Suppose you have declared n registers in your circuit, but at the end you want to measure only m qubits. Here the same is done using an example, where out of 4 registers, you need to measure only 2 qubits.

```
from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit, execute, Aer
q = QuantumRegister(4)
# we need only 2 classical bits as we are measuring only 2 qubits
c = ClassicalRegister(2)
qc = QuantumCircuit(q,c)
# affecting the 2 qubits, that I am planning to measure
qc.h(q[0])
qc.x(q[3])
# Measuring the first and last qubit only
qc.measure([0,3],[1,0])
job = execute(qc,Aer.get_backend('qasm_simulator'),shots=1000)
counts = job.result().get_counts(qc)
print(counts)
```

The above code results in

```
{'01': 503, '11': 497}
```

In short, you need to provide the array of Qubits’ indices that you want to measure in the first parameter of `qc.measure`

. And also, don’t make the mistake of declaring more classical registers than required.

#### Why [1,0] as second parameter instead of simply putting `c`

?

In classical bits, counting starts from the right. If there are 2 bits, say `10`

, then `0`

is the 0th bit and `1`

is the 1st bit. Now if we provide simply `c`

as the second parameter in `qc.measure`

, by default `q[0]`

is mapped to the least significant bit - 0th bit(the right most bit). And that would result in output as `{'10': 503, '11': 497}`

because the right most bit(`q[0]`

) gives 0 and 1 with nearly equal probability.

This leads to confusion because we passed `[0,3] `

in the first parameter and while reading the output we expect the outcome of `q[0]`

to be on the left most side rather than the right most side. To avoid this confusion, we map the outcome of q[0] to the most significant bit, which is the right most classical bit - 1st bit in this case.

**NOTE**: Alternatively you could pass [3,0] as first parameter and c as second parameter in `qc.measure`

, but I didn’t like the idea of disturbing the order of qubits.

## Credits:

It was really frustrating for me, when I couldn’t find how to do this on the google. Thanks to Jack Woehr for helping me out with this thing in Qiskit slack.