Finding difference between two rows

I have a 6696502 x 2 table which shows energy in MW for a period of time. I want to add a column which is the difference in energy for each time period so column 3 row 2 shows the difference in values for row 2 and 1 in column 2.

I have the following code, however it is very slow to compute given my table is so large. if true

if true
  % code
for n = 1:countReg
          if n == 1
              diffReg = 0;
          else
              diffReg = RegCol(n) - RegCol(n-1);
          end
          newRegTable.ChangeReg(n) = diffReg;       
      end
end

Is there any way to do this so it does not take so much time?

Answer 1

[Guillaume]

yourtable.ChangeReg = [0; diff(yourtable.NameOfFirstColumn)]

[Dooyoung Kim]

Works well! Thank you :)