Writing an ANN in Excel: VBA Type Mismatch Error

Question

So I'm trying to program a basic Artificial Neural Network using Excel VBA. I've been following one example in particular:

https://www.analyticsvidhya.com/blog/2017/05/neural-network-from-scratch-in-python-and-r/

I've been basing the code off of the Python example in the article (code example is toward the bottom of the article). I don't have access to Python unfortunately so I'm trying to do this with VBA. I've done my best to convert the code into a useable format for Excel, however there is an issue I'm encountering and I'm not sure how to solve it:

Here is my VBA code:

Sub ANN()
 Dim X(1010, 1011, 101) As Integer 'Input
 Dim Y(1, 1, 0) As Double 'output for comparison
 Dim E(0, 0, 0) As Double 'Error

'Variable Initialization
 Dim Epoch As Long
 Dim LearnRate As Double
 Dim InputLayer_Neurons() As Integer 'Number of Features in data set
 ReDim InputLayer_Neurons(ArrayLen(X))
 Dim HiddenLayer_Neurons As Integer
 Dim Output_Neurons As Integer
 Dim hidden_layer_input1 As Variant
 Dim hidden_layer_input As Variant
 Dim hiddenlayer_activations As Variant
 Dim output_layer_input1 As Variant
 Dim output_layer_input As Variant
 Dim slope_output_layer As Variant
 Dim slope_hidden_layer As Variant
 Dim d_output As Variant
 Dim Output As Variant
 Dim Wh As Double 'Weight Hidden Layer
 Dim Bh As Double 'Bias Hidden Layer
 Dim Wout As Double 'Weight output Layer
 Dim Bout As Double 'Bias Ouput layer
 Dim i As Long
 Epoch = 5000 'Training Iterations
 LearnRate = 0.1 'Learning Rate
 HiddeLayer_Neurons = 3 'Number of Neurons in Hidden Layer
 Output_Neurons = 1 'Number of Neurons at output layer

'Weight & Bias Initialization
 Wh = Application.WorksheetFunction.RandBetween(InputLayer_Neurons, HiddenLayer_Neurons)
 Bh = Application.WorksheetFunction.RandBetween(1, HiddenLayer_Neurons)
 Wout = Application.WorksheetFunction.RandBetween(HiddenLayer_Neurons, Output_Neurons)
 Bout = Application.WorksheetFunction.RandBetween(1, Output_Neurons)

For i = 0 To Epoch

'Forward Propagation
 hidden_layer_input1 = WorksheetFunction.MMult(X, Wh)
 hidden_layer_input = hidden_layer_input1 + Bh
 hiddenlayer_activations = Sigmoid_Activation(hidden_layer_input)
 output_layer_input1 = WorksheetFunction.MMult(hiddenlayer_activations, Wout)
 output_layer_input = output_layer_input1 + Bout
 Output = Derivatives_Sigmoid(output_layer_input)

'Backpropagation
 E = Y - Output
 slope_output_layer = Derivatives_Sigmoid(Output)
 slope_hidden_layer = Derivatives_Sigmoid(hiddenlayer_activations)
 d_output = E * slope_output_layer
 Error_at_hidden_layer = WorksheetFunction.MMult(d_output, Transpose(Wout))
 d_hiddenlayer = Error_at_hidden_layer * slope_hidden_layer
 Wout = Wout + WorksheetFunction.MMult(Transpose(hiddenlayer_activations), d_output) * LearnRate
 Bout = Bout + WorksheetFunction.Sum(d_ouput) * LearnRate
 Wh = Wh + WorksheetFunction.MMult(Transpose(X), d_hiddenlayer) * LearnRate
 Bh = Bh + WorksheetFunction.Sum(d_hiddenlayer) * LearnRate

Next

Debug.Print Output

End Sub

Function Sigmoid_Activation(X) As Variant
 Sigmoid_Activation = 1 / (1 + Application.WorksheetFunction.Power(-X))
 End Function

Function Derivatives_Sigmoid(X) As Double
 Derivatives_Sigmoid = X * (1 - X)
 End Function

Public Function ArrayLen(arr As Variant) As Integer
 ArrayLen = UBound(arr) - LBound(arr) + 1
 End Function

my Issue is:

In the section under Backpropagation. I am getting a type mismatch error on the line:

E = Y – Output

I suppose this is because in VBA there is no built in function for subtracting a Double type value (Output) from an array (Y). Even though Output is declared as a variant, I believe the value it will hold will contain a floating point value. I'm not sure if this is the reason for the conflict. It seems you can do this in Python however, which is probably why it is used for scientific calculations.

In any case, what would be the best way to resolve this?

Answer 1

Unfortunately VBA does not support direct operations on arrays. Instead you have to loop :(

If you really want to do it with VBA you could have a look at this CodeReview post .

If you want to do it using Excel, but cannot accpet the limitations of VBA, there are a few Python tools allowing to program Excel using Python. A quick search will get you plenty, including https://www.xlwings.org/ which has been on my list of things to try for much too long.