How to add values in rows and pass the result to the next column using pandas?

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I have a pandas dataframe with the values as follows:

Start  end   Depart  Duration  
  A     B      5           2
  B     C      5           3
  C     D      5           6
  D     E      5           4

I want the output as shown below. I need to add rows of 'Depart' and 'Duration' to form a new column 'Arrive' and pass the result value to the next row in Depart column and then add with Duration to form a value in Arrive and repeat the process again.

I also need to verify if the column values 'Start' is same as the 'End' value in previous row.

Start  end     Depart   Arrive
  A     B        5          7      
  B     C        7          10        
  C     D        10         16
  D     E        16         20

I tried something like this:

if np.where(d.Start == d.End):
    d['Arrive'] = d['Depart'] + d['Duration']
    d['Depart'] = d.Arrive.shift(1)
    print(d)

The output is as follows. It still takes the original Depart value as 5 and adds up to give the result. Where am I going wrong?

  Start End  Depart  Duration  Arrive
0     A   B     NaN         2       7
1     B   C     7.0         3       8
2     C   D     8.0         6      11
3     D   E    11.0         4       9
pandas dataframe python
1 Answers

Try using cumsum:

depart = df['Depart'] + df['Duration'].cumsum().shift(1).fillna(0).astype(int)
arrival = df['Depart'] + df['Duration'].cumsum()

df['Depart'] = depart
df['Arrival'] = arrival
df = df.drop('Duration', axis=1)

Output:

>>> df
  Start end  Depart  Arrival
0     A   B       5        7
1     B   C       7       10
2     C   D      10       16
3     D   E      16       20
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