Condition within for loop is ambiguous

November 8, 2021

I am trying to run a conditional for loop on a dataframe. If the condition is not met, then the last row is appended to the result dataframe.

import pandas as pd
import numpy as np

df = pd.DataFrame(np.random.randint(100, size=(100,3)), columns=['Value1', 'Value2', 'Value3'])

split_df = [df[i:i+12] for i in range(0, len(df))]

result = []
for i in split_df:
    if i == 0:
        result = split_df[i]
    else:
        last_row = split_df[i].tail(1)
        result = result.append(last_row, ignore_index=True)

For If i == 0:, I get the following error:

ValueError: The truth value of a DataFrame is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

How do I get around this?

>Solution :

i is a DataFrame, because you’re looping over a list of DataFrames. You probably want to write the loop like this:

for i, df in enumerate(split_df):
    if i == 0:
        result = df
    else:
        last_row = df.tail(1)
        result.append(last_row, ignore_index=True)

Also note that you don’t want to write result = result.append(...) – rather, you’d just write result.append(...) because append() modifies the loop in-place, and it doesn’t return anything.