十三、Loss Functions

一、Loss Functions损失函数

损失函数的作用：
1，损失函数就是实际输出值和目标值之间的差
2，由这个差便可以通过反向传播对之后的数据进行更新
Loss Functions官网给的API
里面由很多种损失函数，不同的损失函数有其不同的用途及表达式

二、L1Loss损失函数

torch.nn.L1Loss(size_average=None, reduce=None, reduction=‘mean’)
由官网给的各参数使用可知，size_average和reduce参数都已经被弃用了
而reduction有三个模式，none、mean、sum

import torch
from torch.nn import L1Loss

input = torch.tensor([1,2,3],dtype=torch.float32)
target = torch.tensor([4,5,6],dtype=torch.float32)

input = torch.reshape(input,(1,1,1,3))
target = torch.reshape(target,(1,1,1,3))

loss_1 = L1Loss(reduction='sum')
result_1 = loss_1(input,target)
print(result_1)#tensor(9.)   4-1 + 5-2 + 6-3 = 9

loss_2 = L1Loss(reduction='mean')
result_2 = loss_2(input,target)
print(result_2)#tensor(3.)   (4-1 + 5-2 + 6-3) / 3 = 3

loss_3 = L1Loss(reduction='none')
result_3 = loss_3(input,target)
print(result_3)#tensor([[[[3., 3., 3.]]]])
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三、MSELoss均方误差损失函数

torch.nn.MSELoss(size_average=None, reduce=None, reduction=‘mean’)

import torch

input = torch.tensor([1,2,3],dtype=torch.float32)
target = torch.tensor([4,5,6],dtype=torch.float32)

input = torch.reshape(input,(1,1,1,3))
target = torch.reshape(target,(1,1,1,3))

loss_mse = torch.nn.MSELoss()
result = loss_mse(input,target)
print(result)#tensor(9.)   [(4-1)^2 + (5-2)^2 + (6-3)^2] / 3 = 9
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四、CrossEntropyLoss交叉熵损失函数

torch.nn.CrossEntropyLoss(weight=None, size_average=None, ignore_index=- 100, reduce=None, reduction=‘mean’, label_smoothing=0.0)

假设x为三类：狗0、猫1、猪2，对应的输出概率为(1.0,2.0,3.0)
最后的真实目标结果为猪，对应的是(2)

import torch

x = torch.tensor([1.0,2.0,3.0])
target = torch.tensor([2])

x = torch.reshape(x,(1,3))#将x变成(N,C)形式 ，1维，3类
print(x.shape)#torch.Size([1, 3])

loss_cross = torch.nn.CrossEntropyLoss()
result = loss_cross(x,target)

print(result)#tensor(0.4076)

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