title: How do create new column in csv file using python by shifting one row
body: <p>I have CSV file like below. It is huge file with thousands of records. </p>
  
  <p>input.csv</p>
  
  <pre><code>No;Val;Rec;CSR
  0;10;1;1200
  0;100;2;1300
  0;100;3;1300
  0;100;4;1400
  0;10;5;1200
  0;11;6;1200
  </code></pre>
  
  <p>I want to create output.csv file by adding new column "PSR" after 1st column "No". This column value depends on column "PSR" Value. For 1st row, "PSR" shall be zero. From next record on-wards, it depends on "CSR" value in previous row. If present and previous record CSR value is same, then "PSR" shall be zero. If not, PSR value shall have the previous CSR value. For exmple, Value of CSR in 2nd row is 1300 which is different to the value in 1st record ( it is 1200). So PSR value for 2nd row shall be 1200. Where in 2nd and 3rd row, CSR value is same. So PSR value for 3rd row shall be zero. So new value PSR depends on CSR value in present and previous field. </p>
  
  <p>Output.csv</p>
  
  <pre><code>No;PCR;Val;Rec;CSR
  0;0;10;1;1200
  0;1200;100;2;1300
  0;0;100;3;1300
  0;1300;100;4;1400
  0;1400;10;5;1200
  0;0;11;6;1200
  </code></pre>
  
  <p>My Approach:</p>
  
  <ol>
  <li>Use csv.reader and iterate over the objects in a list. Copy 5th column to 2nd column in list.  Shift it one row down.</li>
  <li>Then check the values in 2nd and 5th column (PCR and CSR), if both values are same. Replace the PCR value with zero.</li>
  </ol>
  
  <p>I have problem in getting 1st step coded. I am able to duplicate the column but not able to shift it. Also 2nd step is quite straightforward. </p>
  
  <p>Also, I am not sure whether this approach is correct Any pointers/recommendation would be really helpful.</p>
  
  <p>Note: I am not able to install Pandas on CentOS. So help without this module would be better.</p>
  
  <p>My Code: </p>
  
  <pre><code>with open('input.csv', 'r') as input, open('output.csv', 'w') as output:
          reader = csv.reader(input, delimiter = ';')
          writer = csv.writer(output, delimiter = ';')
          mylist = []                                        
          header = next(reader)                           
          mylist.append(header)
          for rec in reader:
                  mylist.append(rec)                      
                  rec.insert(1, rec[3])
                  mylist.append(rec)
          writer.writerows(mylist)
  </code></pre>