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Neural Networks: Prediction (i.e. the forward pass) Machine Learning Based on slides and material from Geoffrey Hinton, Richard Socher, Dan Roth, 1 Yoav Goldberg, Shai Shalev-Shwartz and Shai Ben-David, and others Neural Networks What is a


  1. Neural Networks: Prediction (i.e. the forward pass) Machine Learning Based on slides and material from Geoffrey Hinton, Richard Socher, Dan Roth, 1 Yoav Goldberg, Shai Shalev-Shwartz and Shai Ben-David, and others

  2. Neural Networks • What is a neural network? • Predicting with a neural network • Training neural networks • Practical concerns 2

  3. This lecture • What is a neural network? • Predicting with a neural network • Training neural networks • Practical concerns 3

  4. Let us consider an example network output We will use this example network as to introduce the general principle of how to make predictions with a neural network. 4

  5. Let us consider an example network output Naming conventions for this example Inputs: x • Hidden: z • 𝑧 Output: y • 1 𝑨 ! 𝑨 " 1 𝑦 ! 𝑦 " 5

  6. Let us consider an example network output Naming conventions for this example Inputs: x • Hidden: z • 𝑧 Output: y • 1 𝑨 ! 𝑨 " Bias feature, always 1 1 𝑦 ! 𝑦 " 6

  7. Let us consider an example network output Naming conventions for this example Inputs: x • Hidden: z • 𝑧 Output: y • Sigmoid activations 1 𝑨 ! 𝑨 " Bias feature, always 1 1 𝑦 ! 𝑦 " 7

  8. Let us consider an example network output Naming conventions for this example Inputs: x • Linear activation Hidden: z • 𝑧 Output: y • Sigmoid activations 1 𝑨 ! 𝑨 " Bias feature, always 1 1 𝑦 ! 𝑦 " 8

  9. Let us consider an example network output Naming Convention for Weights -.)/0- - 1.20) 𝑧 𝑥 ()*+,-* #$% 𝑥 "! #$% 𝑥 &! #$% 𝑥 !! 1 𝑨 ! 𝑨 " ' 𝑥 &! ' 𝑥 "" 1 𝑦 ! 𝑦 " 9

  10. Let us consider an example network output Naming Convention for Weights -.)/0- - 1.20) 𝑧 𝑥 ()*+,-* #$% 𝑥 "! #$% 𝑥 &! #$% 𝑥 !! #$% 𝑥 &! 1 𝑨 ! 𝑨 " From neuron #0 ' 𝑥 &! to neuron #1 in ' 𝑥 "" output layer 1 𝑦 ! 𝑦 " 10

  11. How to predict: The forward pass Given an input x , how is the output predicted output 𝑧 #$% 𝑥 "! #$% 𝑥 &! #$% 𝑥 !! 1 𝑨 ! 𝑨 " ' 𝑥 &! ' 𝑥 "" 1 𝑦 ! 𝑦 " 11

  12. The forward pass Given an input x , how is the output predicted output 𝑧 #$% 𝑥 "! #$% 𝑥 &! # + 𝑥 !! # 𝑦 ! + 𝑥 $! # 𝑦 $ ) #$% 𝑥 !! z ! = 𝜏(𝑥 "! 1 𝑨 ! 𝑨 " ' 𝑥 &! ' 𝑥 "" 1 𝑦 ! 𝑦 " 12

  13. The forward pass Given an input x , how is the output predicted output 𝑧 # + 𝑥 !$ # 𝑦 ! + 𝑥 $$ # 𝑦 $ ) 𝑨 $ = 𝜏(𝑥 "$ #$% 𝑥 "! #$% 𝑥 &! # + 𝑥 !! # 𝑦 ! + 𝑥 $! # 𝑦 $ ) #$% 𝑥 !! z ! = 𝜏(𝑥 "! 1 𝑨 ! 𝑨 " ' 𝑥 &! ' 𝑥 "" 1 𝑦 ! 𝑦 " 13

  14. The forward pass Given an input x , how is the output predicted output % + 𝑥 !! % 𝑨 ! + 𝑥 $! % 𝑨 $ output y = 𝑥 "! 𝑧 # + 𝑥 !$ # 𝑦 ! + 𝑥 $$ # 𝑦 $ ) 𝑨 $ = 𝜏(𝑥 "$ #$% 𝑥 "! #$% 𝑥 &! # + 𝑥 !! # 𝑦 ! + 𝑥 $! # 𝑦 $ ) #$% 𝑥 !! z ! = 𝜏(𝑥 "! 1 𝑨 ! 𝑨 " ' 𝑥 &! ' 𝑥 "" 1 𝑦 ! 𝑦 " 14

  15. The forward pass Given an input x , how is the output predicted output % + 𝑥 !! % 𝑨 ! + 𝑥 $! % 𝑨 $ output y = 𝑥 "! 𝑧 # + 𝑥 !$ # 𝑦 ! + 𝑥 $$ # 𝑦 $ ) 𝑨 $ = 𝜏(𝑥 "$ #$% 𝑥 &! # + 𝑥 !! # 𝑦 ! + 𝑥 $! # 𝑦 $ ) #$% 𝑥 !! z ! = 𝜏(𝑥 "! 1 𝑨 ! 𝑨 " ' 𝑥 &! ' 𝑥 "" 1 𝑦 ! 𝑦 " 15

  16. The forward pass output Given an input x , how is the output predicted % + 𝑥 !! % 𝑨 ! + 𝑥 $! % 𝑨 $ output y = 𝑥 "! 𝑧 # + 𝑥 !$ # 𝑦 ! + 𝑥 $$ # 𝑦 $ ) 𝑨 $ = 𝜏(𝑥 "$ #$% 𝑥 "! #$% 𝑥 &! # + 𝑥 !! # 𝑦 ! + 𝑥 $! # 𝑦 $ ) #$% 𝑥 !! z ! = 𝜏(𝑥 "! 1 𝑨 ! 𝑨 " In general, before visiting (i.e. computing) the value of a node, visit all nodes that serve as ' 𝑥 &! inputs to it. ' 𝑥 "" 1 𝑦 ! 𝑦 " 16

  17. The forward pass output Given an input x , how is the output predicted % + 𝑥 !! % 𝑨 ! + 𝑥 $! % 𝑨 $ output y = 𝑥 "! 𝑧 # + 𝑥 !$ # 𝑦 ! + 𝑥 $$ # 𝑦 $ ) 𝑨 $ = 𝜏(𝑥 "$ #$% 𝑥 "! #$% 𝑥 &! # + 𝑥 !! # 𝑦 ! + 𝑥 $! # 𝑦 $ ) #$% 𝑥 !! z ! = 𝜏(𝑥 "! 1 𝑨 ! 𝑨 " In general, before visiting (i.e. computing) the value of a node, visit all nodes that serve as inputs to it. ' 𝑥 &! ' 𝑥 "" Questions? 1 𝑦 ! 𝑦 " 17

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