Problem in drawing circuit for conditional measurement using multiple classical registers

[AbeerVaishnav13]

Information

The qc.draw() function does not draw the quantum circuit properly if we change the order of classical registers while initializing the QuantumCircuit() instance if there is a measurement in the circuit that is conditioned on the output of some other classical register.

• Qiskit Terra version:

  {'qiskit-terra': '0.17.4', 'qiskit-aer': '0.8.2', 'qiskit-ignis': '0.6.0', 'qiskit-ibmq-provider': '0.14.0', 'qiskit-aqua': '0.9.2', 'qiskit': '0.27.0', 'qiskit-nature': '0.1.3', 'qiskit-finance': None, 'qiskit-optimization': None, 'qiskit-machine-learning': None}

• Python version: 3.8.6
• Operating system: MacOS

What is the current behavior (and how to reproduce the problem)?

Notice that in the above code, we initialize the quantum circuit as:

qc = QuantumCircuit(q_a, q_b, c_a, c_b)

However, if we exchange the classical registers while initializing, like so:

qc = QuantumCircuit(q_a, q_b, c_b, c_a)

we get this, which is the correct circuit...

What is the expected behavior?

The circuit in the first image should have the arrows going to the classical register c_b and the dots going to c_a, but here both of them go to c_a.

[enavarro51]

I can work on this. It may affect all 3 circuit drawers. Thanks for finding it.

[AbeerVaishnav13]

I can work on this. It may affect all 3 circuit drawers. Thanks for finding it.

Hi @enavarro51 , thanks for taking this up! I checked the "text" circuit drawer and the output of that looked fine to me. However, I didn't check the "latex" option. But anyway you can try both of them out yourself also once :)