C1 W2 A2 propagate

Course Q&A Deep Learning Specialization Neural Networks and Deep Learning
1

This is the error i’m getting.

ValueError Traceback (most recent call last)
in
3 X = np.array([[1., -2., -1.], [3., 0.5, -3.2]])
4 Y = np.array([[1, 1, 0]])
----> 5 grads, cost = propagate(w, b, X, Y)
6
7 assert type(grads[“dw”]) == np.ndarray

in propagate(w, b, X, Y)
32 # YOUR CODE STARTS HERE
33 A = sigmoid(np.dot(w.T,X)+b)
—> 34 cost = (-1/m) * (np.sum(np.dot(Y,(np.log(A))))+(np.dot((1-Y),(np.log(1-A)))))
35
36 # YOUR CODE ENDS HERE

<array_function internals> in dot(*args, **kwargs)

ValueError: shapes (1,3) and (1,3) not aligned: 3 (dim 1) != 1 (dim 0)

2

It is fine to use np.dot to compute the cost, but the problem is Y and A are both 1 x m vectors, so you can’t just dot them “as is”: the dimensions don’t work. You need to understand how matrix multiplication works as a prerequisite here. Here’s another recent thread about this.

1 Like
3

Yes i got it. I did not think it through. Thanks for the help.

4

Glad to hear that you solved it. For future reference, the error message was pretty clear in pointing to what the problem was.