# Troubleshooting OpenQASM
<a name="braket-troubleshooting-openqasm"></a>

This section provides troubleshooting pointers that might be useful when encountering errors using OpenQASM 3.0.

**Topics**
+ [Include statement error](#braket-troubleshooting-openqasm-include-statement-error)
+ [Non-contiguous qubits error](#braket-troubleshooting-openqasm-non-contiguous-qubits-error)
+ [Mixing physical qubits with virtual qubits error](#braket-troubleshooting-openqasm-mixing-physical-and-virtual-qubits-error)
+ [Requesting result types and measuring qubits in the same program error](#braket-troubleshooting-openqasm-result-types-measuring-qubits-error)
+ [Classical and qubit register limits exceeded error](#braket-troubleshooting-openqasm-register-limits-error)
+ [Box not preceded by a verbatim pragma error](#braket-troubleshooting-openqasm-box-not-preceded-by-pragma-error)
+ [Verbatim boxes missing native gates error](#braket-troubleshooting-openqasm-verbatim-box-missing-native-gates-error)
+ [Verbatim boxes missing physical qubits error](#braket-troubleshooting-openqasm-verbatim-box-missing-physical-qubits-error)
+ [The verbatim pragma is missing "braket" error](#braket-troubleshooting-openqasm-pragma-naming-error)
+ [Single qubits cannot be indexed error](#braket-troubleshooting-openqasm-qubit-index-error)
+ [The physical qubits in a two qubit gate are not connected error](#braket-troubleshooting-openqasm-disconnected-physical-qubits-error)
+ [Local simulator support warning](#braket-troubleshooting-openqasm-local-simulator-support-warning)

## Include statement error
<a name="braket-troubleshooting-openqasm-include-statement-error"></a>

Braket currently doesn't have a standard gate library file to be included in OpenQASM programs. For example, the following example raises a parser error.

```
OPENQASM 3;
include "standardlib.inc";
```

This code generates the error message: `No terminal matches '"' in the current parser context, at line 2 col 17.` 

## Non-contiguous qubits error
<a name="braket-troubleshooting-openqasm-non-contiguous-qubits-error"></a>

Using non-contiguous qubits on devices that `requiresContiguousQubitIndices` be set to `true` in the device capability result in an error.

When running quantum tasks on simulators and IonQ, the following program triggers the error.

```
OPENQASM 3;

qubit[4] q;

h q[0];
cnot q[0], q[2];
cnot q[0], q[3];
```

This code generates the error message: `Device requires contiguous qubits. Qubit register q has unused qubits q[1], q[4].` 

## Mixing physical qubits with virtual qubits error
<a name="braket-troubleshooting-openqasm-mixing-physical-and-virtual-qubits-error"></a>

Mixing physical qubits with virtual qubits in the same program is not allowed and results in an error. The following code generates the error.

```
OPENQASM 3;

qubit[2] q;
cnot q[0], $1;
```

This code generates the error message: `[line 4] mixes physical qubits and qubits registers.` 

## Requesting result types and measuring qubits in the same program error
<a name="braket-troubleshooting-openqasm-result-types-measuring-qubits-error"></a>

Requesting result types and that qubits are explicitly measured in the same program results in an error. The following code generates the error.

```
OPENQASM 3;

qubit[2] q;

h q[0];
cnot q[0], q[1];
measure q;

#pragma braket result expectation x(q[0]) @ z(q[1])
```

This code generates the error message: `Qubits should not be explicitly measured when result types are requested.` 

## Classical and qubit register limits exceeded error
<a name="braket-troubleshooting-openqasm-register-limits-error"></a>

Only one classical register and one qubit register are allowed. The following code generates the error.

```
OPENQASM 3;

qubit[2] q0;
qubit[2] q1;
```

This code generates the error message: `[line 4] cannot declare a qubit register. Only 1 qubit register is supported.` 

## Box not preceded by a verbatim pragma error
<a name="braket-troubleshooting-openqasm-box-not-preceded-by-pragma-error"></a>

All boxes must be preceded by a verbatim pragma. The following code generates the error.

```
box{
rx(0.5) $0;
}
```

This code generates the error message: `In verbatim boxes, native gates are required. x is not a device native gate.` 

## Verbatim boxes missing native gates error
<a name="braket-troubleshooting-openqasm-verbatim-box-missing-native-gates-error"></a>

Verbatim boxes should have native gates and physical qubits. The following code generates the native gates error.

```
#pragma braket verbatim
box{
x $0;
}
```

This code generates the error message: `In verbatim boxes, native gates are required. x is not a device native gate.` 

## Verbatim boxes missing physical qubits error
<a name="braket-troubleshooting-openqasm-verbatim-box-missing-physical-qubits-error"></a>

Verbatim boxes must have physical qubits. The following code generates the missing physical qubits error.

```
qubit[2] q;

#pragma braket verbatim
box{
rx(0.1) q[0];
}
```

This code generates the error message: `Physical qubits are required in verbatim box.` 

## The verbatim pragma is missing "braket" error
<a name="braket-troubleshooting-openqasm-pragma-naming-error"></a>

You must include “braket” in the verbatim pragma. The following code generates the error.

```
#pragma braket verbatim           // Correct
#pragma verbatim                  // wrong
```

This code generates the error message: `You must include “braket” in the verbatim pragma` 

## Single qubits cannot be indexed error
<a name="braket-troubleshooting-openqasm-qubit-index-error"></a>

Single qubits cannot be indexed. The following code generates the error.

```
OPENQASM 3;

qubit q;
h q[0];
```

This code generates the error: `[line 4] single qubit cannot be indexed.` 

However, single qubit arrays can be indexed as follows:

```
OPENQASM 3;

qubit[1] q;
h q[0];   // This is valid
```

## The physical qubits in a two qubit gate are not connected error
<a name="braket-troubleshooting-openqasm-disconnected-physical-qubits-error"></a>

To use physical qubits, first confirm that the device uses physical qubits by checking `device.properties.action[DeviceActionType.OPENQASM].supportPhysicalQubits` and then verify the connectivity graph by checking `device.properties.paradigm.connectivity.connectivityGraph` or `device.properties.paradigm.connectivity.fullyConnected`.

```
OPENQASM 3;

cnot $0, $14;
```

This code generates the error message: `[line 3] has disconnected qubits 0 and 14` 

## Local simulator support warning
<a name="braket-troubleshooting-openqasm-local-simulator-support-warning"></a>

The `LocalSimulator` supports advanced features in OpenQASM that may not be available on QPUs or on-demand simulators. If your program contains language features specific only to the `LocalSimulator`, as seen in the following example, you will receive a warning.

```
qasm_string = """
qubit[2] q;

h q[0];
ctrl @ x q[0], q[1];
"""
qasm_program = Program(source=qasm_string)
```

This code generates the warning: `This program uses OpenQASM language features only supported in the LocalSimulator. Some of these features may not be supported on QPUs or on-demand simulators.

For more information on supported OpenQASM features, explore the page [Advanced feature support for OpenQASM on the Local Simulator](braket-openqasm-supported-features.md#braket-openqasm-supported-features-advanced-feature-local-simulator).