Numpy : Add a 1d array (row) to an empty 2d array in Numpy Python

Question

I don't know the number of rows and columns of a 2d array (a) I need in advance:

a = np.empty( (0,0), dtype=np.uint8 )
a.ndim
2

I managed to convert each line I read from a file to an 1d array of bytes called line

I want to add/append each line to a, so I tried :

np.vstack( (a,line) )

but I get : ValueError: all the input arrays must have same number of dimensions

And if I call a=np.append(a,line), the number of dimensions becomes 1

Can you help ?

Have you tried gathering all the 1d arrays into a list and calling np.array on it? — Anis
– Anis, Commented Jul 3, 2017 at 15:09
@Anis What do you mean by gathering and how do I do that ? — SebMa
– SebMa, Commented Jul 3, 2017 at 15:11
Where does line come from? It may be possible to simplify your solution. — cs95
– cs95, Commented Jul 3, 2017 at 15:13
@cᴏʟᴅsᴘᴇᴇᴅ line is an array of uint8, example : [201, 81, 237, 230] — SebMa
– SebMa, Commented Jul 3, 2017 at 15:15
And where do you get it from? Is it part of a bigger list? Or do you call it inside some function? — cs95
– cs95, Commented Jul 3, 2017 at 15:16

cs95 · Accepted Answer · 2017-07-03 15:20:14Z

2

You're on the right track with np.vstack. The only requirement is that the arrays being stacked must have the same number of columns. Take a look at this:

array = None # initialise array to None first

for ...: # now inside the loop
    line = ... # read line from somewhere and get something like [201, 81, 237, 230]

    if array is None:
        array = line
    else: 
        array = np.vstack((array, line))

In terms of performance, this is actually a little more wasteful than just creating an array at the end out of a list. This is the problem with numpy arrays - they're immutable in terms of dimensions.

answered Jul 3, 2017 at 15:20

cs95

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3 Comments

SebMa Over a year ago

@COLDSPEED You are absolutely right, I tried both solutions and the one from Tom Wyllie is both faster and simpler.

cs95 Over a year ago

@SebMa Cheers. :)

SebMa Over a year ago

@COLDSPEED Bye :-)

Tom Wyllie · Accepted Answer · 2017-07-03 15:10:02Z

1

Numpy arrays cannot be appended to. The easiest way for you to do what you are trying to achieve would be to use a list and call np.asarray() on it.

a = []
# Build list from file
a = np.asarray(a)

answered Jul 3, 2017 at 15:10

Tom Wyllie

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