Fill a dataframe column with empty arrays

Question

I need to fill a pandas dataframe column with empty numpy arrays. I mean that any row has to be an empty array. Something like

df['ColumnName'] = np.empty(0,dtype=float)

but this don't work because it tries to use every value of the array and assign one value per row.

I tried then

for k in range(len(df)):
     df['ColumnName'].iloc[k] = np.empty(0,dtype=float)

but still no luck. Any advice ?

Now, what do you mean by empty? Have you read the docs on np.empty? — Divakar
– Divakar, Commented Oct 27, 2017 at 13:12
I mean that every row of the dataframe has to contain an array object, unitialisated and of zero length. — Uge
– Uge, Commented Oct 27, 2017 at 13:14
Do simple search before asking stackoverflow.com/questions/31466769/… — BENY
– BENY, Commented Oct 27, 2017 at 14:55
What's the intended use of this column? That kind of array is pretty useless. — hpaulj
– hpaulj, Commented Oct 27, 2017 at 16:51
Possible duplicate of Add column of empty lists to DataFrame — Shihe Zhang
– Shihe Zhang, Commented Oct 28, 2017 at 2:53

Bharath M Shetty · Accepted Answer · 2017-10-27 13:24:03Z

2

You can repeat the np.empty into number of rows and then assign them to the column. Since it aint a scalar it cant be directly assigned like df['x'] = some_scalar.

df = pd.DataFrame({'a':[0,1,2]})

df['c'] = [np.empty(0,dtype=float)]*len(df)

Output :

  a   c
0  0  []
1  1  []
2  2  []

edited Oct 27, 2017 at 13:24

answered Oct 27, 2017 at 13:19

Bharath M Shetty

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3 Comments

Uge Over a year ago

Yes ! Thanks a lot

Bharath M Shetty Over a year ago

Glad to help. Dont forget to vote and accept the answer if it was helpful.

mp252 Over a year ago

For some reason, this keeps making the lists dictionary for me. Might be to do with the version I am using of pandas/numpy, will check it out

vkt · Accepted Answer · 2021-10-30 13:18:16Z

2

You can also use a simple comprehension

df = pd.DataFrame({'a':[0,1,2]})
df['c'] = [[] for i in range(len(df))]

Output

   a   c
0  0  []
1  1  []
2  2  []

answered Oct 30, 2021 at 13:18

vkt

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