CODEWITHSUNDEEP
pythonpandasdataframe
constantLeaner
constantLeaner

Reputation: 167

Pandas: How to assign values from a Python list to a column in dataframe when sizes are unequal

I have a list abc[] of size x and I have a data-frame whose shape is 2x. Now, I want to assign the values from list abc[] to a new column in data frame.

When the size of DF is equal or less than the list, I just say:

df['NewCol'] = abc[:df.shape[0]]

When the size of the df is more than the list (in this case twice), I do a for like below:

for i,rowData in df.iterrows():
  i = i-1
  j = i/2
  df['NewCol'].iloc[i] = abc[j]

Here the size of df is exactly twice the size of list. And I will always have the case where the size of df is either twice/thrice the list. So that one entry can be matched to two or three consecutive entries.

Is there any faster way to achieve this?

Upvotes: 1

Views: 711

Answers (2)

FlorianGD
FlorianGD

Reputation: 2436

You could use numpy.repeat to repeat your list, as you are sure there will always be an integer.

import numpy as np
import pandas as pd
df = pd.DataFrame({'a':np.arange(6)})
abc = [4, 5, 6]
df['NewCol'] = np.repeat(abc, len(df)/len(abc))
df
   a  NewCol
0  0       4
1  1       4
2  2       5
3  3       5
4  4       6
5  5       6

If you prefer to have the list repeated as a whole, you can use np.tile :

df['NewCol2'] = np.tile(abc, len(df)/len(abc))
df
   a  NewCol  NewCol2
0  0       4        4
1  1       4        5
2  2       5        6
3  3       5        4
4  4       6        5
5  5       6        6

Upvotes: 0

Alexander
Alexander

Reputation: 109626

df = pd.DataFrame(np.random.randn(4, 3), columns=list('ABC'))
abc = ['a', 'b']

I will always have the case where the size of df is either twice/thrice the list.

multiplier = len(df) / len(abc)  # Should be 2 or 3 per above condition.
df = df.assign(NewCol=[val for val in abc for _ in range(multiplier)])

>>> df
          A         B         C NewCol
0 -0.262760  1.898977  2.265480      a
1  0.552906  2.144316 -0.942272      a
2 -1.429635 -0.060660  0.756665      b
3 -0.658036 -1.056586  1.458374      b

Upvotes: 1

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