Gradient Checking doubt - course 2

In the video about gradient checking is said that we “concatenate all the parameters b[l] W[l] y one vector theta” and then, theta(i) is used.
I’m not sure if theta(i) stands for a whole parameter (vector o matrix) W (or b), or instead it stands form uno of the components of this parameters.

In other words, if you have
W=[1, 2, 3
4, 5, 6]
b = [7, 8]
theta(1) would be [[1, 2, 3], [4, 5, 6]] or 1?

Thanks in advance!

You can examine the source code for the routines dictionary_to_vector and vector_to_dictionary to see the details of how the conversions are done (see the FAQ Thread for how to find and open the relevant file). But it is creating theta as a simple vector with all the elements of the various W and b values concatenated. That means that theta[i] is one element from one of the various W or b values.