Control flow operations: conditionals and loops

When building complex models such as recurrent neural networks you may need to control the flow of operations through conditionals and loops. In this section we introduce a number of commonly used control flow ops.

Let’s assume you want to decide whether to multiply to or add two given tensors based on a predicate. This can be simply implemented with tf.cond which acts as a python “if” function:

a = tf.constant(1)
b = tf.constant(2)

p = tf.constant(True)

x = tf.cond(p, lambda: a + b, lambda: a * b)

print(tf.Session().run(x))

Since the predicate is True in this case, the output would be the result of the addition, which is 3.

Most of the times when using TensorFlow you are using large tensors and want to perform operations in batch. A related conditional operation is tf.where, which like tf.cond takes a predicate, but selects the output based on the condition in batch.

a = tf.constant([1, 1])
b = tf.constant([2, 2])

p = tf.constant([True, False])

x = tf.where(p, a + b, a * b)

print(tf.Session().run(x))

This will return [3, 2].

Another widely used control flow operation is tf.while_loop. It allows building dynamic loops in TensorFlow that operate on sequences of variable length. Let’s see how we can generate Fibonacci sequence with tf.while_loops:

n = tf.constant(5)

def cond(i, a, b):
    return i < n

def body(i, a, b):
    return i + 1, b, a + b

i, a, b = tf.while_loop(cond, body, (2, 1, 1))

print(tf.Session().run(b))

This will print 5. tf.while_loops takes a condition function, and a loop body function, in addition to initial values for loop variables. These loop variables are then updated by multiple calls to the body function until the condition returns false.

Now imagine we want to keep the whole series of Fibonacci sequence. We may update our body to keep a record of the history of current values:

n = tf.constant(5)

def cond(i, a, b, c):
    return i < n

def body(i, a, b, c):
    return i + 1, b, a + b, tf.concat([c, [a + b]], 0)

i, a, b, c = tf.while_loop(cond, body, (2, 1, 1, tf.constant([1, 1])))

print(tf.Session().run(c))

Now if you try running this, TensorFlow will complain that the shape of the the fourth loop variable is changing. So you must make that explicit that it’s intentional:

i, a, b, c = tf.while_loop(
    cond, body, (2, 1, 1, tf.constant([1, 1])),
    shape_invariants=(tf.TensorShape([]),
                      tf.TensorShape([]),
                      tf.TensorShape([]),
                      tf.TensorShape([None])))

This is not only getting ugly, but is also somewhat inefficient. Note that we are building a lot of intermediary tensors that we don’t use. TensorFlow has a better solution for this kind of growing arrays. Meet tf.TensorArray. Let’s do the same thing this time with tensor arrays:

n = tf.constant(5)

c = tf.TensorArray(tf.int32, n)
c = c.write(0, 1)
c = c.write(1, 1)

def cond(i, a, b, c):
    return i < n

def body(i, a, b, c):
    c = c.write(i, a + b)
    return i + 1, b, a + b, c

i, a, b, c = tf.while_loop(cond, body, (2, 1, 1, c))

c = c.stack()

print(tf.Session().run(c))

TensorFlow while loops and tensor arrays are essential tools for building complex recurrent neural networks. As an exercise try implementing beam search using tf.while_loops. Can you make it more efficient with tensor arrays?

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