I have trouble understanding the model I'm trying to create.
I have few questions so I'll explain my model first and what I'm trying to do:

I have created sequences of data (input and ouput of the model) 7 timesteps each so the input would be the values of the days of a certain week and the output is the values of the days the following week (**so input1 of my model has an imput shape of (7,1) same as the output**).
I also have prepared another input list that has some extra features like holiday flag and weather condition for the **following week** so the model's second input2 has the shape (7,7) each example.
This is the full model summary:

full model summary

After the lstm layers and a fully connected NN, I tried to concatenate the two inputs together, basically, I want to concatenate input2 to the output of the layer dense_1. So I'm concatenating (7,1) shape with the second input 'input_2' of shape (7,7)

My questions:

1- The outputs of my dense layers confuse me I thought they would be in the shape (None, number of units) but they seem to be (None, 7 , number of units) there is always the 2nd dimension "7" which i don't understand.

dense layers

2- For the concatenation part, since i'm adding a 7x7 input to a 7x1 input i was expecting to have 49+7=56 units connected to the next dense layer but the number of parameters tells me it's not the case being just 14*120+120=1800


3- I was thinking of having 7 units in the last layer ( 1 for each day to predict ) but had to make it 1 so it would output (7,1) and for it to work, there is something i'm clearly missing.

3rd question

This is the code for model definition:

input1 = tf.keras.layers.Input(shape=(7,1),name="input_1")
x = tf.keras.layers.Conv1D(120, kernel_size=5,strides=1,activation="relu",
                           padding="causal",input_shape=[7, 1])(input1)
x = tf.keras.layers.Bidirectional(tf.keras.layers.LSTM(210, return_sequences=True),name="LSTM_1")(x)
x = tf.keras.layers.Bidirectional(tf.keras.layers.LSTM(210, return_sequences=True,name="LSTM_2"))(x)
x = tf.keras.layers.Dense(60,activation="relu",name="dense_1_1")(x)
x = tf.keras.layers.Dense(30,activation="relu",name="dense_1_2")(x)
x = tf.keras.layers.Dense(7,name="dense_1_3")(x)

input2 = tf.keras.layers.Input(shape=(7,7),name="input_2")
concat = tf.keras.layers.concatenate([x, input2],name="concat_1")
x = tf.keras.layers.Dense(120,activation="selu",name="dense_2_1")(concat)
x = tf.keras.layers.Dense(90,activation="selu",name="dense_2_2")(x)
x = tf.keras.layers.Dense(60,activation="selu",name="dense_2_3")(x)
output = tf.keras.layers.Dense(1)(x)

model = tf.keras.Model([input1, input2], output)

These are the dimensions of the inputs (input1 & input2) and the output



1 Answer 1


From Keras docs: https://keras.io/api/layers/core_layers/dense/

Input shape

N-D tensor with shape: (batch_size, ..., input_dim). The most common situation would be a 2D input with shape (batch_size, input_dim).

Output shape

N-D tensor with shape: (batch_size, ..., units). For instance, for a 2D input with shape (batch_size, input_dim), the output would have shape (batch_size, units).

If the input to the layer has a rank greater than 2, then Dense computes the dot product between the inputs and the kernel along the last axis of the inputs and axis 1 of the kernel (using tf.tensordot). For example, if input has dimensions (batch_size, d0, d1), then we create a kernel with shape (d1, units), and the kernel operates along axis 2 of the input, on every sub-tensor of shape (1, 1, d1) (there are batch_size * d0 such sub-tensors). The output in this case will have shape (batch_size, d0, units).

The input to dense_1_1 is (None, 7, 420). The Dense layer will be applied to every [:,i,:] for in 1..7 slice of the input producing an output of size (None, 7, 60)

If you're expecting an output with shape (None, 60) you can reshape the input to (None, 420*7) but that would not make sense in this case since you would want to apply the Dense layer to each time slice of your input.

  • $\begingroup$ I understand better now but this still confuses me when you said "The Dense layer will be applied to every [:,i,:] for in 1..7 slice of the input producing an output of size (None, 7, 60)" why is the number of parameters 420*60 + 60 = 25260 I thought it should have been (420*60 +60) * 7*, it is as if all the timesteps share the same 420 * 60 weights shouldn't it be 420 * 60 weight for each timestep, I have tried to wrap the dense_1_* layers with a TimeDistributed wrapper but the number of parameters is exactly the same? $\endgroup$
    – wazzup
    Commented Oct 16, 2020 at 15:05
  • 1
    $\begingroup$ You're correct. The timesteps do share the weights. It would make sense to apply the same transformation to each timestep because each of them represent the same features but at a different point of time. I have edited the answer to include a snippet from the Keras Docs that may give some insight $\endgroup$ Commented Oct 16, 2020 at 16:54
  • $\begingroup$ Thanks, I think now I get that part. Now for the concatenation part if you have any idea, lets say I change dense_1_3 to output (None,7,1) representing the value of each timestep to predict, now I want to concatenate a (7,7) input representing 7 features of the 7 days to predict so they are not really the same timesteps ,rather the timesteps of the days to predict , do you reckon flattening the (7,1) output and concatenating a (49) vector to the flattened (7) vector is the way to go instead of concatenating the (7,7) to the (7,1) ? $\endgroup$
    – wazzup
    Commented Oct 16, 2020 at 17:35

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