如何使用 Pandas/Python 在特定列中的每个项目行上使用 function

Question

I have a simple DataFrame in Pandas that one of the column contains date in this format day-month-year. I need to make another column that contains which weekday is that. I wrote this function that works with simple argument like '12-3-1999':

def convert_date_to_weekday(date_string):
  # convert string to date object
  date_object = datetime.strptime(date_string, '%d-%m-%Y').date()
  # convert date object to weekday string
  print(date_object.strftime("%A"))

Unfortunetely this doesn't work:

df['Weekday'] = convert_date_to_weekday(df['Date'])

How to make it work?

Answer 1

You can use apply , which also a recommended way with pandas

def convert_date_to_weekday(date_string):
  # convert string to date object
  date_object = datetime.strptime(date_string, '%d-%m-%Y').date()
  # convert date object to weekday string
  # print(date_object.strftime("%A"))
  return date_object.strftime("%A") 


df['Weekday'] = df['Date'].apply(lambda x: convert_date_to_weekday(x)))

Answer 2

try

df['Weekday'] = df['Date'].apply(lambda x: convert_date_to_weekday(x))

and use return in your function instead of print

Answer 3

Try:

def convert_date_to_weekday(date_string):
  # convert string to date object
  date_object = datetime.strptime(date_string, '%d-%m-%Y').date()
  # convert date object to weekday string
  return date_object.strftime("%A")

Along with your apply line.

Answer 4

You probably want to use apply() method of the pandas dataframe:

df['Weekday'] = df['Date'].apply(convert_date_to_weekday)

But your convert_date_to_weekday should return the result instead of printing.

Answer 5

You can use the apply() function.

df['Weekday'] = df['Date'].apply(lambda d: convert_date_to_weekday(d))

Also, change your print(date_object.strftime("%A")) to return date_object.strftime("%A")