Create Pandas dataframe from numpy array and use first column of the array as index

Question

I have a numpy array (a):

array([[ 1. ,  5.1,  3.5,  1.4,  0.2],
[ 1. ,  4.9,  3. ,  1.4,  0.2],
[ 2. ,  4.7,  3.2,  1.3,  0.2],
[ 2. ,  4.6,  3.1,  1.5,  0.2]])

I would like to make a pandas dataframe with values=a, columns=A,B,C,D and index= to the first column of my numpy array, finally it should look like this:

       A    B    C    D
  1  5.1  3.5  1.4  0.2
  1  4.9  3.0  1.4  0.2
  2  4.7  3.2  1.3  0.2
  2  4.6  3.1  1.5  0.2

I am trying this:

    df = pd.DataFrame(a, index=a[:,0], columns=['A', 'B','C','D'])

and I get the following error:

ValueError: Shape of passed values is (5, 4), indices imply (4, 4)

Any help?

EdChum · Accepted Answer · 2015-10-13 09:17:57Z

11

You passed the complete array as the data param, you need to slice your array also if you want just 4 columns from the array as the data:

In [158]:
df = pd.DataFrame(a[:,1:], index=a[:,0], columns=['A', 'B','C','D'])
df

Out[158]:
     A    B    C    D
1  5.1  3.5  1.4  0.2
1  4.9  3.0  1.4  0.2
2  4.7  3.2  1.3  0.2
2  4.6  3.1  1.5  0.2

Also having duplicate values in the index will make filtering/indexing problematic

So here a[:,1:] I take all the rows but index from column 1 onwards as desired, see the docs

answered Oct 13, 2015 at 9:17

EdChum

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