tf.while_loop(cond, body, loop_vars, shape_invariants=None, parallel_iterations=10, back_prop=True, swap_memory=False, name=None)

tf.while_loop(cond, body, loop_vars, shape_invariants=None, parallel_iterations=10, back_prop=True, swap_memory=False, name=None)

See the guide: Control Flow > Control Flow Operations

Repeat body while the condition cond is true.

cond is a callable returning a boolean scalar tensor. body is a callable returning a (possibly nested) tuple, namedtuple or list of tensors of the same arity (length and structure) and types as loop_vars. loop_vars is a (possibly nested) tuple, namedtuple or list of tensors that is passed to both cond and body. cond and body both take as many arguments as there are loop_vars.

While cond evaluates to true, body is executed.

In addition to regular Tensors or IndexedSlices, the body may accept and return TensorArray objects. The flows of the TensorArray objects will be appropriately forwarded between loops and during gradient calculations.

For correctness, tf.while_loop() strictly enforces shape invariants for the loop variables. A shape invariant is a (possibly partial) shape that is unchanged across the iterations of the loop. An error will be raised if the shape of a loop variable after an iteration is determined to be more general than or incompatible with its shape invariant. For example, a shape of [11, None] is more general than a shape of [11, 17], and [11, 21] is not compatible with [11, 17]. By default (if the argument shape_invariants is not specified), it is assumed that the initial shape of each tensor in loop_vars is the same in every iteration. The shape_invariants argument allows the caller to specify a less specific shape invariant for each loop variable, which is needed if the shape varies between iterations. The tf.Tensor.set_shape function may also be used in the body function to indicate that the output loop variable has a particular shape. The shape invariant for SparseTensor and IndexedSlices are treated specially as follows:

a) If a loop variable is a SparseTensor, the shape invariant must be TensorShape([r]) where r is the rank of the dense tensor represented by the sparse tensor. It means the shapes of the three tensors of the SparseTensor are ([None], [None, r], [r]). NOTE: The shape invariant here is the shape of the SparseTensor.dense_shape property. It must be the shape of a vector.

b) If a loop variable is an IndexedSlices, the shape invariant must be a shape invariant of the values tensor of the IndexedSlices. It means the shapes of the three tensors of the IndexedSlices are (shape, [shape[0]], [shape.ndims]).

while_loop implements non-strict semantics, enabling multiple iterations to run in parallel. The maximum number of parallel iterations can be controlled by parallel_iterations, which gives users some control over memory consumption and execution order. For correct programs, while_loop should return the same result for any parallel_iterations > 0.

For training, TensorFlow remembers the tensors that are produced in the forward inference but needed in back propagation. These tensors can be a main source of memory consumption and often cause OOM problems when training on GPUs. When the flag swap_memory is true, we swap out these tensors from GPU to CPU. This for example allows us to train RNN models with very long sequences and large batches.

Args:

  • cond: A callable that represents the termination condition of the loop.
  • body: A callable that represents the loop body.
  • loop_vars: A (possibly nested) tuple, namedtuple or list of numpy array, Tensor, and TensorArray objects.
  • shape_invariants: The shape invariants for the loop variables.
  • parallel_iterations: The number of iterations allowed to run in parallel. It must be a positive integer.
  • back_prop: Whether backprop is enabled for this while loop.
  • swap_memory: Whether GPU-CPU memory swap is enabled for this loop.
  • name: Optional name prefix for the returned tensors.

Returns:

The output tensors for the loop variables after the loop. When the length of loop_vars is 1 this is a Tensor, TensorArray or IndexedSlice and when the length of loop_vars is greater than 1 it returns a list.

Raises:

  • TypeError: if cond or body is not callable.
  • ValueError: if loop_vars is empty.

Example:

i = tf.constant(0)
c = lambda i: tf.less(i, 10)
b = lambda i: tf.add(i, 1)
r = tf.while_loop(c, b, [i])

Example with nesting and a namedtuple:

import collections
Pair = collections.namedtuple('Pair', 'j, k')
ijk_0 = (tf.constant(0), Pair(tf.constant(1), tf.constant(2)))
c = lambda i, p: i < 10
b = lambda i, p: (i + 1, Pair((p.j + p.k), (p.j - p.k)))
ijk_final = tf.while_loop(c, b, ijk_0)

Example using shape_invariants:

i0 = tf.constant(0)
m0 = tf.ones([2, 2])
c = lambda i, m: i < 10
b = lambda i, m: [i+1, tf.concat([m, m], axis=0)]
tf.while_loop(
    c, b, loop_vars=[i0, m0],
    shape_invariants=[i0.get_shape(), tf.TensorShape([None, 2])])

Defined in tensorflow/python/ops/control_flow_ops.py.