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An artificial neural network
is a network that is designed on the brain and is designed to perform explicit
tasks 1.
An artificial neural network (ANN), are simulations performed on computers to
execute particular tasks including classification, pattern recognition, and
clustering 1.

ANNs are comprised of
various nodes which can accept data as inputs and output simple operations on
the input data. The nodes imitate biological neurons in a human brain. The
outcome of the operations made by the nodes is then succeeded to other neurons.
The production at each node is referred to activation
or node value. 2

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Each link is associated with a weight which would then be
multiplied by its respective input. An ANN is competent of learning by means of
varying the values of the weights 2.
The following diagram exemplifies a simple ANN:

The input layer encloses neurons that accept input on which the network
will learn. The output layer is the final
layer which comprehends of the units that respond to the information about how
the network learned the tasks. The middle layer, also referred to as the hidden layer, converts the input into
something useful, that the output unit can use. 1

A popular educating
algorithm used in Neural Network is backpropagation
which is the training algorithm 2
and it is an addition of gradient based delta learning rule 1. If the program
finds an error, the error is propagated backward from output to the input layer
through the hidden layer 1.
The network weights are then altered by the algorithm in order to deliver the desired
output for a specific input 2.
The algorithm is concluded when the error is acceptably low.

The assignments’ aim is to
learn how ANNs operate and how they can be employed using a high-level
language. An ANN that learns a Boolean function is implemented. The neural
network has five input neurons, four hidden neurons and three output neurons.
The sigmoid function transfer function is used. An Error Back Propagation
algorithm is implemented in order to train weights. The last step is plotting a
graph using the library ‘matplotlib’. This assignment is implemented using the HLL

Programming Documentation

this section, one could find the list of variables used, a description of each
line of code and a proof of the running programming.




the dataset matrix to be inputted. Each row in the matrix is a training


the dataset matrix to be outputted. Each row in the matrix is a training


is the primary layer of the Neural Network, which is itemised by the data


is the subsequent layer of the Neural Network which is also referred to as
the hidden layer.


is the third layer of the Neural Network, this shows the output data.


is the first layer of synapses, which connects lay0 to lay1.


second layer of synapses, connecting lay1 to lay2.



            import numpy as np

This line of code
imports the linear algebra library, ‘numpy’.

            import matplotlib.pyplot as plt

We also import the
matplotlib as plt in order to plot the graph.

Sigmoid Function

def nonlin(x, deriv=False):



    return 1/(1*np.exp(-x))

In the above code the
sigmoid function is represented, where if true is passed then the derivative of
the sigmoid would be calculated, this is one of the necessary assets of the
sigmoid function that the outputs of the function can be utilised to
generate its derivative. If the sigmoid’ output is equal to ‘out’, then the derivative
would be

However, if false occurs
then the derivative is not going to be calculated. The derivative is needed
when the error is being calculated in the backpropagation. The sigmoid function
will be run in every neuron. 

The sigmoid
function plots any denomination to another denomination which is between 1 and 0.
A sigmoid function is defined by the following formula:

It is a
mathematical function and has a curve shaped like an ‘S’ as shown in the below

The Input

I = np.array(0,0,0,0,0, 0,1,1,0,0, 1,0,0,1,1, 1,1,1,1,1)

Here we are
initialising the input dataset as a numpy matrix. Each column relates to one of
the input nodes, hence there are four nodes as inputs to
the network and five training examples.

The Output

O = np.array(0,0,0, 1,0,0, 1,0,1, 0,1,0)

This sets the output
dataset, where the data is being produced horizontally with three rows and four
columns. The rows each represent a
training example and the columns each represent an
output node, therefore, it can be said that the
network has three inputs and one output.



Seeding is done
in order to start at the dame point each time the neural network runs. This is
done as it would be simpler to observe how the modifications have an influence on
the neural network.


W0 = 2*np.random.random((5,3)) – 1

W0 = 2*np.random.random((5,4)) – 1

W1 = 2*np.random.random((4,3)) – 1

The variable ‘W0’
represents the weight matrix, which would be a five by three matrix. Later on
the weights of the first layer are generated randomly again in order to
generate a five by four matrix. The second layer of weights is a four by three
matrix of weights. The one at the end is the bias.


for iter in range(epochs):

A for loop is used to
iterate numerous times in order to augment the network to the dataset. Here we
are continuously inputting our data and updating the weights over time to


lay0 = I

lay1 = nonlin(, W0))

= nonlin(, W1))

Lay0 is our input layer and
it is the first layer of the Neural Network. Here we are doing a matrix
multiplication where we are multiplying the synapse, which is also known as the
weight, by the layers and the result is then passed through the sigmoid


lay2_error = O – lay2

(k % 10000) == 0:

print (‘Error:’ + str(np.mean(np.abs(lay2_error))))

In the backpropagation, the
algorithm tries to reduce the error each time the loop is run. The error we try
to reduce is where the prediction is inaccurate. The guess lay2 is subtracted
from the true answer O and the answer is stored in lay2_error which will show
how well the network did. Printing is done every then thousand step to see how
well the network is doing.

The following screenshot
shows how the output of the errors would look when the program is run. Since
the “epochs” variable is set to 60000 and the errors are being executed every
1000, a total of six errors should be shown. Errors should be getting closer to
zero the closer the iterations get to 60000.

Delta Calculations

lay2_delta = lay2_error *
nonlin(lay2, deriv=True)

lay1_error =

= lay1_error * nonlin(lay1, deriv=True)

Delta is the difference in
the quantity every time the loop is run. Here we are calculating deltas as the
data moves through the layers as the sigmoid function is applied to all of the

Updating of Weights/Synapses

W1 +=


This line of code calculates
the synapse updates for each synapse for each training example.

Printing of the Output

print ()

print (‘The final output after training: ‘)


Together with the error
printing every then thousand steps, we are also printing out the final output
which would be stored in lay2.

The result is shown in the
following figure.

Plotting the Graph

nonlin(lay2,deriv=True), ‘ro’)

The final step was to plot
the graph. Here we are plotting the deltas of the final layer as the data moves
through the layers and the sigmoid function is applied to it against the
sigmoid function.

The yielded graph is shown
in the following screenshot. The “lay2_delta” is on the x-axis and the
“nonlin(lay2, deriv=Ture)” is on the y-axis:



Source Code

this section one would find the source code of the program that has been built
for the neural network and is as explained in the previous sections.

import numpy as np

import matplotlib.pyplot as plt


# Number
of iterations

epochs =


the sizes for the input layer, hidden layer and the output layer respectively

hiddenLayerSize, outputLayerSize = 5, 4, 3



sigmoid function

#if true
is passed then the derivative of the sigmoid would be calculated

false is passed it is not going to be calculated. The derivative is needed when

is being calculated in the backpropagation

#This is
the sigmoid function that is going to be run in every neuron

def nonlin(x,deriv=False):


        return x*(1-x)

    return 1/(1+np.exp(-x))


# input
dataset & output dataset

I = np.array(0,0,0,0,0, 0,1,1,0,0,
1,0,0,1,1, 1,1,1,1,1)

O = np.array(  0,0,0,    
1,0,0,     1,0,1,     0,1,0)


random numbers to make calculation

to start at the same point each time (good for debugging)





#a five
by four matrix and the one is the bias

W0 = 2*np.random.random((5,4)) – 1

#A four
by three matrix and the one is the bias

W1 = 2*np.random.random((4,3)) – 1





inputting our data and updating the weights over time to backpropagation

for j in range(epochs):


    # Feed forward
through layers 0, 1, and 2

    lay0 = I

#this is matrix multiplication,
multiplying the synapse W0 by layer 0 and the synapse W1 by layer 1

    lay1 = nonlin(,W0))

    lay2 = nonlin(,W1))



    #tries to reduce the error each time the
loop is run

    #where the prediction is bad

    lay2_error = O – lay2


    #printing every
10000 steps to see how well it is doing

    if (j% 10000) == 0:

#this will print out our error,
using the absolute value    function of

#to make sure that it is a positive number. After this is done

#we would get the mean of that and print it as a string

        print (“Error:
” + str(np.mean(np.abs(lay2_error))))


#Delta calculations

#The difference in the quantity
every time and we are going to   

#as data moves through the layers as
the sigmoid function is applied to

#all of them elementary
multiplication is done between the layer and the

#derivative, which is set to true,
the result from this multiplication is

#then multiplied by the layer error.
The multiplication for the slopes with

#the error results in reducing the
error of high confidence predictions

    lay2_delta = lay2_error*nonlin(lay2,deriv=True)

    lay1_error =   

    lay1_delta = lay1_error *


#updating the weights/synapses

    W1 +=

    W0 +=


print ()

print (“The final output after training:”)





plt.plot(lay2_delta, nonlin(lay2,deriv=True), ‘ro’)

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