# chapter 3 - It Starts with a Tensor

Oct 20, 2023

- "floating point numbers are the way a network deals with information"
- "In the context of deep learning, tensors refer to the generalization of vectors and matrices to an arbitrary number of dimensions,"
- unlike numpy arrays, torch tensors have the ability to run on GPUs, distribute operations to multiple computers, and track the graph of computations that created them

- notes on using tensors in this ipython notebook
- squeezing and unsqueezing
- broadcasting
- named tensors
- which will not be used in the book, due to their experimental nature

- Tensor element types
- the tensor documentation lists the availble types
`float32`

default`float16`

can be useful, as it is a default data type on modern GPUs and often the extra precision of 32 bits does not buy you useful training results

- tensors used as indexes into other tensors are expected to be
`int64`

- predicates on tensors produce
`bool`

result tensors - notebook with examples

## Types of operations

- The types of operations
- The torch docs are thorough, but here's an overview:
- Creation operations
`ones`

,`zeros`

,`rand`

,`from_numpy`

,`range`

,`linspace`

,`full`

(fill with a given value)

- Pointwise operations
- return a new tensor by applying a function to each element independently
`abs`

,`cos`

,`add`

, bitwise ops,`pow`

- Reduction operations
- aggregate values by iterating through tensors
`mean`

,`std`

,`norm`

,`all`

,`quantile`

- Comparison operations
- functions for evaluating numerical predicates over tensors
`equal`

,`max`

- Spectral operations
- functions for transofrming in and operating in the frequency domain
- All the stuff from DSP I don't understand!
`hamming_window`

,`stft`

(short-time fourier transform)

- Other operations
`clone`

,`cross`

,`flatten`

,`histogram`

,`renorm`

,`trace`

(sum of elements of diagonal)

- BLAS and LAPACK operations
- fast matrix math

## Storage

- values in tensors are allocaetd in contiguous chunks of memory managed by
`torch.Storage`

- storage is 1d, and a tensor is a view of storage capable of idnexing into it with an offset and strides
- as such, multiple tensors can index the same storage

- notebook with more info and some storage examples
- tensors are defined by three metadata
**size**is a tuple indicating how many elements across each dimension the tensor represents**offset**is the index in the storage corresponding to the first element in the tensor**stride**is the number of elements in the storage that need to be skipped over to obtain the next element along each dimension

- in a 2d tensor, accessing element
`(i,j)`

is:`offset + (stride[0]*i) + (stride[1]*j)`

- operations that don't reallocate
`storage`

are cheap- transposition or extracting a subtensor, for example

- You can use
`clone()`

to allocate a tensor with new storage - There is a shorthand for
`transpose`

-`t`

- but it doesn't work with specifying dimensions - it expects a 2d tensor

- You can transpose in multiple dimensions by specifying which dimensions should be transposed
- A tensor whose values are laid out in storage from the rightmost dimension onwards is called
`contiguous`

- What is the "rightmost dimension"? I don't understand this at all
- contiguous tensors have improved efficiency because we can iterate them efficiently
- improved cache locality

- Some functions only work on contiguous tensors
`is_contiguous`

will tell us if our tensor is or is not- a transpose of a contiguous tensor will not be contiguous
- we can use the
`contiguous`

method to obtain a contiguous tensor from a non-contiguous one- It will reallocate

- The final example in the section helps me understand a bit better what contiguous means, so I'll copy it in here from the notebook referenced above

In [23]: p = torch.tensor([[4,1],[5,3],[2,1.0]])
...: p_t = p.t()
...: p_t, p_t.storage(), p_t.stride(), p_t.is_contiguous()
Out[23]: (tensor([[4., 5., 2.],
[1., 3., 1.]]),
4.0
1.0
5.0
3.0
2.0
1.0
[torch.storage.TypedStorage(dtype=torch.float32, device=cpu) of size 6],
(1, 2),
False)
In [24]: p_t_c = p_t.contiguous()
...: p_t_c, p_t_c.storage(), p_t_c.stride()
Out[24]: (tensor([[4., 5., 2.],
[1., 3., 1.]]),
4.0
5.0
2.0
1.0
3.0
1.0
[torch.storage.TypedStorage(dtype=torch.float32, device=cpu) of size 6],
(3, 1))

## Moving tensors to the GPU

- You can specify the
`device`

as a keyword argument to the tensor - devices
`cuda`

is the device for nvidia GPUs`MPS`

is the device for M1 macs (Metal Pixel Shader)- this issue tracks coverage for torch operaions on the MPS device
- the gaps are still significant

`cpu`

is the device for the CPU

- To copy a CPU tensor to the GPU, use
`to`

with the`device`

kwarg - check out the notebook

## Numpy compatibility

- call
`numpy()`

on a tensor to get a numpy array out- will share storage with the tensor

- call
`torch.from_numpy(array)`

to get a tensor out- will also share storage
- note that numpy defaults to float64, while float32 or float16 are best for us, so you might want to change the dtype

## serializing tensors

- calling
`torch.save(tensor, filename)`

will save the tensor to a pickle at`filename`

- can also pass a file handle as the second arg

`torch.load(filename | file_handle)`

will return the tensor- Both of those methods save it in a pickle format, which is not great for interop
- we can use
`h5py`

to save tensors as HDF5 - book doesn't mention parquet, but torch seems to have libraries for it