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Introduction

I assume that the reader has a basic knowledge about neural networks. I will just explain how to build one with TensorFlow. I will not explain what a neural network is.

We will approximate sinus as a toy example. Luckily, we have every possible label provided because the function is well known.

Let’s start right away.

Code

First, we obviously have to import TensorFlow and NumPy. Additionally, we will use matplotlib to see how well we approximated sinus by comparing the outcome of the network to the original function visually.

import tensorflow as tf
import numpy as np
import matplotlib.pyplot as plt


Layer

We also need a placeholder for the input into the unit. Like in the last tutorial:

# initialize placeholder for data input
with tf.name_scope("Input"):
x = tf.placeholder(dtype=tf.float32, shape=[None, 1], name="x")


Notice that the first dimension of the placeholder assigns a None. This is a practical trick to allow working with batches. This just means that we don’t want to set a fixed number of how many data points we can feed inside this node. So we can input a vector of size $(N, 1)$ for $N \in \mathbb{N}$ and get $N$ calculated values from the network. Assuming the cache of your device is sufficient.

If you want to take a look at what happens when you print x…

<tf.Tensor 'x:0' shape=(?, 1) dtype=float32>


… you can see that the internal representation of the None is a ?.

To emphasize the computation graph representation of a calculation, I will now create the layers in a loop. The considered network is relatively small, so we could also create it by hand. However, it can be as huge as you want it to be.

# think about the dimensions of your neural net (trial & error)
layer = [1, 50, 50, 1]

# hidden layer in a loop
with tf.name_scope("Hidden"):
out = x
for i in range(len(layer) - 2):
with tf.name_scope("Layer"):
# random initialization of weight and bis
W = tf.Variable(
tf.random_normal([layer[i], layer[i + 1]], dtype=tf.float32),
name="W")
b = tf.Variable(
tf.random_normal([1, layer[i + 1]], dtype=tf.float32),
name="b")

# matrix operation
out = tf.matmul(out, W) + b
# activation function
out = tf.nn.relu(out)

# summarize histogram
tf.summary.histogram("W", W)
tf.summary.histogram("b", b)

# output layer
with tf.name_scope("Output"):
# random initialization of weight and bis
W = tf.Variable(
tf.random_normal([layer[-2], layer[-1]], dtype=tf.float32),
name="W")
b = tf.Variable(
tf.random_normal([1, layer[-1]], dtype=tf.float32),
name="b")

# matrix operation
out = tf.matmul(out, W) + b

# no activation for the output

# summarize histogram
tf.summary.histogram("W", W)
tf.summary.histogram("b", b)


A look at the namescopes lets us assume that we build a graph like that:

The list layer represents the number of units in each layer of the network.

• 1 input,
• 2 x 50 hidden,
• and 1 output unit.

We create TensorFlow variables that need to be assigned with a initial value. tf.matmul obviously performs a matrix multiplication and tf.nn.relu represents the rectified linear unit activation function.
Note: We also could have taken sigmoid (tf.nn.sigmoid) or any other activation function that pops into your mind.

Each layer contains a summary operation (for histograms) and the output layer has no activation function for convenience. Everything else is the same for every layer.

The interested reader also may notice that the vertices of the graph contain information about the dimension of the operations. Please ignore the nodes loss and trainer for now. We will elaborate on that later in this tutorial. First we have to make sure that TensorFlow understood what we wanted to do in the loop.

This looks just perfect. Let’s proceed with creating a loss for the little network.

Loss & Training

We have a beautiful computation right now, but in order to really approximate something we need a loss function that tells the network how it has done so far. We will make it easy for us and take the Euclidean distance of label and output.

# loss function
with tf.name_scope("Loss"):
# supervised input labels
with tf.name_scope("label"):
y = tf.placeholder(dtype=tf.float32, shape=[None, 1], name="y")
# euclidean distance as loss
with tf.name_scope("distance"):
loss = tf.subtract(out, y, name="difference")
loss = tf.abs(loss, name="absolute")
loss = tf.reduce_mean(loss)

# summarise scalar
tf.summary.scalar('loss', loss)


Note that the shape of y (labels) also contains a None. So we can process more than just one label when we need to. Figure out your hyperparameters on your own. There is no theory behind it. Just experience. Always remember that an AI engineer is like a slot machine addict!
Let’s see what we have build:

Looks nice, it compares the output with the label and calculates the Euclidean distance. It also calculates a mean for two reasons. First, we want to have scalar values for the summary of the loss function. Second, remember that the placeholders for x and y are very flexible and could evaluate multiple instances of the data set in one run.

The last operation is very easy for a user but a masterpiece of the developers of TensorFlow. Look how I can command the network to learn with respect to the lossfunction and the architecture:

with tf.name_scope("trainer"):


That is also the explanation why loss and trainer sneaked inside the graph representation of the previous samples. We just decide what the lossfunction is that we want to minimize with the Adam Optimizer. TensorFlow automatically adapts the variables that influence the outcome via backpropagation. In the last lesson we learned that TensorFlow can automatically calculate gradients. This is way more exciting.
You can also decide which variables should be learnable and which not. But we won’t go into detail about this since it would be too much for this tutorial.

Computation

Before we start the TensorFlow session let us get over and done with the bureaucracy first.

# for tensorboard usability just merge all summaries into one node
summary_op = tf.summary.merge_all()

# operation for initialize variables
init_op = tf.global_variables_initializer()

# determine training length and batch size
train_len = 5000
batch = 2000


Like in the last tutorials we merge all summaries. Additionally, this time we use a global variable initializer. If we don’t do that we can’t work with the model. Imagine it similar to a placeholder where we first have to feed values inside to get a reasonable output. Furthermore, we introduce train_len and batch which represent the epochs of training and the batchsize for the upcoming TensorFlow session:

with tf.Session() as sess:
# run operation for variable initialization
sess.run(init_op)
# create a writer
writer = tf.summary.FileWriter("./graphs", sess.graph)
for i in range(train_len):
# randomly train with trainingset
x_data = np.float32(np.random.uniform(-1, 1, (1, batch))).T
y_data = np.float32(np.sin(4 * x_data))

# run training operation
sess.run(train_op, feed_dict={x: x_data, y: y_data})

# summarize
summary = sess.run(summary_op, feed_dict={x: x_data, y: y_data})


Basically, we are executing the initialization operation init_op, creating a writer and loop over a uniformly random data set of labeled sinus values between -1 and 1. For that we are executing the training operation train_op by providing it with labeled data. In the end we are doing summaries that we can view here:

We also created histograms to measure the weights:

These can be very helpful for debugging your model. There is no theory behind it. However, if you see that all your weights are zero, maybe you made a wrong initialization. If you notice that your bias or weights are jumping around without convergence, maybe you took not enough or too small layers. You have to develop a feeling for what they should look like. Start with easy models, play around a bit and slowly increase the size of your models.

Inference

Now we want to see how the trained model is doing regarding its task. Here we basically have to feed the computation graph with x-values and compare the output to the labels in an easy, understandable way. Note that the following is still done in the same session. Otherwise the variables wouldn’t be saved. Advanced saving and restoring will be the topic of our next tutorial.

    # inference
# check if everything worked out fine by visualizing equidistant testpoints
samples = 1000
x_data = np.float32(np.linspace(-1, 1, samples))[None, :].T
y_data = np.float32(np.sin(4 * x_data))
_out = sess.run(out, feed_dict={x: x_data})
writer.close()

# plot
fig = plt.figure(1)

As you can see we are just feeding our x with x_data, no labels needed this time. The validation set contains 1000 equidistant labeled sinus values. Note: we never actually trained on that set before we randomly made our choice. The comparison with matplotlib looks like that: