Feed output of neural networks into other network in tensorflow

Question

I have two well trained neural networks, shown as:

Model: "sequential"
_________________________________________________________________
 Layer (type)                Output Shape              Param #   
=================================================================
 dense (Dense)               (None, 42)                1806      
                                                                 
 dense_1 (Dense)             (None, 128)               5504      
                                                                 
 dense_2 (Dense)             (None, 256)               33024     
                                                                 
 dense_3 (Dense)             (None, 512)               131584    
                                                                 
 dense_4 (Dense)             (None, 256)               131328    
                                                                 
 dense_5 (Dense)             (None, 128)               32896     
                                                                 
 dense_6 (Dense)             (None, 64)                8256      
                                                                 
 dense_7 (Dense)             (None, 32)                2080      
                                                                 
 dense_8 (Dense)             (None, 28)                924       
                                                                 
=================================================================
Total params: 347,402
Trainable params: 347,402
Non-trainable params: 0
_________________________________________________________________
Model: "sequential"
_________________________________________________________________
 Layer (type)                Output Shape              Param #   
=================================================================
 dense (Dense)               (None, 28)                812       
                                                                 
 dense_1 (Dense)             (None, 128)               3712      
                                                                 
 dense_2 (Dense)             (None, 256)               33024     
                                                                 
 dense_3 (Dense)             (None, 512)               131584    
                                                                 
 dense_4 (Dense)             (None, 256)               131328    
                                                                 
 dense_5 (Dense)             (None, 128)               32896     
                                                                 
 dense_6 (Dense)             (None, 64)                8256      
                                                                 
 dense_7 (Dense)             (None, 32)                2080      
                                                                 
 dense_8 (Dense)             (None, 4)                 132       
                                                                 
=================================================================
Total params: 343,824
Trainable params: 343,824
Non-trainable params: 0

And I want to combine them such that I replace the input layer of the second network with output layer of the first network. How can I do this in tensorflow?

Answer 1

The concatenate function can connect 2 neural networks. You just have to be careful with the dimensions between them.

Here is a pseudo-code:

from tensorflow.keras.layers import Concatenate, Dense, Input
from tensorflow.keras.models import Model

import numpy as np

input_a = ...
layer_a_1 = ...
layer_a_n = ...
output_a = Dense(1)(layer_a_n)
input_b = Input(shape=(28,))
concat_2 = Concatenate()([output_a, input_b])
layer_b_1 = Dense(1)(concat_2)
...
layer_b_n = ...
output_b = Dense(1)(layer_b_n)
model = Model(inputs=[input_a], outputs=[output_b])

model.compile(loss='mse', optimizer='sgd')
hist = model.fit([input_a], output_b, epochs=500)