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How does a Neural network work? Its the basis of deep learning and the reason why image recognition, chatbots, self driving cars, and language translation work! In this video, i’ll use python to code up a neural network in just 4 minutes using just the numpy library, capable of doing matrix mathematics.

Code for this video:

https://github.com/llSourcell/Make_a_neural_network

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2 Great Neural Net Tutorials:

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1. https://medium.com/technology-invention-and-more/how-to-build-a-simple-neural-network-in-9-lines-of-python-code-cc8f23647ca1#.l51z38s7f

2. https://iamtrask.github.io/2015/07/12/basic-python-network/

Awesome Tutorial Series on Neural Networks:

http://lumiverse.io/series/neural-networks-demystified

The Canonical Machine Learning Course:

https://www.coursera.org/learn/machine-learning

Curious just how inspired neural networks are from brain architecture? Take some time to learn about the human brain! This is my favorite intro to neuroscience course:

https://www.mcb80x.org/

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Ok I built it, it works but like what do I do with it now lol can I somehow use this to make a chatbot I previously coded better?

Did u started like this? And understood forward propagation, back propagation, gradient descent….. In just 4 mins?

See the funny thing is there is no reason to make this 4min long

Why l2_error (line 34) is different from l1_error? (Line 41)

Thanks you all. 🙂

Won’t ReLU give better performance over sigmoid in this case.

Can someone help explain why we need this code?

if (deriv == True):

return (x * (1 – x))

what version of python are u using?

Recently ran this in Python3 after fixing many of the bugs. I keep outputting 0.47, 0.48, 0.54 and 0.54. Does anyone know why this is? My predicted output is nowhere close to the actual output, and my error rate doesn’t decrease each iteration — it stays at 0.4964

I think I have it working, at least it compiles now… :

Siraj’s 4 minute Neural Network!

# 3 layer neural net:

# Input Hidden Output

# Layer Layer Layer

#

# O

# O x x

# x O x

# O synapses0 synapses1 O

# x O x

# O x x

# O

#

#

#

#I had to run this in Anaconda since pip said numpy was already installed but stupid IDLE was complaining it wasn’t.

#also I guessed a lot of the error fixes… so there could still be problems.

import numpy as np

#sigmoid function: any value gets squashed to between 0 and 1

#Sigmoid function is used at every neuron to generate probabilities out of numbers

def nonlin(x,deriv):

if(deriv==True):

return x*(1-x)

return 1/(1+np.exp(-x))

#input data initialised as a matrix

# each row is a different training example with three input neurons each

# each column is a different nueron

X= np.array([[0,0,1],

[0,1,1],

[1,0,1],

[1,1,1]])

#output data set: four examples, so one output neuron each

y = np.array([[0],

[1],

[1],

[0]])

np.random.seed(1) # always use the same random number sequence (deterministic)

#for the following code in orer to help with debugging

#2 synapses matrices for a three layer network

#Synapses are the connections between each neuron in one layer to each neuron in the next layer

#each synaps has a random weight assigned to it.

syn0 = 2*np.random.random((3,4))-1 #3 inputs to 4 intermediate neurons

syn1 = 2*np.random.random((4,1))-1 #4 intermediate neurons to 1 output neuron

#training

for j in range (60000): #for loop that iterates over the training code to optimise the network for the given data set

layer0 = X # create our first layer which is out input data

layer1 = nonlin(np.dot(layer0, syn0),deriv=False) #prediction layer. perform matrix multiplication

#between each layer and its synaps then run our sigmoid function on each value in the matrix to

#create our next layer which is a prediction of the output data.

layer2 = nonlin(np.dot(layer1,syn1),deriv=False) # then we do the same thing on this layer to get a more refined prediction

#now that we have a prediction, let’s compare it to the expected data:

l2_error = y – layer2 # get the error rate

if(j %1000)==0:

print (“Error:” + str(np.mean(np.abs(l2_error))) )# print error rate at a set interval for monitoring purposes.

#next multiply our error rate by our sigmoid function of the prediction to get the derivative

#of our output prediction from layer 2, giving us a delta which we will use to reduce our error rate

#of our predictions when we update our synapses every iteration.

l2_delta = l2_error*nonlin(layer2, deriv=True)

#next we see how much layer 1 contributed to the errors in layer 2, this is called back propogation

# we get this error by multiplying layer2’s delta by syn1’s transpose

l1_error = l2_delta.dot(syn1.T)

#then we’ll get layer 1’s delta by multiplying its error by the resutl of our sigmoid function

#the function is used to get the derivative of layer 1

l1_delta = l1_error*nonlin(layer1,deriv=True)

#now that we have deltas for each of our layers, we can use them to update our synaps weights

#to reduce the error rate more and more.

#update weights – algorithm: Gradient Descent

syn1 += layer1.T.dot(l2_delta)

syn0 += layer0.T.dot(l1_delta)

print (“Outpu after training”)

print (layer2)

God, I wish I understood how to code…. O.o

idk why but that feels like 4 years

sensei neromation review

Great Work .. respect

Very cool. I like how you home brewed this.

I have a interesting idea for a program, but i have no idea of how about actually programming something like this, im wanting to make it for facebook users

Where are the bias ????

eh, maybe you should have done this in 40 minutes, not 4

how to read what he wrote ar 0:55 -> 1/(1+np,exp(-x)) I guess np is the Euler’s number, but I don’t understand the expresion… I don’t code in Python…

your video looks cool but they are useless if someone wanaa learn from it seriously..

Your code on” https://github.com/llSourcell/Make_a_neural_network” is for only one neuron. Do you have another code for multiple neuron.

I HAVE NEVER SEEN A THING WITH THAT MUCH INFORMATION PER TIME HOLY MOLY THIS IS SO COOL.

Can we get an example to show how bias fits in? …please?

instructions unclear, i now have *FoOT fUnGeS*

Thanks, very helpful.

Hey Siraj, i would like to be able to make you some questions cause im trying to implement an AI for a game but im a little stuck… where can i write you other than YT comments…??

Wait

How did you run it? You imported numpy as numpy

None of the np.s should have worked

You used a comma instead of a dot

It might work, but you cheated kinda

i have programmed before, but only learn python 2 days ago,

I dont understand a thing he said …