Values diplicated in ln(x_t/Xt_1) as growth rate

March 8, 2023

I am trying get a growth rate in a dataframe using: ln(x1_t/x1_t-1) but I can´t get because I have repetead values.

This is a example:

data = [['t1',9],['t2',16],['t3', 20], ['t4', 28]]
df = pd.DataFrame(data,columns=['Name','Cash_Flow'],dtype=float)

for row in range(len(df)):
    df['ln(CF_t/CF_t-1)'] = mt.log(df.iloc[row, 1] / df.iloc[row-1, 1])

The result is:


   Name Cash_Flow   ln(CF_t/CF_t-1)
0   t1  9.0         0.336472
1   t2  16.0        0.336472
2   t3  20.0        0.336472
3   t4  28.0        0.336472

>Solution :

You are overwriting the entire ln(CF_t/CF_t-1) column in each iteration of the loop. You need to update only the current row value for ln(CF_t/CF_t-1)

import pandas as pd
import numpy as np

data = [['t1',9],['t2',16],['t3', 20], ['t4', 28]]
df = pd.DataFrame(data,columns=['Name','Cash_Flow'],dtype=float)

df['ln(CF_t/CF_t-1)'] = np.log(df['Cash_Flow'] / df['Cash_Flow'].shift(1))

print(df)

output:

 Name  Cash_Flow  ln(CF_t/CF_t-1)
0   t1        9.0              NaN
1   t2       16.0         0.575364
2   t3       20.0         0.223144
3   t4       28.0         0.336472