How to Iterate over rows and insert a value based on previous row to a new column

Question

I have a few DataFrame's which I am trying to add a new column with calculated values from the previous row and current row.
the problem is I get the same result for each row (when it needs to be different)

I made a function that gets a CSV turns it to df and do the changes.

def index_csv_update(file_path):
    df = pd.read_csv(file_path)
    df.drop(["Adj Close"], axis=1, inplace=True)
    df.drop(["Volume"], axis=1, inplace=True)
    print(df.head())
    start = 0
    for i, row in df.iterrows():
        first_row = df.iloc[start, 4]
        try:
            second_row = df.iloc[start + 1, 4]
        except IndexError:
            second_row = 'null'
        if not second_row == 'null':
            df['Daily Change %'] = np.float((second_row/first_row)-1)
        start += 1
    df.to_csv(file_path, index=False)

print result:

         Date   Open   High    Low  Close  Daily Change %
0  2018-07-09  13.02  13.22  12.60  12.69        0.011575
1  2018-07-10  12.52  13.21  11.93  12.64        0.011575
2  2018-07-11  14.05  14.15  13.09  13.63        0.011575
3  2018-07-12  13.07  13.33  12.42  12.58        0.011575
4  2018-07-13  12.39  12.97  11.62  12.18        0.011575

on the Daily Change % column there should be different numbers. I can't find the problem,
please help thanks.

Answer 1

you see when you are using

df['Daily Change %'] = np.float((second_row/first_row)-1)

You creating a new column with the value .. what u need to use is loc or iloc.

import pandas as pd
import numpy as np
df = pd.DataFrame({'foo': [1,2,3,3], 'bar': [1.2,2.3,3.5,1.3], 'foo2':[None, None, None, None]})
print(df)

start=0
for i, row in df.iterrows():
        first_row = df.iloc[start, 1]
        try:
            second_row = df.iloc[start + 1, 1]
        except IndexError:
            second_row = 'null'
        if not second_row == 'null':
            df.iloc[start,2] = np.float((second_row/first_row)-1)
        start += 1
print(df)

outputs:

   foo  bar  foo2
0    1  1.2  None
1    2  2.3  None
2    3  3.5  None
3    3  1.3  None


   foo  bar      foo2
0    1  1.2  0.916667
1    2  2.3  0.521739
2    3  3.5 -0.628571
3    3  1.3      None