# [SOLVED] How to subtract Nth rows from a certain row within a column multiple times

## Issue

I want to be able to subtract a specific number of rows from another specific set of rows multiple times. I assume I would do this with a for loop but I can’t figure out how exactly to do this.

For example I would like to subtract the yields from the treatment group from the control within the same field. Every 6 rows I would like to subtract the 4-6th row from the 1-3rd rows over and over again

This is the what I have thus far:

`dfArray = df ['Crop_Yield'].to_numpy()`

``````  `for i in range(len(dfArray('Crop_Yield') //6):`
``````

Thank you in advance for helping me out with this. Sorry if something like this has been asked before, I tired looking but didn’t find anything similar.

Below is an example of the table.

Treatment Field Crop_Yield
Control 1 Top 14
Control 2 Top 18
Control 3 Top 16
Fert A 1 Bottom 12
Fert A 2 Bottom 17
Fert A 3 Bottom 15
Control 4 Top 10
Control 5 Top 13
Control 6 Top 15
Fert B 4 Bottom 17
Fert B 5 Bottom 13
Fert B 6 Bottom 18

## Solution

I hope I’ve understood you correctly:

``````for i in range(len(df) // 6):
df.loc[i * 6 : i * 6 + 2, "Crop_Yield"] -= df.loc[i * 6 + 3 : i * 6 + 5, "Crop_Yield"].to_numpy()

print(df)
``````

Prints:

``````   Treatment     Field  Crop_Yield
0    Control     1 Top           2
1    Control     2 Top           1
2    Control     3 Top           1
3     Fert A  1 Bottom          12
4     Fert A  2 Bottom          17
5     Fert A  3 Bottom          15
6    Control     4 Top          -7
7    Control     5 Top           0
8    Control     6 Top          -3
9     Fert B  4 Bottom          17
10    Fert B  5 Bottom          13
11    Fert B  6 Bottom          18
``````

Answered By – Andrej Kesely

Answer Checked By – Mildred Charles (BugsFixing Admin)