Insert string in pandas framework based on string value?

Question

I have a DataFrame:

df = pd.DataFrame({'B':[2,1,2],'C':['a','b','a']})
  B C
0 2 'a'
1 1 'b'
2 2 'a'

I want to insert a line above any occurrence of "b", that is, a duplicate of this line, but from "b" changed to "c", so I get the following:

  B C
0 2 'a'
1 1 'b'
1 1 'c'
2 2 'a'

In my life, I cannot figure out how to do this.

Answer 1

Here is one way to do this:

duplicates = df[df['C'] == 'b'].copy()
duplicates['C'] = 'c'
df.append(duplicates).sort_index()

Answer 2

Work at the NumPy level, here is the vector approach -

arr = df.values
idx = np.flatnonzero(df.C=='b')
newvals = arr[idx]
newvals[:,df.columns.get_loc("C")] = 'c'
out = np.insert(arr,idx+1,newvals,axis=0)
df_index = np.insert(np.arange(arr.shape[0]),idx+1,idx,axis=0)
df_out = pd.DataFrame(out,index=df_index)

Run Example -

In [149]: df
Out[149]: 
   B  C
0  2  a
1  1  b
2  2  d
3  4  d
4  3  b
5  8  a
6  4  a
7  2  b

In [150]: df_out
Out[150]: 
   0  1
0  2  a
1  1  b
1  1  c
2  2  d
3  4  d
4  3  b
4  3  c
5  8  a
6  4  a
7  2  b
7  2  c