# 代码代写｜Assignment 4: Introduction to neural networks

## 这是一篇来自美国的关于构建简单的神经网络来执行各种机器学习任务的代码代写

This assignment will have you building simple neural networks to perform a variety of machine learning tasks.

This project was developed at UC Berkeley (http://ai.berkeley.edu), and modified for CISC-352 (including a new rubric).

Installation

For this project, you will need to install the following two libraries:

• numpy, which provides support for large multi-dimensional arrays
• matplotlib, a 2D plotting library

CONDA

conda install -c anaconda numpy

conda install -c conda-forge matplotlib

PIP

pip install numpy

pip install matplotlib

Note: If you are using Python 3.x, you may need to change pip to pip3.

To test that everything has been installed, run:

If numpy and matplotlib are installed correctly, you should see a window pop up where a line segment spins in a circle:

Introduction

Files to Edit and Submit: You will fill in portions of models.py during the assignment. Please do not change the other files in this distribution.

Files you should read but NOT

edit:

nn.py     Neural network mini-library

Files you will not edit:

backend.py   Backend code for various machine

(we will use our own auto-grader code that mirrors what you are provided).

Important: Marks will be deducted for poor code quality. Anywhere from 0.5- 1.5pt per problem, depending on the severity of the issues. Please use comments where it makes sense, and try to keep your code clear and easy for the TAs to understand.

Note: Read Neural Network Tips and Tutorial before attempting the question.

Question 1 (1.5 points): Perceptron

In this part, you will implement a binary perceptron. Your task will be to complete the implementation of the PerceptronModel class in models.py.

For the perceptron, the output labels will be either 1 or -1 , meaning that data points (x, y) from the dataset will have y be a nn.Constant node that contains either 1 or -1 as its entries.

We have already initialized the perceptron weights self.w to be a 1 × dimensions parameter node. The provided code will include a bias feature inside x_point when needed, so you will not need a separate parameter for the bias.

Your tasks are to: 1. Implement the run(self, x_point) method. This should compute the dot product of the stored weight vector and the given input,returning an nn.DotProduct object.

1. Implement get_prediction(self, x_point), which should return 1 if the dot product is non-negative or -1 otherwise. You should use nn.as_scalar to convert a scalar Node into a Python floating-point number.

Write the train_model(self) method. This should repeatedly loop over the data set and make updates on examples

1. Write the train_model(self) method. This should repeatedly loop over the data set and make updates on examples that are misclassified. Use the update method of the nn.Parameter class to update the weights. When an entire pass over the data set is completed without making any mistakes,100% training accuracy has been achieved, and training can terminate.

In this project, the only way to change the value of a parameter is by calling parameter.update(multiplier, direction), which will perform the update to the weights:

weights ← weights + multiplier * direction

The direction argument is a Node with the same shape as the parameter, and the multiplier argument is a Python scalar.

Note: the autograder should take at most 20 seconds or so to run for a correct implementation. If the autograder is taking forever to run, your code probably has a bug.

Question 2 (2 points): Non-linear Regression

For this question, you will train a neural network to approximate sin(x) over [-2*Pi , 2*Pi]

You will need to complete the implementation of the RegressionModel class in models.py. For this problem, a relatively simple architecture should suffice (see Neural Network Tips for architecture tips. Use nn.SquareLoss as your loss.

1. Implement RegressionModel.__init__ with any needed initialization
2. Implement RegressionModel.run to return a batch_size × 1 node that represents your model’s prediction.
1. Implement RegressionModel.get_loss to return a loss for given inputs and target outputs.

Your implementation will receive full points if it gets a loss of 0.02 or better, averaged across all examples in the dataset. You may use the training loss to determine when to stop training (use nn.as_scalar to convert a loss node to a Python number). Note that it should take the model a few minutes to train.

Question 3 (2.5 points): Digit Classification

For this question, you will train a network to classify handwritten digits from the MNIST dataset.

Each digit is of size 28 × 28 pixels, the values of which are stored in a 784- dimensional vector of floating point numbers. Each output we provide is a 10-dimensional vector which has zeros in all positions, except for a one in the position corresponding to the correct class of the digit.

Complete the implementation of the DigitClassificationModel class in models.py. The return value from DigitClassificationModel.run() should be a batch_size × 10 node containing scores, where higher scores indicate a higher probability of a digit belonging to a particular class (0-9). You should use nn.SoftmaxLoss as your loss. Do not put a ReLU activation after the last layer of the network.

In addition to training data, there is also validation data and a test set. You can use dataset.get_validation_accuracy() to compute validation accuracy for your model, which can be useful when deciding whether to stop training. The test set will be used by the autograder.

To receive points for this question, your model should achieve an accuracy of at least 97% on the test set. For reference, our implementation consistently achieves an accuracy of near 98% on the validation data after training for around 10-15 epochs. Note that the test grades you on test accuracy, while you only have access to validation accuracy – so if your validation accuracy meets the 97% threshold, you may still fail the test if your test accuracy does not meet the threshold. Therefore, it may help to set a slightly higher stopping threshold on validation accuracy, such as 97.5% or 98%.

Neural Network Tips

Building Neural Nets

Throughout the applications portion of the project, you’ll use the framework provided in nn.py to create neural networks to solve a variety of machine learning problems. A simple neural network has layers, where each layer performs a linear operation (just like perceptron). Layers are separated by a non-linearity,which allows the network to approximate general functions. We’ll use the ReLU operation for our non-linearity, defined as relu(x) = max(x,0). For example, a simple two-layer neural network for mapping an input row vector x to an output vector f(x) would be given by the function:

f(x) = relu(x * W1 + b1) * W2 + b2

where we have parameter matrices W1 and W2 and parameter vectors b1 and b2 to learn during gradient descent. W1 will be an i × h matrix, where i s the dimension of our input vectors x , and h is the hidden layer size. b1 will be a size h vector. We are free to choose any value we want for the hidden size ( we will just need to make sure the dimensions of the other matrices and vectors agree so that we can perform the operations). Using a larger hidden size will usually make the network more powerful (able to fit more training data), but can make the network harder to train (since it adds more parameters to all the matrices and vectors we need to learn), or can lead to overfitting on the training data.

We can also create deeper networks by adding more layers, for example a three-layer net:

f(x) = relu( relu(x * W1 + b1) * W2 + b2) * W3 + b3

Note on Batching For efficiency,

You will be required to process whole batches of data at once rather than a single example at a time. This means that instead of a single input row vector x with size i , you will be presented with a batch of b inputs represented as a b × i matrix X . We provide an example for linear regression to demonstrate how a linear layer can be implemented in the batched setting.

Note on Randomness

The parameters of your neural network will be randomly initialized, and data in some tasks will be presented in shuffled order. Due to this randomness,it’s possible that you will still occasionally fail some tasks even with a strong architecture – this is the problem of local optima! This should happen very rarely, though – if when testing your code you fail the autograder twice in a row for a question, you should explore other architectures.

Practical tips

• Be systematic. Keep a log of every architecture you’ve tried, what the hyperparameters (layer sizes, learning rate, etc.) were, and what the resulting performance was. As you try more things, you can start seeing patterns about which parameters matter. If you find a bug in your code,be sure to cross out past results that are invalid due to the bug.
• Start with a shallow network (just two layers, i.e. one non-linearity).

Deeper networks have exponentially more hyperparameter combinations, and getting even a single one wrong can ruin your performance. Use the small network to find a good learning rate and layer size; afterwards you can consider adding more layers of similar size.

• If your learning rate is wrong, none of your other hyperparameter choices matter. You can take a state-of-the-art model from a research paper, and change the learning rate such that it performs no better than random. A learning rate too low will result in the model learning too slowly, and a learning rate too high may cause loss to diverge to infinity. Begin by trying different learning rates while looking at how the loss decreases over time.
• Smaller batches require lower learning rates. When experimenting with different batch sizes, be aware that the best learning rate may be different depending on the batch size.
• Refrain from making the network too wide (hidden layer sizes too large) If you keep making the network wider accuracy will gradually decline, and computation time will increase quadratically in the layer size – you’re likely to give up due to excessive slowness long before the accuracy falls too much. The full autograder for all parts of the project takes 2-12 minutes to run with staff solutions; if your code is taking much longer you should check it for efficiency.
• If your model is returning Infinity or NaN, your learning rate is probably too high for your current architecture.
• Recommended values for your hyperparameters:

Hidden layer sizes: between 10 and 400.

Batch size: between 1 and the size of the dataset. For Q2 and Q3, we require that total size of the dataset be evenly divisible by the batch size.

Learning rate: between 0.001 and 1.0.

Number of hidden layers: between 1 and 3. E-mail: vipdue@outlook.com  微信号:vipnxx 