Subtracting a value in a column based on condition

Question

Here is the set up of my dataframe:

import pandas as pd

a=pd.DataFrame([[101,'RK',1,'01','A',200],[101,'FET01',1,'02','B',10],
                [101,'CS',1,'01','C',300],[101,'AS',1,'03','D',250],
                [101,'FET02',1,'04','E',15],[102,'DG',1,'05','F',200],
                [103,'GH',3,'06','G',150],[103,'FET01',3,'08','H',12]],
                columns=['Inv','Item','Order','Warehouse','Module','Amt'])

Output:

     Inv    Item    Order   Warehouse   Module    Amt
0   101      RK       1       01          A       200
1   101    FET01      1       02          B       10
2   101      CS       1       01          C       300
3   101      AS       1       03          D       250
4   101    FET02      1       04          E       15
5   102      DG       1       05          F       200
6   103      GH       3       06          G       150
7   103    FET01      3       08          H       12

I would like to substract the Amt of an Item from it's corresponding FET Amt . For eq, as Item RK in the first row has Item FET01 beneath it, the Amt for RK should be changed to (200-10). In the 3rd row, Item CS does not have FET beneath it, the Amt should not change. In the fourth row, there is FET beneath Item AS , the Amt should be subtracted from it's FET which is (250-15).

The final output should be:

    Inv    Item    Order   Warehouse   Module    Amt
0   101      RK       1       01          A       190
1   101    FET01      1       02          B       10
2   101      CS       1       01          C       300
3   101      AS       1       03          D       235
4   101    FET02      1       04          E       15
5   102      DG       1       05          F       200
6   103      GH       3       06          G       138
7   103    FET01      3       08          H       12

Answer 1

You could do

In [1333]: check = df.Item.str.startswith('FET').shift(-1).fillna(False)

In [1334]: df.loc[check, 'Amt'] = df.Amt.diff(-1)

In [1325]: df
Out[1325]:
   Inv   Item  Order  Warehouse Module    Amt
0  101     RK      1          1      A  190.0
1  101  FET01      1          2      B   10.0
2  101     CS      1          1      C  300.0
3  101     AS      1          3      D  235.0
4  101  FET02      1          4      E   15.0
5  102     DG      1          5      F  200.0
6  103     GH      3          6      G  138.0
7  103  FET01      3          8      H   12.0

---

Details

In [1336]: check
Out[1336]:
0     True
1    False
2    False
3     True
4    False
5    False
6     True
7    False
Name: Item, dtype: bool

Answer 2

You can do this by simply iterating through your dataframe, checking what is the value of 'Item' in the next row, and subtracting if it contains 'FET':

for ii, row in a.iterrows():
    if ii+1 >= len(a):
        break
    if 'FET' in a.iloc[ii+1]['Item']:
        a.set_value( ii, 'Amt', a.iloc[ii]['Amt']-a.iloc[ii+1]['Amt'] )

print a