add dimension to an xarray DataArray

Question

I need to add a dimension to a DataArray, filling the values across the new dimension. Here's the original array.

a_size = 10
a_coords = np.linspace(0, 1, a_size)

b_size = 5
b_coords = np.linspace(0, 1, b_size)

# original 1-dimensional array
x = xr.DataArray(
    np.random.random(a_size),
    coords=[('a', a coords)])

I guess I could create an empty DataArray with the new dimension and copy the existing data in.

y = xr.DataArray(
    np.empty((b_size, a_size),
    coords=([('b', b_coords), ('a', a_coords)])
y[:] = x

A better idea might be to be to use concat. It took me a while to figure out how to specify both the dims and the coords for the concat dimension, and none of these options seem great. Is there something I'm missing that can makes this version cleaner?

# specify the dimension name, then set the coordinates
y = xr.concat([x for _ in b_coords], 'b')
y['b'] = b_coords

# specify the coordinates, then rename the dimension
y = xr.concat([x for _ in b_coords], b_coords)
y.rename({'concat_dim': 'b'})

# use a DataArray as the concat dimension
y = xr.concat(
    [x for _ in b_coords],
    xr.DataArray(b_coords, name='b', dims=['b']))

Still, is there a better way to do this than either of the two above options?

Community · Accepted Answer · 2020-05-29 17:35:57Z

14

If DA is your data array with length DimLen, you can now use expand_dims:

DA.expand_dims({'NewDim':DimLen})

edited May 29, 2020 at 17:35

CommunityBot

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answered Oct 2, 2019 at 10:14

Q-man

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Comments

hackz · Accepted Answer · 2017-07-12 17:58:37Z

4

Because of the way that math is applied over new dimensions I like to multiply in order to add new dimensions.

identityb = xr.DataArray(np.ones_like(b_coords), coords=[('b', b_coords)])
y = x * identityb

answered Jul 12, 2017 at 17:58

hackz

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1 Comment

susopeiz Over a year ago

Quick solution although it leads to lose the available attributes in x

J.Warren · Accepted Answer · 2020-05-29 08:46:55Z

4

Using .assign_coords method will do it. However you can't assign coordinates to a non-existant dimension, the way to do as a one liner is:

y = x.expand_dims({b_coords.name: b_size}).assign_coords({b_coords.name: b_coords})

edited May 29, 2020 at 8:46

answered May 5, 2020 at 18:26

J.Warren

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1 Comment

Téo Over a year ago

When I did this I got "ValueError: dimension [name] already exists as a scalar variable", but if I swap the assign and expand, it works fine!

chuaxr · Accepted Answer · 2019-12-16 02:54:32Z

1

With reference to the syntax of the question:

y = x.expand_dims({"b": b_coords})

answered Dec 16, 2019 at 2:54

chuaxr

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