# Wiki: Lesson 1

#248

Can someone please explain the “binary_loss” in the last couple of cells of lesson 1?

``````def binary_loss(y, p):
return np.mean(-(y * np.log(p) + (1-y)*np.log(1-p)))
``````
1. Why is it the “binary” loss? Does that just mean that it is normalized between 0 and 1?
2. What is the “`y`” here? It is not the `y` from `log_preds,y = learn.TTA()`, because it crashes if I use that “`y`
3. I see that that when a list is made to call this function, the variable is called “`acts`”, what does this stand for? I realise that it is `y`, but I don’t understand.
4. How would I get y from the confusion matrix? I got inf when I tried using `probs` - so maybe what is `probs` then…

Also, one really dumb question, but I just want to verify I understand - what does “precompute” actually precompute when set to True?

Thanks!

(Pierre Guillou) #249

## Check your understanding of the lesson 1

Check your understanding of the lesson 2 >>>

(post original in portuguese at Deep Learning Brasília - Lição 1)

Hi guys,

I did watch again the video of the lesson 1 (part 1) to get the whole image and I took notes of the vocabulary used by @jeremy.

Let’s play ! OK ?
Can you give a definition / a url / an explanation for all the followings terms and expressions ?

If yes, you are done with the first lesson !!!

PS : you do not want to test yourself or you want to check your answers ? Go to the blog post “Deep Learning 2: Part 1 Lesson 1” of @hiromi : " super travail !!! "

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Wiki: Lesson 2
#250
1. Binary loss represents the loss function for a binary classification problem. This does not necessarily mean that the loss itself is normalized from 0 to 1.

2. y here represents the labels for the examples that the loss is calculated for. For example, if picture 1 is a dog and picture 2 is a cat, then y = [1, 0] (assuming 1 represents dog and 0 represents cat). p represents the probability that the example is a dog (1), output by the model.

3. I’m guessing that acts stands for actuals, as in the actual labels.

4. I’m not sure why you would want to get y from the confusion matrix. As I understand, the confusion matrix is a visualization of the model’s predictions so that you can see which categories your model performs well on and which ones it performs poorly on.

Setting precompute to True ensures that the model uses precomputed activations for the model. This means that the model will use the activations that were precomputed during training except for the last layer. This is because with little data, it will be difficult to properly train the whole model, but training only the last layer is easier to do.

(BixQu) #252

At minute 49:27 in the video, I see a function S(x) = 1/(1-exp(x)). Is that an activation function? I seems to look like a Sigmoid, but that is 1/(1+exp(-x)).

(Reshama Shaikh) #253

Here are instructions for creating a keypair

You will want to replace `aws_fastai_gpu.pem` with your keypair name which will be `id_rsa.pub`

#254

Thanks @reshama for your paperspace setup instructions. Extremely useful!

#255

Did a `git pull` and `conda env update` this evening and now lesson1.ipynb (for cell 29) gives `AttributeError: 'ConvLearner' object has no attribute 'data_path'`.

(edit)
Found this post forums.fast.ai 15606 which fixed the problem. Thanks Tal!

(Mathieu) #256

What is the relationship between epoch and batch size? How to set batch size correctly?

At 1:19 the teacher is talking about epoch and batch size, at each epoch we take a batch size of 64…

I noticed I was unable to plot the learning rate learn.sched.plot() until I set the batch size to 6 for my 200 images (100 each of each type) with a setting of 75% training 15% valid. When I inspected the current batch size learn.data.bs it was already set to 64 before I changed it for my dataset.

Thanks for clarifications!

(Ivan Oboth) #258

Thanks a bunch @duanev, i’d got the same issue, thanks for sharing the link to solution.

(Ivan Oboth) #260

Hi prairieguy,
I get an error “selenium.common.exceptions.WebDriverException: Message: Process unexpectedly closed with status: 1” when i run the script. Could i be doing something wrong?

(ashish johnson) #261

thanks for the terms and expressions !! It is well appreciated and keep doing it

(Shivaraj Bakale) #262

Why do we always have to find the local minimum in Deep learning algorithms ? What purpose does it serve

(Stas Bekman) #263

The issue with np.mean() and the call to accuracy_np(probs, y) failing as it was getting passed a one-dim array:

`AxisError: axis 1 is out of bounds for array of dimension 1`

update: for some reason pip wasn’t loading the latest version of fastai - I replaced it with pulling directly from github and it all works now. So it was a false alarm.

(Alex) #264

Ideally we would want to find a global minimum of our loss function which should represent “how far away” we are from our desired values. But in practice we may end up with overfitting.
From this paper: https://arxiv.org/abs/1412.0233

We empirically verify several hypotheses regarding
learning with large-size networks:
• For large-size networks, most local minima are equivalent and yield similar performance on a test set.
• The probability of finding a “bad” (high value) local minimum is non-zero for small-size networks and decreases quickly with network size.
• Struggling to find the global minimum on the training set (as opposed to one of the many good local ones) is not useful in practice and may lead to overfitting.

(shweta ) #265

Hi everyone,
I have written my first blog post on Dogs Vs Cats Classification, please have a look

give suggestion for further improvement
thanks!!

(This Connection is Not Secure) #266

Hi just some feedback. I was following an older version of this course a while ago, and I found that much, much easier to follow than this version.

The old one had a few utility methods and stuff (“utils.py” and “vgg16.py”!), but this new one comes with thousands of lines of “helpful” code in the fastai library, way too much to casually understand without a lot of work.

Now I feel like I’m not learning how to use keras or theano or tensorflow or pytorch, I’m just investing a lot of time into learning your made-for-this-course framework.

I’m willing to work hard, but if I put in the work to understand the fastai library it’s not transferrable or useful. I’d much rather have to slowly build up over time all the code for image-loading, transforming, model creation, etc. Then at least that effort teaches me something that’s useful in the future.

As helpful as the fastai library is, it’s not likely to be used outside of this course. Rather than learn it, I’d like to learn how to do those things myself.

(Vamsi Uppala) #267

Can we please move the link to auto generate test data to a more prominent position in the wiki? I didn’t pay enough attention to this link until I actually spent considerable time to find web scrapers, download and arrange images into folders and be satisfied with my hours of work before realizing there was an easier way to do it.

(Kofi Asiedu Brempong) #268

In lesson 1 around the 29th minute, @jeremy says that you could download some pictures, change the path to point to those pictures and use the same lines of code to train the neural network to recognize those images too.

I wanted to train it to recognize minerals so I downloaded some pictures of 2 minerals and changed the path to point to the folder containing them but I’m getting some errors with the code.

(Rahul) #269

I remember there was a link to a pdf where someone had made notes commenting in the Jupiter notebook itself and explaining the codes. Anyone know where I can find that?

(Dusten) #270

This was just posted two days ago, May 15 2018, and it was very helpful in getting Fast.ai course work up and running in Sagemaker.

https://aws.amazon.com/blogs/machine-learning/running-fast-ai-notebooks-with-amazon-sagemaker/