使用pandas重复数据框中的项目

Question

I have the following dataFrame:

id  z2  z3  z4  
1   2   a   fine 
2   7   b   good
3   9   c   delay
4   30  d   cold

I am going to generate a data frame by repeating each item in a row twice except items in column z4 (that should not be repeated). How I can do it using python and pandas.

The output should be like this:

id  z1  z3  z4
1   2   a   fine 
1   2   a   
1   2   a   
2   7   b   good
2   7   b   
2   7   b   
3   9   c   delay
3   9   c   
3   9   c   
4   30  d   cold
4   30  d   
4   30  d   

Answer 1

Another way to do this is to use indexing: Notice that df.iloc[[0, 1, 2, 3]*2, :3] will give you two copies of the first three columns.

This can then be appended to the original df . Remove the NA . Then sort on index values and reset index (dropping the old index). All of which can be chained:

df.append(df.iloc[[0, 1, 2, 3]*2, :3]).fillna('').sort_index().reset_index(drop=True)

which produces:

    id  z2 z3     z4
0    1   2  a   fine
1    1   2  a       
2    1   2  a       
3    2   7  b   good
4    2   7  b       
5    2   7  b       
6    3   9  c  delay
7    3   9  c       
8    3   9  c       
9    4  30  d   cold
10   4  30  d       
11   4  30  d

Answer 2

groupby and apply will do the trick:

def func(group):
    copy = group.copy()
    copy['z4'] = ""
    return pd.concat((group, copy, copy))

df.groupby('id').apply(func).reset_index(drop=True)


    id  z2 z3     z4
0    1   2  a   fine
1    1   2  a       
2    1   2  a       
3    2   7  b   good
4    2   7  b       
5    2   7  b       
6    3   9  c  delay
7    3   9  c       
8    3   9  c       
9    4  30  d   cold
10   4  30  d       
11   4  30  d