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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]# Difference between array( [1,0,1] ) and array( [ [1,0,1] ] )

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Thanks Edmondo, Stephen, Mats and Steven you for the tips, I studied linear algebra many years ago and I remember only a few rudiments. But I was trying to visualize (in a geometric way) how the numpy represents arrays, and what the geometrical meaning of the transpose operation made by numpy. I think I understood a little bit more. The number of nested brackets indicates the number of array dimensions. the vector ( [1,2] ) is one-dimensional, but the vector ( [ [1,2] ] ) is two-dimensional. v_1 = np.array( [1,2] ) > v_1.shape (2,) > v_1 v_1 > v_1 array( [1, 2] ) > v_2 = np.array( [ [1,2] ] ) > v_2.shape (1, 2) And it does not make sense to transpose a one-dimensional array. > v_1.T array( [1, 2] ) > v_2.T array( [ [1], ???????????? [2] ] ) Anothe example: vector_1 = np.array( [?? 1,?? 2,?? 3,?? 4,?? 5,?? 6,?? 7,?? 8? ] ) ????????????????????????????????? ^ vector_2 = np.array( [??? [1, 2, 3, 4],??? [5, 6, 7, 8]? ]? ) ????????????????????????????????? ^? ^ vector_3 = np.array( [?? [?? [1,2],? [3,4]? ], [? [5,6],?? [7,8] ]? ]? ) ????????????????????????????????? ^ ^ ^ > vector_1 array([1, 2, 3, 4, 5, 6, 7, 8]) > vector_2 array( [ [1, 2, 3, 4], ???????????? [5, 6, 7, 8] ] ) > vector_3 array( [ [ [1, 2], ?????????????? [3, 4] ], ???????????? [ [5, 6], ?????????????? [7, 8] ] ] ) And looking for some tutorial about geometric aspects of matrices and the geometric meaning of the transpose I found that transposed is "mirrored along the diagonal" at: https://www.coranac.com/documents/geomatrix/ >vector_1.T array([1, 2, 3, 4, 5, 6, 7, 8]) > vector_2.T array( [ [1, 5], ???????????? [2, 6], ???????????? [3, 7], ???????????? [4, 8] ] ) > vector_3.T array( [ [ [1, 5], ?????????????? [3, 7]], ???????????? [ [2, 6], ?????????????? [4, 8] ] ] ) Thank you, Markos Em 21-06-2019 07:44, edmondo.giovannozzi at gmail.com escreveu: > Every array in numpy has a number of dimensions, > "np.array" is a function that can create an array numpy given a list. > > when you write > vector_1 = np.array([1,2,1]) > you are passing a list of number to thet function array that will create a 1D array. > As you are showing: > vector_1.shape > will return a tuple with the sizes of each dimension of the array that is: > (3,) > Note the comma thta indicate that is a tuple. > While if you write: > vector_2 = np.array([[1,2,3]]) > You are passing a list of list to the function array that will instruct it to crete a 2D array, even though the size of the first dimension is 1: > vector_2.shape > (1,3) > It is still a tuple as you can see. > Try: > vector_3 = np.array([[1,2,3],[4,5,6]]) > And you'll see that i'll return a 2D array with a shape: > vector_3.shape > (2,3) > As the external list has 2 elements that is two sublists each with 3 elements. > The vector_2 case is just when the external list has only 1 element. > > I hope it is more clear now. > Cherrs, > > > > > > > Il giorno venerd? 21 giugno 2019 08:29:36 UTC+2, Markos ha scritto: >> Hi, >> >> I'm studying Numpy and I don't understand the difference between >> >>>>> vector_1 = np.array( [ 1,0,1 ] ) >> with 1 bracket and >> >>>>> vector_2 = np.array( [ [ 1,0,1 ] ] ) >> with 2 brackets >> >> The shape of vector_1 is: >> >>>>> vector_1.shape >> (3,) >> >> But the shape of vector_2 is: >> >>>>> vector_2.shape >> (1, 3) >> >> The transpose on vector_1 don't work: >> >>>>> vector_1.T >> array([1, 0, 1]) >> >> But the transpose method in vector_2 works fine: >> >>>>> vector_2.T >> array([[1], >> ?????? [0], >> ?????? [1]]) >> >> >> I thought that both vectors would be treated as an matrix of 1 row and 3 >> columns. >> >> Why this difference? >> >> Any tip? >> >> Thank you, >> Markos

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