平方 (x^2) 近似的神经网络 [英] Neural network for square (x^2) approximation
问题描述
我是 TensorFlow 和数据科学的新手.我做了一个简单的模块,应该弄清楚输入和输出数字之间的关系.在这种情况下,x 和 x 平方.Python中的代码:
I'm new to TensorFlow and Data Science. I made a simple module that should figure out the relationship between input and output numbers. In this case, x and x squared. The code in Python:
import numpy as np
import tensorflow as tf
# TensorFlow only log error messages.
tf.logging.set_verbosity(tf.logging.ERROR)
features = np.array([-10, -9, -8, -7, -6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8,
9, 10], dtype = float)
labels = np.array([100, 81, 64, 49, 36, 25, 16, 9, 4, 1, 0, 1, 4, 9, 16, 25, 36, 49, 64,
81, 100], dtype = float)
model = tf.keras.Sequential([
tf.keras.layers.Dense(units = 1, input_shape = [1])
])
model.compile(loss = "mean_squared_error", optimizer = tf.keras.optimizers.Adam(0.0001))
model.fit(features, labels, epochs = 50000, verbose = False)
print(model.predict([4, 11, 20]))
我尝试了不同数量的单元,并添加更多层,甚至使用relu
激活函数,但结果总是错误.它适用于 x 和 2x 等其他关系.这里有什么问题?
I tried a different number of units, and adding more layers, and even using the relu
activation function, but the results were always wrong.
It works with other relationships like x and 2x. What is the problem here?
推荐答案
您犯了两个非常基本的错误:
You are making two very basic mistakes:
- 您的超简单模型(具有单个单元的单层网络)根本不符合神经网络的要求,更不用说深度学习"模型了(因为您的问题已被标记)
- 同样,您的数据集(只有 20 个样本)也非常小
当然可以理解,如果神经网络要解决像 x*x
这样简单"的问题,就需要具有一定的复杂性;它们真正闪耀的地方是在接受大型训练数据集时.
It is certainly understood that neural networks need to be of some complexity if they are to solve problems even as "simple" as x*x
; and where they really shine is when fed with large training datasets.
尝试解决此类函数近似的方法不仅仅是列出(少数可能的)输入,然后将其连同所需的输出一起输入模型;请记住,NN 是通过示例而不是符号推理来学习的.而且例子越多越好.在类似情况下,我们通常会生成大量示例,然后将其提供给模型进行训练.
The methodology when trying to solve such function approximations is not to just list the (few possible) inputs and then fed to the model, along with the desired outputs; remember, NNs learn through examples, and not through symbolic reasoning. And the more examples the better. What we usually do in similar cases is to generate a large number of examples, which we subsequently feed to the model for training.
话虽如此,这里是一个相当简单的 Keras 3 层神经网络演示,用于逼近函数 x*x
,使用 [-50, 50]
:
Having said that, here is a rather simple demonstration of a 3-layer neural network in Keras for approximating the function x*x
, using as input 10,000 random numbers generated in [-50, 50]
:
import numpy as np
import keras
from keras.models import Sequential
from keras.layers import Dense
from keras.optimizers import Adam
from keras import regularizers
import matplotlib.pyplot as plt
model = Sequential()
model.add(Dense(8, activation='relu', kernel_regularizer=regularizers.l2(0.001), input_shape = (1,)))
model.add(Dense(8, activation='relu', kernel_regularizer=regularizers.l2(0.001)))
model.add(Dense(1))
model.compile(optimizer=Adam(),loss='mse')
# generate 10,000 random numbers in [-50, 50], along with their squares
x = np.random.random((10000,1))*100-50
y = x**2
# fit the model, keeping 2,000 samples as validation set
hist = model.fit(x,y,validation_split=0.2,
epochs= 15000,
batch_size=256)
# check some predictions:
print(model.predict([4, -4, 11, 20, 8, -5]))
# result:
[[ 16.633354]
[ 15.031291]
[121.26833 ]
[397.78638 ]
[ 65.70035 ]
[ 27.040245]]
嗯,还不错!请记住,NN 是函数逼近器:我们不应该期望它们完全重现函数关系,也不要知道"4
和-4
应该是相同的.
Well, not that bad! Remember that NNs are function approximators: we should expect them neither to exactly reproduce the functional relationship nor to "know" that the results for 4
and -4
should be identical.
让我们在 [-50,50]
中生成一些新的随机数据(请记住,出于所有实际目的,这些是模型的未见数据)并绘制它们,连同原始图片,以获得更一般的图片:
Let's generate some new random data in [-50,50]
(remember, for all practical purposes, these are unseen data for the model) and plot them, along with the original ones, to get a more general picture:
plt.figure(figsize=(14,5))
plt.subplot(1,2,1)
p = np.random.random((1000,1))*100-50 # new random data in [-50, 50]
plt.plot(p,model.predict(p), '.')
plt.xlabel('x')
plt.ylabel('prediction')
plt.title('Predictions on NEW data in [-50,50]')
plt.subplot(1,2,2)
plt.xlabel('x')
plt.ylabel('y')
plt.plot(x,y,'.')
plt.title('Original data')
结果:
嗯,可以说它确实看起来像是一个很好的近似......
Well, it arguably does look like a good approximation indeed...
您还可以查看此线程以获得正弦近似值.
You could also take a look at this thread for a sine approximation.
最后要记住的是,尽管即使使用相对简单的模型,我们也确实得到了不错的近似值,但我们应该不期望的是外推,即[-50, 50]
之外的良好表现;有关详细信息,请参阅我在 深度学习不擅长拟合训练范围之外的简单非线性函数吗?
The last thing to keep in mind is that, although we did get a decent approximation even with our relatively simple model, what we should not expect is extrapolation, i.e. good performance outside [-50, 50]
; for details, see my answer in Is deep learning bad at fitting simple non linear functions outside training scope?
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