# Lesson 3 official topic

Hey guys this is Ajay, i have a doubt regarding the weights of a model. so basically the sources i came through including fast ai’s book says weights are some kind of parameters which says how important a feature is in the neural network, and an another sections says weights are tunable parameters which govern’s the model’s learning. so what weights are actually?

I don’t have excel and Apple’s numbers app doesn’t have the functionality needed for gradient descent so I tried doing this in Python.

Hello,
Can someone please explain this excerpt from the Jupyter notebook on gradient descent as -

The basic idea is this: if we know the gradient of our `mae()` function with respect to our parameters, `a` , `b` , and `c` , then that means we know how adjusting (for instance) `a` will change the value of `mae()` . If, say, `a` has a negative gradient, then we know that increasing `a` will decrease `mae()` . Then we know that’s what we need to do, since we trying to make `mae()` as low as possible.

Specifically, I did not understand -

If, say, `a` has a negative gradient, then we know that increasing `a` will decrease `mae()` .

How did we derive this conclusion? How can `a` have a negative gradient? `a` is just a fixed number in the quadratic right?

I did not understand how a negative sign of the gradient should intuitively mean that we need to increase our parameters. If anyone can provide an explanation, it would be really helpful!

Hi,

Here, when we say `a`’s gradient, we mean gradient of `loss` with respect to `a`. `loss` is calculated from calling `mae`. In Pytorch, we can type `a.grad` to get the gradient of `loss` with respect to `a`.

Also, negative gradient means negative slope, so we increase the parameter. If it doesn’t make sense, it might be helpful to watch videos on calculus on Khan Academy Unit 2, which covers derivatives.

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I’m struggling to figure out why Jeremy changed total loss function from sum of individual losses to average of them in the Titanic excel example. Does anyone know why?

The “sum of ReLu” function made sense but I found it tricky to find combinations of parameters to demonstrate the point that Jeremy made that it can be used to create any arbitrary squiggly line. In the end I figured out a good set of parameters to approximate a quadratic function using a ReLu sum function and using this to initialize the parameters, I could fit a nice line to the quadratic using a sum of 100 ReLus. Code is here (bottom half of the notebook) and here’s what it looks like:

How familiar are you with derivatives? If you aren’t, I do recommend brushing up on how they function.

To answer your question about your confusion, when we take the gradient of a parameter, we are looking to see how much the value (here that would be mae()) would change each time we slightly increase a, b, or c separately (similar to how a derivative works). The conclusion is baked into the math itself. If ‘a’ had a positive gradient then we would decrease it to lower mae(). And the code also does this by subtracting the gradient times the learning rate.

Another syntax question:

I looked up res.max(a) and I found that it simply returns the maximum value of the input a. How is it that res.max(tensor(0.0)) doesn’t only return the max, but actually replaces any negative values?

How to call learn.predict() ?

Hello guys,

I created a learner following fastbook notebook 04_mnist_basics.ipynb to distinguish photo 3 and photo 7.
All the steps are good except calling learn.predict().
What data type should be sent to predict() ? A image or a file path ?
I tried all those, but both return error.

Here is my basic code.

``````# download data
path = untar_data(URLs.MNIST_SAMPLE)
Path.BASE_PATH = path

# generate paths
threes = (path/'train'/'3').ls().sorted()
sevens = (path/'train'/'7').ls().sorted()

seven_tensors = [tensor(Image.open(o)) for o in sevens]
three_tensors = [tensor(Image.open(o)) for o in threes]
stacked_sevens = torch.stack(seven_tensors).float()/255
stacked_threes = torch.stack(three_tensors).float()/255
valid_3_tens = torch.stack([tensor(Image.open(o))
for o in (path/'valid'/'3').ls()])
valid_3_tens = valid_3_tens.float()/255
valid_7_tens = torch.stack([tensor(Image.open(o))
for o in (path/'valid'/'7').ls()])
valid_7_tens = valid_7_tens.float()/255

# train data set
train_x = torch.cat([stacked_threes, stacked_sevens]).view(-1, 28*28)
train_y = tensor([1]*len(threes) + [0]*len(sevens)).unsqueeze(1)
dset = list(zip(train_x,train_y))

# valid data set
valid_x = torch.cat([valid_3_tens, valid_7_tens]).view(-1, 28*28)
valid_y = tensor([1]*len(valid_3_tens) + [0]*len(valid_7_tens)).unsqueeze(1)
valid_dset = list(zip(valid_x,valid_y))

# loss function
def mnist_loss(predictions, targets):
predictions = predictions.sigmoid()

# function to calculate our validation accuracy:
def batch_accuracy(xb, yb):
preds = xb.sigmoid()
correct = (preds>0.5) == yb
return correct.float().mean()

learn = Learner(dls, nn.Linear(28*28,1), opt_func=SGD,
loss_func=mnist_loss, metrics=batch_accuracy)
learn.fit(10, lr=lr)

# predict on filepath failed
learn.predict(threes[0])
> TypeError: object of type 'PosixPath' has no len()

# predict on image failed too
learn.predict(PILImage.create(threes[0]))
> TypeError: object of type 'PILImage' has no len()
``````

How can I predict?

This is an interesting problem, I haven’t found a solution but here’s what I’ve found.

Here is the source code for `learn.predict`:

``````def predict(self, item, rm_type_tfms=None, with_input=False):
dl = self.dls.test_dl([item], rm_type_tfms=rm_type_tfms, num_workers=0)
inp,preds,_,dec_preds = self.get_preds(dl=dl, with_input=True, with_decoded=True)
i = getattr(self.dls, 'n_inp', -1)
inp = (inp,) if i==1 else tuplify(inp)
dec = self.dls.decode_batch(inp + tuplify(dec_preds))[0]
dec_inp,dec_targ = map(detuplify, [dec[:i],dec[i:]])
res = dec_targ,dec_preds[0],preds[0]
if with_input: res = (dec_inp,) + res
return res
``````

I took that source code, pasted it into a cell and ran the following (I ran your provided code first which gives me `train_x`):

``````item = train_x[0]

dl = learn.dls.test_dl([item], rm_type_tfms=None, num_workers=0)
inp,preds,_,dec_preds = learn.get_preds(dl=dl, with_input=True, with_decoded=True)
i = getattr(learn.dls, 'n_inp', -1)
inp = (inp,) if i==1 else tuplify(inp)
dec = learn.dls.decode_batch(inp + tuplify(dec_preds))[0]
dec_inp,dec_targ = map(detuplify, [dec[:i],dec[i:]])
res = dec_targ,dec_preds[0],preds[0]
``````

This gives the following error:

``````IndexError: too many indices for tensor of dimension 0
``````

for the following line of code:

``````inp,preds,_,dec_preds = learn.get_preds(dl=dl, with_input=True, with_decoded=True)
``````

I found this Forums post which although isn’t technically related to your situation, I thought I would give it a try (`unsqueeze` the `train_x[0]` value):

``````item = train_x[0].unsqueeze(dim=0)

dl = learn.dls.test_dl([item], rm_type_tfms=None, num_workers=0)
inp,preds,_,dec_preds = learn.get_preds(dl=dl, with_input=True, with_decoded=True)
i = getattr(learn.dls, 'n_inp', -1)
inp = (inp,) if i==1 else tuplify(inp)
dec = learn.dls.decode_batch(inp + tuplify(dec_preds))[0]
dec_inp,dec_targ = map(detuplify, [dec[:i],dec[i:]])
res = dec_targ,dec_preds[0],preds[0]
``````

This resolved the initial error but gave a new error:

``````AttributeError: 'list' object has no attribute 'decode_batch'
``````

Caused by the following line:

``````dec = learn.dls.decode_batch(inp + tuplify(dec_preds))[0]
``````

Looking at your `DataLoaders`, it doesn’t have a `decode_batch` attribute (I’m not sure why):

Here is a colab notebook with the code.

Dropping a few more related links that I found—note the last link where they have had the same issue but no resolution:

Not sure if any of this helps but hopefully you can find a solution to this.

Thank you!

although the problem still unresolved, I found a clue too.

looks like activation() and decodes() need to be created properly for loss_func().

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Hugging Face & Github Synchronization

I am able to push my app.py, requirement.txt and all other require files to my Hugging Face Space. But I want to know how can we push those file simultaneously to Github repo also. I am using Github Desktop.