【Pytorch学习笔记】2.Pytoch基础

Dear 丶 2022-09-04 05:54 350阅读 0赞

### 文章目录 ###

*  4.基本数据类型
 *   *  4.1 All is about Tensor
     *  4.2 How to denote string
     *  4.3 Data type
     *  4.4 Type check
     *  4.5 Dimension / rank
     *  4.6 Mixed
 *  5.创建Tensor
 *   *  5.1 Import from numpy
     *  5.2 Import from List
     *  5.3 uninitialized
     *  5.4 set default type
     *  5.5 rand / rand\_like, randint, randn
     *  5.6 full
     *  5.7 arrange / range
     *  5.8 linspace / logspace
     *  5.9 Ones / zeros / eye
     *  5.10 randperm
 *  6.索引与切片
 *   *  6.1 ...
     *  6.2 select by mask
     *  6.3 select by flatten index
 *  7.维度变换
 *   *  7.1 View / reshape
     *  7.2 Sequeeze / unsqueeze
     *  7.3 Expand / repeat
     *  7.4 Transpose / t / permute
 *  8. Broadcasting
 *  9.拼接与拆分
 *   *  9.1 Cat
     *  9.2 Stack
     *  9.3 split: by len
     *  9.4 Chunk: by num
 *  10.基本运算
 *  11.数据统计
 *  12.高级OP
 *   *  12.1 Where
     *  12.2 Gather

根据龙良曲Pytorch学习视频整理，视频链接：  
[【计算机-AI】PyTorch学这个就够了！][-AI_PyTorch]  
[(好课推荐)深度学习与PyTorch入门实战——主讲人龙良曲][PyTorch]

# 4.基本数据类型 #

## 4.1 All is about Tensor ##

<table> 
 <thead> 
  <tr> 
   <th>python</th> 
   <th>PyTorch</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>Int</td> 
   <td>IntTensor of size()</td> 
  </tr> 
  <tr> 
   <td>float</td> 
   <td>FloatTensor of size()</td> 
  </tr> 
  <tr> 
   <td>Int array</td> 
   <td>IntTensor of size[d1,d2,…]</td> 
  </tr> 
  <tr> 
   <td>Float array</td> 
   <td>FloatTensor of size[d1,d2…]</td> 
  </tr> 
  <tr> 
   <td>string</td> 
   <td>–</td> 
  </tr> 
 </tbody> 
</table>

## 4.2 How to denote string ##

*  One-hot：\[0, 1, 0, 0,…\]
 *  Embedding：Word2vec，glove

## 4.3 Data type ##

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2tlaXZlbl8_size_16_color_FFFFFF_t_70]

## 4.4 Type check ##

a = torch.randn(2, 3)
    print(a)    # tensor([[ 0.3765, 0.6532, -0.8246],
                # [ 1.3567, -0.3963, 0.1719]])
    print(a.type())     # torch.FloatTensor
    print(type(a))      # <class 'torch.Tensor'>
    print(isinstance(a, torch.FloatTensor)) # True
    
    # 数据位置对数据类型的影响
    print(isinstance(a, torch.cuda.FloatTensor))    # False
    a = a.cuda()
    print(isinstance(a, torch.cuda.FloatTensor))    # True

## 4.5 Dimension / rank ##

*  Dimension 0 ：Loss
 *  Dimension 1 ：Bias和Linear Input
 *  Dimension 2 ：Linear Input batch
 *  Dimension 3 ：RNN Input Batch
 *  Dimension 4 ：CNN\[b, c, h, w\]

## 4.6 Mixed ##

a = torch.rand(2, 3, 4, 5)
    print(a.size())     # torch.Size([2, 3, 4, 5])
    print(a.shape)      # torch.Size([2, 3, 4, 5])
    print(len(a.shape)) # 4
    print(a.dim())      # 维度：4
    print(a.numel())    # 具体的数据大小：120

# 5.创建Tensor #

## 5.1 Import from numpy ##

a = np.ones(2)
    print(a)            # [1. 1.]
    print(torch.from_numpy(a))  # tensor([1., 1.], dtype=torch.float64)

## 5.2 Import from List ##

a = torch.tensor([2, 3.2])
    print(a)    # tensor([2.0000, 3.2000])

`torch.Tensor()`接受数据的类型  
`torch.tensor()`接收现成的数据

## 5.3 uninitialized ##

初始化的数据可能会非常大或者非常小，不建议使用

*  torch.empty()
 *  torch.FloatTensor(d1, d2, d3)
 *  torch.IntTensor(d1, d2, d3)

## 5.4 set default type ##

print(torch.tensor([1.2, 3]).type())    # torch.FloatTensor
    torch.set_default_tensor_type(torch.DoubleTensor)
    print(torch.tensor([1.2, 3]).type())    # torch.DoubleTensor

## 5.5 rand / rand\_like, randint, randn ##

推荐使用这个随机初始化

a = torch.rand(2, 1)
    print(a)    # tensor([[0.1746],
                # [0.6831]])
    print(torch.rand_like(a))   # tensor([[0.5452],
                                # [0.4782]])
    print(torch.randint(1, 5, [2, 1]))  # tensor([[3],
    # 限定了最小值和最大值，左闭右开 # [4]])

`torch.randn()` 输出随机数服从正态分布(0, 1)  
`torch.normal(mean=torch.full([10], 0), std=torch.arrange(1, 0, -0.1))` 自定义正态分布均值和标准差

## 5.6 full ##

print(torch.full([], 7))    # 标量：tensor(7)
    print(torch.full([1], 7))   # 张量：tensor([7])

## 5.7 arrange / range ##

print(torch.arange(0, 10, 2))   # tensor([0, 2, 4, 6, 8])
    print(torch.range(0, 10, 2))    # 不建议使用，tensor([ 0., 2., 4., 6., 8., 10.])

## 5.8 linspace / logspace ##

print(torch.linspace(0, 10, steps=3))    # 等差均分为steps份：tensor([ 0., 5., 10.])
    print(torch.logspace(0, -1, steps=5))    # 对数均分为steps份：tensor([1.0000, 0.5623, 0.3162, 0.1778, 0.1000])

## 5.9 Ones / zeros / eye ##

print(torch.ones(1, 2))     # 全1矩阵：tensor([[1., 1.]])
    print(torch.zeros(1, 2))    # 全0矩阵：tensor([[0., 0.]])
    print(torch.eye(2))         # 单位矩阵：tensor([[1., 0.],
                                # [0., 1.]])

## 5.10 randperm ##

a = torch.rand(4, 3)
    print(a)
    idx = torch.randperm(3)     # 随机生成索引
    print(idx)
    print(a[[1, 0, 2]])
    """ tensor([[0.1708, 0.2821, 0.8163], [0.8898, 0.6628, 0.2350], [0.3743, 0.4281, 0.5309], [0.4996, 0.7259, 0.5485]]) tensor([1, 0, 2]) tensor([[0.8898, 0.6628, 0.2350], [0.1708, 0.2821, 0.8163], [0.3743, 0.4281, 0.5309]]) """

# 6.索引与切片 #

索引和切片的方式同python `list[start:end:steps]`

*  Indexing
 *  select first / last N
 *  select by steps
 *  select by specific index

## 6.1 … ##

连续全部取值

a = torch.rand(4, 3, 28, 28)
    print(a[0, 1].shape)    # torch.Size([28, 28])
    print(a[0, 0, 2, 4])    # tensor(0.9485)
    print(a.index_select(2, torch.arange(8)).shape)         # torch.Size([4, 3, 8, 28])
    print(a[:,:1,...].shape)       #torch.Size([4, 1, 28, 28])

## 6.2 select by mask ##

x = torch.randn(3, 4)
    print(x)
    mask = x.ge(0.5)	# 元素大于0.5的
    print(mask)
    print(torch.masked_select(x, mask))
    """ tensor([[-1.8692, 0.9687, -0.4978, 0.7836], [-2.5662, 0.0487, 0.3978, -0.3676], [-1.5896, -0.1129, -1.9687, 0.5585]]) tensor([[False, True, False, True], [False, False, False, False], [False, False, False, True]]) tensor([0.9687, 0.7836, 0.5585]) """

## 6.3 select by flatten index ##

reshape为一维后取索引

src = torch.tensor([[4, 3, 5],
    					[6, 7, 8]])
    print(torch.take(src, torch.tensor([0, 2, 5])))       # tensor([4, 5, 8])

# 7.维度变换 #

## 7.1 View / reshape ##

view的size前后必须一致，view的size要有实际意义，避免产生数据污染

a = torch.rand(4, 1, 28, 28)
    print(a.view(4, 28*28).shape)   # torch.Size([4, 784])
    print(a.unsqueeze(4).shape)     # torch.Size([4, 1, 28, 28, 1])

## 7.2 Sequeeze / unsqueeze ##

unsqueeze插入一个维度，不会改变数据本身  
squeeze若给定的挤压维度为1则被挤压掉，若不为1则保持原来数据维度不变

b = torch.tensor([1, 2])
    print(b.unsqueeze(-1))  # tensor([[1],
                            # [2]])
    print(b.unsqueeze(0))   # tensor([[1, 2]])
    
    c = torch.rand(1, 31, 1, 1)
    print(c.shape)                  # torch.Size([1, 31, 1, 1])
    print(c.squeeze().shape)        # torch.Size([31])
    print(c.squeeze(0).shape)       # torch.Size([31, 1, 1])
    print(c.squeeze(1).shape)       # torch.Size([1, 31, 1, 1])

## 7.3 Expand / repeat ##

*  Expand(broadcasting)
 *  repeat(memory copied)

a = torch.rand(1,32,1,1)
    print(a.shape)                          # torch.Size([1, 32, 1, 1])
    print(a.expand(4,32,14,14).shape)       # torch.Size([4, 32, 14, 14])
    print(a.repeat(4,32,1,1).shape)         # torch.Size([4, 1024, 1, 1])

## 7.4 Transpose / t / permute ##

t()只能转置二维矩阵，否则报错

a = torch.rand(2,3)
    print(a.t().shape)      # torch.Size([3, 2])
    
    b = torch.rand(4,3,28,28)
    print(b.transpose(1,3).transpose(1,2).shape)    # torch.Size([4, 28, 28, 3])
    print(b.permute(0,2,3,1).shape)                 # torch.Size([4, 28, 28, 3])

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2tlaXZlbl8_size_16_color_FFFFFF_t_70 1]

# 8. Broadcasting #

*  Expand
 *  without copying data

key idea

*  insert 1 dim ahead
 *  Expand dims with size 1 to same size  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2tlaXZlbl8_size_16_color_FFFFFF_t_70 2]  
    Why broadcasting
 *  for actual demanding
 *  memory consumption

Is it broadcating-able?

*  Match from **Last** dim  
    if current dim=1, expand to same  
    if either has no dim, insert one dim and expand to same  
    otherwise, NOT broadcasting-able

# 9.拼接与拆分 #

## 9.1 Cat ##

只能在维度不同的dim上进行cat

a = torch.rand(4, 32, 8)
    b = torch.rand(5, 32, 8)
    print(torch.cat([a, b], dim=0).shape)	# torch.Size([9, 32, 8])

## 9.2 Stack ##

会创建新的维度，两个维度必须一致

a = torch.rand(32, 8)
    b = torch.rand(32, 8)
    print(torch.stack([a, b], dim=0).shape)	# torch.Size([2, 32, 8])

## 9.3 split: by len ##

a = torch.rand(4, 32, 8)
    aa, bb = a.split(2, dim=0)
    print(aa.shape, bb.shape)	# torch.Size([2, 32, 8]) torch.Size([2, 32, 8])
    cc, dd = a.split([3, 1], dim=0)
    print(cc.shape, dd.shape)	# torch.Size([3, 32, 8]) torch.Size([1, 32, 8])

## 9.4 Chunk: by num ##

a = torch.rand(4, 32, 8)
    aa, bb = a.chunk(2, dim=0)
    print(aa.shape, bb.shape)	#torch.Size([2, 32, 8]) torch.Size([2, 32, 8])

# 10.基本运算 #

*  Add / minus / multiply / divide  
    `torch.add()` `torch.sub()` `torch.mul()` `torch.div()`
 *  Matmul矩阵乘法  
    `torh.mm()` only for 2d  
    `torch.matmul()`  
    `@`
 *  Pow  
    `tensor.pow(n)` = `tensor**(n)`
 *  Sqrt / rsqrt  
    `sqrt()`开平方  
    `rsqrt()`开平方的倒数
 *  Exp log  
    `torch.exp()`  
    `torch.log()`
 *  Approximation  
    `tensor.floor()` `tensor.ceil()` `tensor.round()`  
    `tensor.trunc()`整数部分 `tensor.frac()`小数部分
 *  clamp  
    gradient clipping

import torch
    import numpy as np
    
    grad = torch.rand(2, 3) * 15
    print(grad)
    print(grad.min())
    print(grad.median())
    print(grad.max())
    print(grad.clamp(10))
    print(grad.clamp(0, 10))
    """ tensor([[11.9217, 3.4733, 1.3133], [ 7.1854, 13.8410, 13.8098]]) tensor(1.3133) tensor(7.1854) tensor(13.8410) tensor([[11.9217, 10.0000, 10.0000], [10.0000, 13.8410, 13.8098]]) tensor([[10.0000, 3.4733, 1.3133], [ 7.1854, 10.0000, 10.0000]]) """

# 11.数据统计 #

*  norm-p [范数（简单的理解）、范数的用途、什么是范数1][1]

import torch
    import numpy as np
    
    a = torch.full([8], 1.0)
    print(a)
    b = a.view(2, 4)
    print(b)
    c = a.view(2, 2, 2)
    print(c)
    
    print(a.norm(1), b.norm(1), c.norm(1))
    print(a.norm(2), b.norm(2), c.norm(2))
    
    print(b.norm(1, dim=1))
    print(b.norm(2, dim=1))
    
    print(c.norm(1, dim=0))
    print(c.norm(2, dim=0))
    """ tensor([1., 1., 1., 1., 1., 1., 1., 1.]) tensor([[1., 1., 1., 1.], [1., 1., 1., 1.]]) tensor([[[1., 1.], [1., 1.]], [[1., 1.], [1., 1.]]]) tensor(8.) tensor(8.) tensor(8.) tensor(2.8284) tensor(2.8284) tensor(2.8284) tensor([4., 4.]) tensor([2., 2.]) tensor([[2., 2.], [2., 2.]]) tensor([[1.4142, 1.4142], [1.4142, 1.4142]]) """

*  `tensor.mean()`均值
 *  `tensor.prod()`累乘
 *  `tensor.sum()` 累加
 *  `tensor.argmin()`, `tensor.argmax()` 最大、小值索引
 *  `dim=n, keepdim=True` 设置维度，保持维度不变
 *  kthvalue, topk  
    `tensor.topk(n, dim=, largest=False)` 某一维度上最小的前n个  
    `tensor.kthvalue(n, dim=)` 某一维度上最小的第n个
 *  compare

a = torch.rand(2, 3)
    print(a)
    print(torch.gt(a, 0.5))		# 大于0.5
    print(torch.eq(a, a))
    print(torch.equal(a, a))
    """ tensor([[0.5229, 0.7868, 0.0872], [0.3480, 0.4691, 0.2402]]) tensor([[ True, True, False], [False, False, False]]) tensor([[True, True, True], [True, True, True]]) True """

# 12.高级OP #

## 12.1 Where ##

`torch.where(condition, x, y)`

*  Return a tensor of elements selected from either x or y, depending on condition.
 *  where相较于python的if else优点是可以部署在GPU上，实现高度并行

## 12.2 Gather ##

`torch.gather(input, dim, index, out=None)`

*  Gathers values along an axis specified by dim.
 *  retrieve global label  
    argmax(pred) to get relative labeling  
    On some condtion, our label is dinstinct from relative labeling

import torch
    import numpy as np
    
    prob = torch.randn(4, 10)
    idx = prob.topk(dim=1, k=3)
    print(idx)
    """ torch.return_types.topk( values=tensor([[1.3319, 0.8338, 0.4407], [2.0854, 0.9303, 0.2608], [1.7225, 1.0136, 0.5523], [1.3715, 1.3115, 1.0049]]), indices=tensor([[2, 9, 3], [1, 3, 8], [3, 5, 0], [3, 7, 1]])) """
    
    label = torch.arange(10) + 100
    print(label)
    """ tensor([100, 101, 102, 103, 104, 105, 106, 107, 108, 109]) """
    
    print(torch.gather(label.expand(4, 10), dim=1, index=idx[1].long()))
    """ tensor([[102, 109, 103], [101, 103, 108], [103, 105, 100], [103, 107, 101]]) """

[-AI_PyTorch]: https://www.bilibili.com/video/BV1Rv411y7oE?p=1
[PyTorch]: https://www.bilibili.com/video/BV1mh41167e3?p=1
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[1]: https://blog.csdn.net/a6333230/article/details/87860875