qml.for_loop

for_loop([start, ]stop[, step])

A qjit() compatible for-loop for PennyLane programs. When used without qjit(), this function will fall back to a standard Python for loop.

This decorator provides a functional version of the traditional for-loop, similar to jax.cond.fori_loop. That is, any variables that are modified across iterations need to be provided as inputs/outputs to the loop body function:

• Input arguments contain the value of a variable at the start of an iteration.

• output arguments contain the value at the end of the iteration. The outputs are then fed back as inputs to the next iteration.

The final iteration values are also returned from the transformed function.

The semantics of for_loop are given by the following Python pseudo-code:

def for_loop(start, stop, step, loop_fn, *args):
    for i in range(start, stop, step):
        args = loop_fn(i, *args)
    return args

Unlike jax.cond.fori_loop, the step can be negative if it is known at tracing time (i.e., constant). If a non-constant negative step is used, the loop will produce no iterations.

Note

This function can be used in the following different ways:

1. for_loop(stop): Values are generated within the interval [0, stop)

2. for_loop(start, stop): Values are generated within the interval [start, stop)

3. for_loop(start, stop, step): Values are generated within the interval [start, stop), with spacing between the values given by step

Parameters

• start (int, optional) – starting value of the iteration index. The default start value is 0

• stop (int) – upper bound of the iteration index

• step (int, optional) – increment applied to the iteration index at the end of each iteration. The default step size is 1

Returns

A wrapper around the loop body function. Note that the loop body function must always have the iteration index as its first argument, which can be used arbitrarily inside the loop body. As the value of the index across iterations is handled automatically by the provided loop bounds, it must not be returned from the function.

Return type

Callable[[int, …], …]

See also

while_loop(), qjit()

Example

dev = qml.device("lightning.qubit", wires=1)

@qml.qnode(dev)
def circuit(n: int, x: float):

    @qml.for_loop(0, n, 1)
    def loop_rx(i, x):
        # perform some work and update (some of) the arguments
        qml.RX(x, wires=0)

        # update the value of x for the next iteration
        return jnp.sin(x)

    # apply the for loop
    final_x = loop_rx(x)

    return qml.expval(qml.Z(0))

>>> circuit(7, 1.6)
array(0.97926626)

for_loop is also qjit() compatible; when used with the qjit() decorator, the for loop will not be unrolled, and instead will be captured as-is during compilation and executed during runtime:

>>> qml.qjit(circuit)(7, 1.6)
Array(0.97926626, dtype=float64)

Note

Please see the Catalyst quickstart guide, as well as the sharp bits and debugging tips page for an overview of using quantum just-in-time compilation.

code/api/pennylane.for_loop

 

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