I want to lock the first value from a groupby in dataframe pandas.

Is that possible?

example:

this is the dataframe

And this is how i would like to have it

Hope someone can help me...

CodePudding user response：

You need to use groupby transform.

If you want to replace by the first value:

df['b'] = df.groupby('a')['b'].transform('first')

or by the min value:

df['b'] = df.groupby('a')['b'].transform('min')

output:

   a    b
0  1  100
1  1  100
2  1  100
3  2  400
4  2  400
5  2  400
6  3  700
7  3  700
8  3  700

CodePudding user response：

This will do it for you

kv = df.groupby(by='first_col')['second_col'].first().to_dict()

def helper(k):
    return kv[k]

df['second_col'] = df['first_col'].apply(lambda x: helper(x))

Note first_col and second_col are your column names