Convert Matlab to Python

Question

I'm converting matlab code to python, and I'm having a huge doubt on the following line of code:

BD_teste = [BD_teste; grupos.(['g',int2str(l)]).('elementos')(ind_element,:),l];

the whole code is this:

BD_teste = [];
por_treino = 0;
for l = 1:k
    quant_elementos_t = int64((length(grupos.(['g',int2str(l)]).('elementos')) * por_treino)/100);
    for element_c = 1 : quant_elementos_t
        ind_element = randi([1 length(grupos.(['g',int2str(l)]).('elementos'))]);
        BD_teste = [BD_teste; grupos.(['g',int2str(l)]).('elementos')(ind_element,:),l];
        grupos.(['g',int2str(l)]).('elementos')(ind_element,:) = [];
    end
end

This line of code below is a structure, as I am converting to python, I used a list and inside it, a dictionary with its list 'elementos':

'g',int2str(l)]).('elementos')

So my question is just in the line I quoted above, I was wondering what is happening and how it is occurring, and how I would write in python.

Thank you very much in advance.

I'm writing an answer, but as general advice, avoid naming a variable l it is easy to confuse it with 1. — Mefitico
– Mefitico, Commented Oct 18, 2019 at 20:32

Mefitico · Accepted Answer · 2019-10-21 11:54:12Z

BD_teste = [BD_teste; grupos.(['g',int2str(l)]).('elementos')(ind_element,:),l];

Is one very weird line. Let's break it down into pieces:

int2str(l) returns the number l as a char array (will span from '1' until k).

['g',int2str(l)] returns the char array g1, then g2 and so on along with the value of l.

grupos.(['g',int2str(l)]) will return the value of the field named g1, g2 and so on that belongs to the struct grupos.

grupos.(['g',int2str(l)]).('elementos') Now assumes that grupos.(['g',int2str(l)]) is itself a struct, and returns the value of its field named 'elementos'.

grupos.(['g',int2str(l)]).('elementos')(ind_element,:) Assuming that grupos.(['g',int2str(l)]) is a matrix, this line returns a line-vector containing the ind_element-th line of said matrix.

grupos.(['g',int2str(l)]).('elementos')(ind_element,:),l appends the number one to the vector obtained before.

[BD_teste; grupos.(['g',int2str(l)]).('elementos')(ind_element,:),l] appends the line vector [grupos.(['g',int2str(l)]).('elementos')(ind_element,:),l] to the matrix BD_teste, at its bottom. and creates a new matrix.

Finally: BD_teste = [BD_teste; grupos.(['g',int2str(l)]).('elementos')(ind_element,:),l];``assignes the value of the obtained matrix to the variableBD_teste`, overwriting its previous value. Effectively, this just appends the new line, but because of the overwriting step, it is not very effective.

It would be recommendable to append with: BD_teste(end+1,:) = [grupos.(['g',int2str(l)]).('elementos')(ind_element,:),l];

Now, how you will rewrite this in Python is a whole different story, and will depend on how you want to define the variable grupos mostly.

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