A NumPy array is a very different data structure from a list and is designed to be used in different ways. Your use of hstack is potentially very inefficient... every time you call it, all the data in the existing array is copied into a new one. (The append function will have the same issue.) If you want to build up your matrix one column at a time, you might be best off to keep it in a list until it is finished, and only then convert it into an array.

e.g.

mylist = []
for item in data:
    mylist.append(item)
mat = numpy.array(mylist)

item can be a list, an array or any iterable, as long as each item has the same number of elements.
In this particular case (data is some iterable holding the matrix columns) you can simply use

mat = numpy.array(data)

(Also note that using list as a variable name is probably not good practice since it masks the built-in type by that name, which can lead to bugs.)

EDIT:

If for some reason you really do want to create an empty array, you can just use numpy.array([]), but this is rarely useful!

Thanks.. the declaration seems to be working. :) But now i'm having trouble entering data and retrieving data from it. · mydata.append((45, 2)) maybe working (doesn't give bugs) but i don't know to check if its gone to where it should since i don't have an extraction method. while googling i got answers like · mydata = myarray.index([3]) mydata = pop.index(3) · and so on... but none of them seem to work for me.... this is what u'm doing now, but that too gives a bug saying "IndexError: list index out of range". · ST=[0.0] · ST.append((3,42.23)) · print("my test val",ST[3])

You can't do exactly what you want in Python (if I read you correctly). You need to put values in for each element of the list (or as you called it, array).

But, try this:

Use with AIa = [0 for x in range(N)]  # N = size of list you want
a[i] = 5  # as long as i < N, you're okay

For lists of other types, use something besides 0. None is often a good choice as well.