Given network architecture, what are the possible ways to define fully connected layer fc1 to have a generalized structure such as nn.Linear($size_of_previous_layer$, 50)?

The main issue arising is due to x = F.relu(self.fc1(x)) in the forward function. After using the flatten, I need to incorporate numerous dense layers. But to my understanding, self.fc1 must be initialized and hence, needs a size (to be calculated from previous layers). How can I declare the self.fc1 layer in a generalized manner?

My Thought: To get the size, I can calculate the size of the outputs from each of Convolution layer, and since I have just 3, it is feasible. But, in case of n layers, how can you get the output size from the final convolutional layer?

class Net(nn.Module):
    def __init__(self):
        super(Net, self).__init__()

        self.conv1 = nn.Conv2d(3, 10, kernel_size=3, padding = 1)
        self.conv2 = nn.Conv2d(10, 20, kernel_size=3, padding = 1)
        self.conv2_drop = nn.Dropout2d(0.4)
        self.conv3 = nn.Conv2d(20, 40, kernel_size=3, padding = 1)
        self.conv3_drop = nn.Dropout2d(0.4)

        self.fc1 = nn.Linear(360, 50)  # self.fc1 = nn.Linear($size_of_previous_layer$, 50)

    def forward(self, x):
        x = F.relu(F.max_pool2d(self.conv1(x), 2))
        x = F.relu(F.max_pool2d(self.conv2_drop(self.conv2(x)), 2))
        x = F.relu(F.max_pool2d(self.conv3_drop(self.conv3(x)), 2))

        x = x.flatten(1)

        x = F.relu(self.fc1(x))
        return F.log_softmax(x)

Input to the following architecture can assumed to be [3, 32, 32] (num_of_channels, height, width).


  • Answers are expected in PyTorch.
  • For single convolutional layer, it is quite easy. The question refers to solve for n consecutive convolutional layers.
  • 1
    $\begingroup$ General programming questions are off-topic here. Please, read our on-topic page: ai.stackexchange.com/help/on-topic. This seems to be a programming issue, so I will close it as off-topic. In any case, you already found an answer to your question, so this should not be a big problem. $\endgroup$ – nbro Jul 1 at 2:08

The question was also asked in Pytorch Forums, where it has been sort of resolved and would love to have further discussions over there.

Link to Pytorch thread: https://discuss.pytorch.org/t/how-is-it-possible-to-get-the-output-size-of-n-consecutive-convolutional-layers/87300

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