# How does an LSTM output the correct dimensions for classes?

Take the below LSTM:

input: 5x1 matrix
hidden units: 256
output size (aka classes, 1 hot vector): 10x1 matrix


It is my understanding that an LSTM of this size will do the following:

$$w_x$$ = weight matrix at $$x$$

$$b_x$$ = bias matrix at $$x$$

activation_gate = tanh($$w_1$$ $$\cdot$$ input + $$w_2$$ $$\cdot$$ prev_output + $$b_1$$)

input_gate = sigmoid($$w_3$$ $$\cdot$$ input + $$w_4$$ $$\cdot$$ prev_output + $$b_2$$)

forget_gate = sigmoid($$w_5$$ $$\cdot$$ input + $$w_6$$ $$\cdot$$ prev_output + $$b_3$$)

output_gate = sigmoid($$w_7$$ $$\cdot$$ input + $$w_8$$ $$\cdot$$ prev_output + $$b_4$$)

The size of the output of each gate should be equal to the number of hidden units, ie, 256. The problem arrises when trying to convert to the correct final output size, of 10. If the forget gate outputs 256, then it is summed with the element wise product of the activation and input gate to find the new state, this will result in a hidden state of size 256. (Also in all my research I have not found anywhere whether this addition is actually addition, or simply appending the two matrices).

So if I have a hidden state of 256, and the output gate outputs 256, doing an element wise product of these two results in, surprise surprise, 256, not 10. If I instead ensure the output gate outputs a size of 10, this no longer works with the hidden state in an element wise product.

How is this handled? I can come up with many ways of doing it myself, but I want an identical replica of the basic LSTM unit, as I have some theories I want to test, and if it is even the slightest bit different it would make the research invalid.

• I found this Quora answer to be helpful in understanding the dimensions at each point in the LSTM cell. Dimensions of LSTM Aug 31 '19 at 5:55
• Thankyou, this is a good source. So from this it leads me to believe that the output of an LSTM must be equal to the number of hidden units multiplied by the batch size? This doesn't sound right Sep 1 '19 at 2:02