I have a pandas dataframe ‘df’ with two columns ‘A’ and ‘B’, I have a function with two arguments
def myfunction(B, A): # do something here to get the result return result
and I would like to apply it row-by-row to df using the ‘apply’ function
df['C'] = df['B'].apply(myfunction, args=(df['A'],))
but I get the error
ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().
whats happening here, it seems it takes df[‘A’] as the whole series! not just the row entry from that series as required.
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Answer
I think you need:
import pandas as pd df = pd.DataFrame({'A':[1,2,3], 'B':[4,5,6]}) print (df) A B 0 1 4 1 2 5 2 3 6 def myfunction(B, A): #some staff result = B + A # do something here to get the result return result df['C'] = df.apply(lambda x: myfunction(x.B, x.A), axis=1) print (df) A B C 0 1 4 5 1 2 5 7 2 3 6 9
Or:
def myfunction(x): result = x.B + x.A # do something here to get the result return result df['C'] = df.apply(myfunction, axis=1) print (df) A B C 0 1 4 5 1 2 5 7 2 3 6 9