Data Types, Sessions and Other Basic Usages of TensorFlow

TensorFlow is a computational framework that allows users to represent computation as a graph. Nodes in this computational graph represents an operation to be performed, which is called “op” in TensorFlow context. An op can take zero or more Tensors and produce zero or more Tensors as it’s output. Tensor can be referred to as a multi-dimensional array. 

In this post I’ll be focusing on Computational graphs, Sessions, Tensors, and Variables.

Typical TensorFlow graph has three main phases, which are Construction phase, Graph Assemble phase and Execution phase. By default TensorFlow provides a computational graph where users can assign ops that needs to be executed without assembling a new graph. 

Operations and Tensors

A Tensor represents the data flow through ops, in other words a value consumed or produced by an op. Tensors does not hold the value of that operation, instead it handles as a symbolic handle to an output of an op. In this perspective, Tensor plays two roles; it can be passed as an input to an op which allows to build the graph and it’s data flow, Secondly it allows to perform computations on values being passed. In below, matrix_1, matrix_2 and product are Tensors.

matrix_1 = tf.constant([[3., 3.]])

matrix_2 = tf.constant([[2.], [2.]])

product = tf.matmul(matrix_1, matrix_2)

Like mentioned previously an operation or op is a node in TensorFlow graph that can both consume or produce zero or more Tensors. Ops performs computations on Tensors. For example,

product = tf.matmul(matrix_1, matrix_2)

Creates an operation of type “matmul” and takes matrix_1 and matrix_2 tensors as inputs to produce product tensor as output.

Graphs

A graph in TensorFlow can be referred as a collection of Ops that linked by Tensors and can be start with ops that doesn’t take any input and pass their output to other ops that performs computations. In below example we have added three ops including two constants and one op to TensorFlow default graph. To launch and get the results we must execute the graph in a TensorFlow session (more on Sessions later).

matrix_1 = tf.constant([[3., 3.]])

matrix_2 = tf.constant([[2.], [2.]])

product = tf.matmul(matrix_1, matrix_2)

sess = tf.Session()

result = sess.run(product)

Sessions

A session contains all operations and tensors of a computational model. It does execute ops and evaluates tensors. The default session can be launched without giving any graph arguments when creating the session. If it’s required to run different graphs in different sessions, the we have to provide which graph should be executed in each session we are creating.

TensorFlow session represents a connection to it’s underline native libraries. So it holds all resources allocated until we close the sessions. Or else we could user session as a context manages.

matrix_1 = tf.constant([[3., 3.]])

matrix_2 = tf.constant([[2.], [2.]])

product = tf.matmul(matrix_1, matrix_2)

sess = tf.Session()

result = sess.run(product)

print(result)

sess.close()

To execute above example with session as a context manager,

matrix_1 = tf.constant([[3., 3.]])

matrix_2 = tf.constant([[2.], [2.]])

product = tf.matmul(matrix_1, matrix_2)

with tf.Session() as sess:

result = sess.run(product)

print(result)

Variables

Variables keeps state across TensorFlow graph executioin and can be considered as in-memory buffers of Tensors. Variables should be explicitly initialized and can be stored in the hard disk to use them later on.

To create a variable we must pass a Tensor as it’s input parameter to the constructor.

x = tf.Variable([1.0, 2.0])

a = tf.constant([3.0, 3.0])

# create a variable that will be initialize to scalar value 0.

state = tf.Variable(0, name="counter")

# variables must be initialized by running an 'init' op after having

# launch the graph. first add the init op to the graph.

init_op = tf.initialize_all_variables()

In another post, we will be dive deep into data flow graphs for better understanding of computation executions.

For source code: https://github.com/isurusiri/tensorflowbasics