The easier way is
vector1 = matrix1[:,0:1]
For the reason, let me refer you to another answer of mine:
When you write something like
a[4]
, that’s accessing the fifth element of the array, not giving you a view of some section of the original array. So for instance, if a is an array of numbers, thena[4]
will be just a number. Ifa
is a two-dimensional array, i.e. effectively an array of arrays, thena[4]
would be a one-dimensional array. Basically, the operation of accessing an array element returns something with a dimensionality of one less than the original array.