什么是神经网络

学习笔记:

The term deep learning refers to training neural networks.
术语"深度学习"指的是训练神经网络.
Let’s start with an example of house price forecasting.
用一个房价预测的例子来说明.
Let’s say you have a data sets with six houses.
比方说有一个六间房子的数据集.
You know the size of the house and the price,
and you wanted to find a function that could predict the price of a house based on the size of the house.
已知房价和房屋面积,想定义一个能够通过房屋面积预测房价的函数.
So if you are familiar with regression,let’s fit a straight line to this data.
熟悉线性回归的人就会用这些数据拟合一条直线.
But the line doesn’t fit, it’s going to make the price negative in the end.
但是直线不合适,它最后会让价格为负.
So let’s draw this line down here,and this line is the function that we want.
所以我们用下面这条线表示我们想要的函数.

Think of this function as a neural network.
把这个函数作为一个神经网络.
You have as the input to the neuro network,the size of the house，
房屋的面积,作为神经网络的输入,
and as the output to the neuro network,the price of the house.
房屋的价格,作为神经网络的输出.
Let’s call it x for area and y for price.
用x表示输入面积,y表示输出价格.
So this little circle which is a single neuron.
这个小圆圈是一个独立的神经元.
It inputs the size,computes this linear fuction and outputs the price.
输入面积,完成线性运算,得出预测价格.
This function is called a ReLU function,which stands for rectified linear unit.
这个函数被称为ReLU函数,全称是"修正线性单元".

If you now have other feautures,you know other things about the house
如果还有一些房屋的其它特征,已知其它信息
such as the number of bedrooms.
例如已知卧室数量.
And it’s really based on the size.
可以和把它和已知的面积关联起来.
These two factors influence family size.
这两个因素影响了所能容纳的家庭人数.
House prices are affected by whether the needs of families can be met.
而家庭人数的需求能否被满足影响了房屋价格.

And the postal code maybe as a feature in many countries.
在很多国家，邮政编码或许能作为一个特征.
It is the symbol of the city and it can be used as the basis to evaluate the degree of walkability.
作为城市的象征,是步行化程度的一个评估依据.
After all, there are people who like highly walkable places.
毕竟有很多人喜欢高度步行化的地方.
At the same time, a postal code can tell the level of schools in an area.
从邮政编码也能看出一个地区的教育水平,或者说是学校质量.
As well as the wealth maybe,
还有富裕程度,
Buying a school district housing is also a prerequisite for getting into a good school.
毕竟很多时候买到学区房也是上一个好学校的前提.
All of these factors determine how much people are willing to pay for a home.
这些因素都决定了人们愿意花多少钱购房.
In this example,x is all these four inputs and y is the price you’re trying to predict.
在这个例子中,x是四个输入,y是预测的价格.
The neural network is expressed as follows.
用神经网络表示如下

You need to give it just the input x and the output y,for number of examples in your training set.
不管训练集多大,需要做的就是输入x,得到输出y,
And all thesee things in middle it will figure out by itselt;
所有的中间过程它都会自己完成.
It just have to get enough x and will do an exact mapping from x to y.
只要给够足够的x,就能完成从x到y的精准映射.
That’s the neural network.
这就是神经网络.