CODEWITHSUNDEEP
pythonpandasdataframe
Xue Xu
Xue Xu

Reputation: 55

Pandas - add multiple values to row inside while loop

I'm working with a dataframe inside a while loop that needs to be updated based on the simulation time. Here's an example:

import pandas as pd
import random

n = 0
df = pd.DataFrame(columns=['A', 'B'])

while n < 10:
  df = df.append({'A': random.uniform(0, 1), 'B': random.uniform(0, 1)}, 
                 ignore_index=True)
  df = df.append({'A': random.uniform(0, 1), 'B': random.uniform(0, 1)}, 
                 ignore_index=True)

  df['New Column'] = n

  print('This value should be added to the new rows only ' + str(n))
  n += 1
  print(df)

This results in the entire new column being updated at once, which is undesired in my case. Here are the first three prints:

This value should be added to the new rows only 0
          A         B  New Column
0  0.242312  0.369978           0
1  0.709112  0.650794           0
This value should be added to the new rows only 1
          A         B  New Column
0  0.242312  0.369978           1    <- should have been 0
1  0.709112  0.650794           1    <- should have been 0
2  0.293787  0.229019           1
3  0.828184  0.917984           1
This value should be added to the new rows only 2
          A         B  New Column
0  0.242312  0.369978           2    <- should have been 0
1  0.709112  0.650794           2    <- should have been 0
2  0.293787  0.229019           2    <- should have been 1
3  0.828184  0.917984           2    <- should have been 1
4  0.644289  0.589050           2
5  0.371361  0.759865           2

Can I do this in one operation or I need to copy to another dataframe and then append that?

Upvotes: 0

Views: 2227

Answers (1)

U13-Forward
U13-Forward

Reputation: 71620

Use the below code:

import pandas as pd
import random

n = 0
df = pd.DataFrame(columns=['A', 'B', 'New Column'])

while n < 10:
  df = df.append({'A': random.uniform(0, 1), 'B': random.uniform(0, 1), 'New Column': n}, 
                 ignore_index=True)
  df = df.append({'A': random.uniform(0, 1), 'B': random.uniform(0, 1), 'New Column': n}, 
                 ignore_index=True)

  print('This value should be added to the new rows only ' + str(n))
  n += 1
  print(df)

Simply just have a New Column from the beginning, and then add the value just like you do with A and B inside the append, I am making New Column also in the same appends.

Your code doesn't work since you're modifying all values on every loop occasion, when you need it for the appended dataframe, which is what I'm doing.

Upvotes: 1

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