Copy column value from a dataframe to another if values are equal

Question

I have two dataframes like that (this is an example because my dataframes are complex) :

lst_p = [['2', 0], ['3', 1], ['4', 0], ['5', 0]]  
df_p = pd.DataFrame(lst_p, columns =['id', 'redness'])



lst_c = [['apple', 2], ['orange', 2], ['banana', 3], ['kiwi', 4], ['cherry', 5]]  
df_c = `pd.DataFrame(lst_c, columns =['name', 'id'])`

My two dataframes don’t have the same length.

As you can see in my second df_c, some ‘id’ appears 2 times. (for id=2)

I would like to create a new column in my df_c that copy the value 'redness' of my df_p if 'id' from my df_c == ‘id’ from my df_p.

I don’t know if it’s very clear…

Thanks a LOT !!!

Asked By: Anelor Guinet

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Source

Answer 1

Use can simply try to convert df_p two column to dictionary any using lambda look for each id’s redness, and create new column.

Code:

df_c['redness'] = df_c['id'].apply(lambda x: pd.Series(df_p.redness.values,index=df_p.id).to_dict()[str(x)])
df_c

Answered By: R. Baraiya

Answer 2

A simple merge will do the trick:

One issue that you have is that in one dataframe your id is of type string,
and in the other dataframe, the id is of type int.

The easiest way to resolve this is to convert the string to int before merge,
and convert back if so desired.

Code:

df_p.id = df_p.id.astype(int)
df_c = pd.merge(df_c, df_p, on=['id'], how='left')

print(df_c)

Output:

Answered By: ScottC

Copy column value from a dataframe to another if values are equal

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