Conditional Concatenation Based on String Value in Column

Question

The python data frame I currently have contains two columns: "EVENT" and "NAME".

EVENT   NAME
A       DEN
B       HAU
C       TOT
D       ABC
E       DEN

I want to implement logic so that my EVENT column is as follows:

EVENT
A_DEN
B
C
D
E_DEN

I want to implement logic that says if "NAME" column contains DEN value then concatenate it to the value in "EVENT" column. Otherwise, leave value as is in "EVENT" column.

I have scoured the internet on how to do this but wasn't able to find anything specific to what I'm trying to accomplish.

cs95 · Accepted Answer · 2018-01-03 18:48:36Z

11

Option 1
You could do this with str.contains/eq to perform the "contains" check, and np.where to conditionally build your result -

df.EVENT = np.where(df.NAME.str.contains('DEN'), df.EVENT + '_' + df.NAME, df.EVENT)

Or,

df.EVENT = np.where(df.NAME.eq('DEN'), df.EVENT + '_' + df.NAME, df.EVENT)

df

   EVENT NAME
0  A_DEN  DEN
1      B  HAU
2      C  TOT
3      D  ABC
4  E_DEN  DEN

Don't forget to import numpy as np.

Option 2
Another method is using pd.Series.mask/pd.Series.where -

df.EVENT = df.EVENT.mask(df.NAME.str.contains('DEN'), df.EVENT + '_' + df.NAME)

Or,

df.EVENT = df.EVENT.where(~df.NAME.str.contains('DEN'), df.EVENT + '_' + df.NAME)

df

   EVENT NAME
0  A_DEN  DEN
1      B  HAU
2      C  TOT
3      D  ABC
4  E_DEN  DEN

Option 3
Yet another option is using loc based indexing with a mask -

m = df.NAME.str.contains('DEN')
df.loc[m, 'EVENT'] += ('_' + df.loc[m, 'NAME'])

df
   EVENT NAME
0  A_DEN  DEN
1      B  HAU
2      C  TOT
3      D  ABC
4  E_DEN  DEN

edited Jan 3, 2018 at 18:48

answered Jan 3, 2018 at 18:25

cs95

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12 Comments

Vaishali Over a year ago

I was just going to post the one with np.where :)

cs95 Over a year ago

@Vaishali np.where is usually the bread and butter for many pandas problems... easy to implement too, one of my favourite numpy functions :-)

Anton vBR Over a year ago

@cᴏʟᴅsᴘᴇᴇᴅ Is it safe to use pd.np.where? or would importing np and np.where be more future proof?

cs95 Over a year ago

@AntonvBR I think np.where would be safer, I use pd.np so op doesn't need to import numpy as np explicitly. Will edit.

PineNuts0 Over a year ago

it worked! I removed the extra set of parenthesis I added in ... thank you so much for your detailed response!

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