be solved symbolically. There are several other points in the mapper where math is used, that can be easily be replaced with sympy.

I'm not sure what you mean. In situations where user code has floats, that error is introduced by the user and we can represent exactly that error without adding more, using Decimal in the parser.

My point is, when possible, we should operate with symbols to avoid adding error.

from qiskit-terra.

Thanks Luciano, I think it is worth investigating using symbols.

from qiskit-terra.

Here is a self-contained test case from @dcmckayibm that triggers the error

from qiskit import QuantumProgram
import numpy as np

coupling_map_test = {0:[0]}
qp = QuantumProgram()
qr = qp.create_quantum_register('q', 1)
cr = qp.create_classical_register('c', 1)
qc = qp.create_circuit('qc1', qregisters=[qr], cregisters=[cr])
qc.u2(np.pi/2, 0, qr[0])
qc.u3(np.pi/2, np.pi/2, 0.0, qr[0])
qp.compile('qc1', coupling_map=coupling_map_test)

from qiskit-terra.

Closing the issue, as it was fixed by #151 - thanks Luciano!

from qiskit-terra.