Calculation of the time difference between two rows

Question

I am trying to calculate the time difference between two lines using shift() , but I am getting an unexpected error. Perhaps I will miss something obvious.

 df['Delta'] = (df.index - df.index.shift(1)) 

This statement creates a ValueError: Cannot shift with no offset . What am I missing?

Answer 1

Two things:

• If you have a DatetimeIndex, shift shifts your data with a period of time. If your index has no frequency, you should provide this with the shift method using the freq keyword (e.g. freq='s' to shift data in one second).
• You cannot subtract two such index objects, as this gives you the operation with difference settings: http://pandas.pydata.org/pandas-docs/stable/indexing.html#set-operations-on-index-objects

If you just want the difference between two consecutive values ​​in the index, you can use the diff method (from a series, a little easier than shift and subtraction):

 df['index_col'] = df.index df['Delta'] = df['index_col'].diff() 

Answer 2

Perhaps vaguely, Series.shift and Index.shift do not exactly do the same, the latter only for a meaningful definition for the TimesSeries. Probably the easiest way is to add your index as a column.

 df['index_col'] = df.index df['Delta']=(df['index_col'] - df['index_col'].shift(1))