Usage of reshape (-1) in numpy or pandas

Example:

z = np.array([[1, 2, 3, 4],
          [5, 6, 7, 8],
          [9, 10, 11, 12],
          [13, 14, 15, 16]])
z.shape
(4, 4)

1.z.reshape (-1) or z.reshape (1, -1) will tile the array horizontally 

z.reshape(-1)
array([ 1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16])

2.z.reshape (-1, 1) Tile the array vertically

z.reshape(-1,1)
 array([[ 1],
        [ 2],
        [ 3],
        [ 4],
        [ 5],
        [ 6],
        [ 7],
        [ 8],
        [ 9],
        [10],
        [11],
        [12],
        [13],
        [14],
        [15],
        [16]])

3.z.reshape(-1, 2)

newshape equals -1, the number of columns equals 2, the number of rows is unknown, and the shape after reshape equals (8, 2)

z.reshape(-1, 2)
 array([[ 1,  2],
        [ 3,  4],
        [ 5,  6],
        [ 7,  8],
        [ 9, 10],
        [11, 12],
        [13, 14],
        [15, 16]])