how to append row vectors to an empty matrix without knowing the size of the matrix using numpy?

rows is a 343x30 matrix of real numbers. Im trying to append row vectors from rows to true rows and false rows but it only adds the first row and doesnt do anything afterwards. Ive tried vstack and also tried putting example as a 2d array ([example]) but it crashed my pycharm. what can I do?

true_rows = []
        true_labels = []
        false_rows = []
        false_labels = []
        i = 0
        for example in rows:
            if question.match(example):
                true_rows = np.append(true_rows , example , axis=0)
                true_labels.append(labels[i])
            else:
                #false_rows = np.vstack(false_rows, example_t)
                false_rows = np.append(false_rows, example, axis=0)
                false_labels.append(labels[i])
            i += 1

Solution 1:

you can use only a simple list to append your rows and then transform this list to numpy array such as :

exemple1 = np.array([1,2,3,4,5])
exemple2 = np.array([6,7,8,9,10])
exemple3 = np.array([11,12,13,14,15])  

true_rows = []
true_rows.append(exemple1)
true_rows.append(exemple2)
true_rows.append(exemple3)
true_rows = np.array(true_rows)

you will get this results:

true_rows = array([[ 1,  2,  3,  4,  5],
                   [ 6,  7,  8,  9, 10],
                   [11, 12, 13, 14, 15]])

you can also use np.concatenate if you want to get one dimensional array like this:

true_rows = np.concatenate(true_rows , axis =0)

you will get this results:

true_rows = array([ 1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15])