title: Calculating summation over months of pandas dataframe
body: <p>I have a pandas dataframe given below:</p>
  <pre><code>ID       Year       R1  R1_f
  KAR1    20201001    1   5
  KAR1    20201101    2   6
  KAR1    20201201    3   7
  KAR1    20210101    4   8
  KAR1    20210201    5   9
  KAR1    20210301    6   10
  KAR1    20210401    7   11
  KAR1    20210501    8   12
  KAR1    20210601    9   13
  KAR1    20210701    10  14
  KAR1    20210801    11  15
  KAR1    20210901    12  16
  KAR2    20201001    4   9
  KAR2    20201101    3   8
  KAR2    20201201    2   7
  KAR2    20210101    1   6
  KAR2    20210201    9   5
  KAR2    20210301    2   4
  KAR2    20210401    6   3
  KAR2    20210501    5   2
  KAR2    20210601    3   1
  KAR2    20210701    30  2
  KAR2    20210801    34  3
  KAR2    20210901    20  4
  </code></pre>
  <p>I need to transform above dataframe as given below:</p>
  <pre><code>ID Year      R1_sum 3m_R1 6m_R1 9m_R1 12m_R1 R1_f 3m_R1_f 6m_R1_f 9m_R1_f 12m_R1_f 
  </code></pre>
  <p>KAR1 20210901   12      33    57    72    78    16    45     81      108      126
  KAR2 20210901   20      84    98    110   119    4     9      15      30        54</p>
  <p>In above output dataframe
  R1_sum is having value equal to value in year 20210901 for both Id's.</p>
  <p>3m_R1 is the summation of values of 3 months 20210901 to 20210701 for column R1</p>
  <p>6m_R1 is the summation of values of 6 months from 20210901 to 20210401 for column R1</p>
  <p>9m_R1 is the summation of values of 9 months from 20210901 to 20210101 for column R1</p>
  <p>12m_R1 is the summation of values of 12 months from 20210901 to 20201001 for column R1</p>
  <p>R1_f is having value equal to value in year 20210901 for both Id's.</p>
  <p>3m_R1_f is the summation of values of 3 months 20210901 to 20210701 for column R1_f</p>
  <p>6m_R1_f is the summation of values of 6 months from 20210901 to 20210401 for column R1_f</p>
  <p>9m_R1_f is the summation of values of 9 months from 20210901 to 20210101 for column R1_f</p>
  <p>12m_R1_f is the summation of values of 12 months from 20210901 to 20201001 for column R1_f</p>
  <p>Please help</p>