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Examples and Intuitions II
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來自 Stanford University 的課程
机器学习
79121 個評分
從本節課中
Neural Networks: Representation
Neural networks is a model inspired by how the brain works. It is widely used today in many applications: when your phone interprets and understand your voice commands, it is likely that a neural network is helping to understand your speech; when you cash a check, the machines that automatically read the digits also use neural networks.
與講師見面
Andrew NgCo-founder, Coursera; Adjunct Professor, Stanford University; formerly head of Baidu AI Group/Google Brain
In this video I'd like to keep working through our example to show how
a Neural Network can compute complex non linear hypothesis.
In the last video we saw how a Neural Network can be used to
compute the functions x1 AND x2, and the function x1 OR
x2 when x1 and x2 are binary, that is when they take on values 0,1.
We can also have a network to compute negation,
that is to compute the function not x1.
Let me just write down the ways associated with this network.
We have only one input feature x1 in this case and the bias unit +1.
And if I associate this with the weights plus 10 and -20,
then my hypothesis is computing this h(x) equals sigmoid (10- 20 x1).
So when x1 is equal to 0, my hypothesis would
be computing g(10- 20 x 0) is just 10.
And so that's approximately 1, and when x is equal to 1,
this will be g(-10) which is approximately equal to 0.
And if you look at what these values are, that's essentially the not x1 function.
Cells include negations, the general idea is to put
that large negative weight in front of the variable you want to negate.
Minus 20 multiplied by x1 and
that's the general idea of how you end up negating x1.
And so in an example that I hope that you can figure out yourself.
If you want to compute a function like this NOT x1 AND NOT x2,
part of that will probably be putting large negative weights in front of x1 and
x2, but it should be feasible.
So you get a neural network with just one output unit to compute this as well.
All right, so this logical function, NOT x1 AND
NOT x2, is going to be equal to 1 if and only if
x1 equals x2 equals 0.
All right since this is a logical function, this says NOT x1 means x1 must
be 0 and NOT x2, that means x2 must be equal to 0 as well.
So this logical function is equal to 1 if and only if both x1 and
x2 are equal to 0 and hopefully you should be able to figure out how to
make a small neural network to compute this logical function as well.
Now, taking the three pieces that we have put together as the network for
computing x1 AND x2, and the network computing for computing NOT x1 AND NOT x2.
And one last network computing for computing x1 OR x2, we should be able
to put these three pieces together to compute this x1 XNOR x2 function.
And just to remind you if this is x1, x2,
this function that we want to compute would have negative examples here and
here, and we'd have positive examples there and there.
And so clearly this will need a non linear decision boundary
in order to separate the positive and negative examples.
Let's draw the network.
I'm going to take my input +1, x1, x2 and create my first hidden unit here.
I'm gonna call this a 21 cuz that's my first hidden unit.
And I'm gonna copy the weight over from the red network, the x1 and x2.
As well so then -30, 20, 20.
Next let me create a second hidden unit which I'm going to call a 2 2.
That is the second hidden unit of layer two.
I'm going to copy over the cyan that's work in the middle, so
I'm gonna have the weights 10 -20 -20.
And so, let's pull some of the truth table values.
For the red network, we know that was computing the x1 and
x2, and so this will be approximately 0 0 0 1,
depending on the values of x1 and x2, and for a 2 2, the cyan network.
What do we know?
The function NOT x1 AND NOT x2, that outputs 1 0 0 0, for
the 4 values of x1 and x2.
Finally, I'm going to create my output node, my output unit that is a 3 1.
This is one more output h(x) and I'm going to copy over the old network for that.
And I'm going to need a +1 bias unit here, so you draw that in, And
I'm going to copy over the weights from the green networks.
So that's -10, 20, 20 and we know earlier that this computes the OR function.
So let's fill in the truth table entries.
So the first entry is 0 OR 1 which can be 1 that makes 0 OR
0 which is 0, 0 OR 0 which is 0, 1 OR 0 and that falls to 1.
And thus h(x) is equal to 1 when either both x1 and x2 are zero or
when x1 and x2 are both 1 and concretely h(x) outputs 1
exactly at these two locations and then outputs 0 otherwise.
And thus will this neural network, which has a input layer,
one hidden layer, and one output layer,
we end up with a nonlinear decision boundary that computes this XNOR function.
And the more general intuition is that in the input layer,
we just have our four inputs.
Then we have a hidden layer,
which computed some slightly more complex functions of the inputs that its shown
here this is slightly more complex functions.
And then by adding yet
another layer we end up with an even more complex non linear function.
And this is a sort of intuition about why neural networks can compute
pretty complicated functions.
That when you have multiple layers you have relatively simple function of
the inputs of the second layer.
But the third layer I can build on that to complete even more complex functions, and
then the layer after that can compute even more complex functions.
To wrap up this video,
I want to show you a fun example of an application of a the Neural Network
that captures this intuition of the deeper layers computing more complex features.
I want to show you a video of that customer a good friend of mine
Yann LeCunj.
Yann is a professor at New York University, NYU and
he was one of the early pioneers of Neural Network reasearch and
is sort of a legend in the field now and his ideas are used in
all sorts of products and applications throughout the world now.
So I wanna show you a video from some of his early work in which he was using
a neural network to recognize handwriting, to do handwritten digit recognition.
You might remember early in this class, at the start of this class I said that
one of the earliest successes of neural networks was trying to use it to read zip
codes to help USPS Laws and read postal codes.
So this is one of the attempts,
this is one of the algorithms used to try to address that problem.
In the video that I'll show you this area here
is the input area that shows a canvasing character shown to the network.
This column here shows a visualization of the features computed by sort of the first
hidden layer of the network.
So that the first hidden layer of the network and so the first hidden layer,
this visualization shows different features.
Different edges and lines and so on detected.
This is a visualization of the next hidden layer.
It's kinda harder to see, harder to understand the deeper, hidden layers, and
that's a visualization of why the next hidden layer is confusing.
You probably have a hard time seeing what's going
on much beyond the first hidden layer, but
then finally, all of these learned features get fed to the upper layer.
And shown over here is the final answer, it's the final predictive value for
what handwritten digit the neural network thinks it is being shown.
So let's take a look at the video.
[MUSIC]
So I hope you enjoyed the video and that this hopefully gave you some intuition
about the source of pretty complicated functions neural networks can learn.
In which it takes its input this image, just takes this input, the raw pixels and
the first hidden layer computes some set of features.
The next hidden layer computes even more complex features and
even more complex features.
And these features can then be used by essentially the final
layer of the logistic classifiers to make accurate predictions
without the numbers that the network sees.
