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Classical Control Flow

Loops and conditionals on classical expressions are useful means to describe reusable building blocks. Qmod has two basic forms - the repeat statement and the if statement.

Classical Repeat

Syntax

repeat ( iteration_variable : count ) { iteration-statements }

def repeat(count: CInt, iteration: QCallable[CInt]) -> None:
    pass

Semantics

  • Invoke the iteration block count times, binding the index variable to the respective iteration number - 0, 1,... count-1.
  • Inside the statement block, use of quantum variables declared outside it is restricted to contexts where the variable is initialized and remains initialized (see Quantum Variables)

Example

The following example defines a useful function - applying the Hadamard function across all qubits in a qubit array - using repeat. Note that a similar function is available in the Classiq open-library.

qfunc my_hadamard_transform(qba: qbit[]) {
  repeat (index: qba.len) {
    H(qba[index]);
  }
}

from classiq import H, QArray, QBit, qfunc, repeat


@qfunc
def my_hadamard_transform(qba: QArray[QBit]):
    repeat(
        count=qba.len,
        iteration=lambda index: H(qba[index]),
    )

Classical If

Syntax

if ( condition ) { then-statements } else { else-statements }

def if_(condition: CBool, then: QCallable, else_: Optional[QCallable] = None) -> None:
    pass

Note that identifiers in Qmod that happen to conflict with Python keywords have _ suffix. This is the case with if_ and else_ in the second function.

Semantics

  • Invoke the then block if condition evaluates to true and otherwise invoke the else block
  • Inside the statement block, use of quantum variables declared outside it is restricted to contexts where the variable is initialized and remains initialized (see Quantum Variables)

Example

qfunc my_conditional_gate(cond: bool, qb: qbit) {
  if (cond) {
    X(qb);
  } else {
    Y(qb);
  }
}

from classiq import CBool, X, Y, QBit, qfunc, if_


@qfunc
def my_conditional_gate(cond: CBool, qb: QBit):
    if_(
        condition=cond,
        then=lambda _: X(qb),
        else_=lambda _: Y(qb),
    )