How to restart loop in pandas using while

[enter image description here][I have ID column and want to add count for ids like in count column in pd dataframe. I want to do that using while loop. I tried this but I am getting never ending loop.
ID:1,1,1,2,2,3,3,3,4,5,6
count must be
COUNT:1,2,3,1,2,1,2,3,1,1,1

i=1
sum=1
n=len(df)
while i < n:
  if df.loc[i,"ID]=df.loc[i-1,"ID"]:
  sum+=1
  df.loc[i,"Count"]=sum
  i+=1
   else:
  i=1

Please help!

>Solution :

groupby, then transform with 'cumcount'.

>>> df = pd.DataFrame({'ID': [1,1,1,2,2,3,3,4,4,4,5,6]})
>>> df 
    ID
0    1
1    1
2    1
3    2
4    2
5    3
6    3
7    4
8    4
9    4
10   5
11   6
>>> df['count'] = df.groupby('ID').transform('cumcount') + 1
>>> df 
    ID  count
0    1      1
1    1      2
2    1      3
3    2      1
4    2      2
5    3      1
6    3      2
7    4      1
8    4      2
9    4      3
10   5      1
11   6      1