I have a dataframe with statistical data that is cumulated with each new row. Every day a new row is added. Now I want to iterate over my column so that each row (starting from last) is substracted from the row above. The new value should be put into a new column.
This is how my dataframe looks and the values in the column 'diff' are my desired outcome:

```
     time        In     diff
0  2017-06-26  7.086      
1  2017-06-27  8.086     1
2  2017-06-28  10.200    2.114

```

this is what I came up with:

```
for x in df['In']:
    df['diff'] = df.iloc[-1] - df.iloc[-2]

```

but thats not it. How do I start the loop from the last row and how do I make the iloc more dynamic? Can someone help?
thank you!