如何将列表/数组中的列填充到空的 Pandas dataframe 中，只有列名

Question

I have a dataframe with several column names. As and when I get data for each column, I have to create rows. I don't have all the row data available in one place. As and when I get data for a column in a particular row, I will fill it

In this example below, I have created an empty dataframe and and I am trying to fill in a particular column with a set of values. This is not working.

import pandas as pd
import numpy as np

col_names = ['ampere', 'freq', 'count']
dataf = pd.DataFrame(columns = col_names)
freq = np.arange(0.6,2.6,0.1).tolist()
#Add the list of frequencies to the frequency column
dataf['freq'].iloc[1:len(freq)] = freq

The error I am getting is

ValueError: cannot copy sequence with size 20 to array axis with dimension 0

Answer 1

You're almost there! There is no need for the iloc -indexing.

So just change your last line to

dataf['freq'] = freq

and it should work as expected.

If you want to keep adding more rows from different lists, as you mentioned in the comment, you can use following code snippet:

import pandas as pd
import numpy as np

col_names = ['ampere', 'freq', 'count']
dataf = pd.DataFrame(columns = col_names)
freq = np.arange(0.6,2.6,0.1).tolist()
#Add the list of frequencies to the frequency column
dataf['freq'] = freq

freq2 = np.arange(7,70,1).tolist()

for i, value in enumerate(freq2, len(dataf)):
    dataf.loc[i] = [np.nan, value, np.nan]

I hope this helps to solve your problem.