Pandas sum column values according to another columns value

One-liner code to sum Pandas second columns according to same values in the first column.

df2 = df1.groupby(df1.columns[0])[df1.columns[1]].sum().reset_index()

For example, applying to a table listing pipe diameters and lenghts, the command will return total lenghts according to each unique diameters.

This functionality is similar to excel's pivot table sum.

Numpy - Accumulated and Incremental series

In Hydrology, it is always needed to deal with time-series of variables, as flow series or precipitation series, with the variable being incremental or accumulated.

Numpy has a great way to transform between accumulated and incremental series.

To accumulate a incremental series use the method

   numpy.cumsum(incrementalSeries)

And to transform a accumulated array to a incremental one, use:

    numpy.diff(accumulatedSeries)