i have a dataframe as :
id|amount|date
20|-7|2017:12:25
20|-170|2017:12:26
20|7|2017:12:27
i want to subtract each row from another for 'amount' column:
the output should be like:
id|amount|date|amount_diff
20|-7|2017:12:25|0
20|-170|2017:12:26|-177
20|7|2017:12:27|-163
i used the code:
df.sort_values(by='date',inplace=True)
df['amount_diff'] = df['invoice_amount'].diff()
and obtained the output as:
id|amount|date|amount_diff
20|-7|2017:12:25|163
20|-170|2017:12:26|-218
20|48|2017:12:27|0
IIUC you need:
df.sort_values(by='date',inplace=True)
df['amount_diff'] = df['amount'].add(df['amount'].shift()).fillna(0)
print (df)
id amount date amount_diff
0 20 -7 2017:12:25 0.0
1 20 -170 2017:12:26 -177.0
2 20 7 2017:12:27 -163.0
Because if want subtract your solution should work:
df.sort_values(by='date',inplace=True)
df['amount_diff1'] = df['amount'].sub(df['amount'].shift()).fillna(0)
df['amount_diff2'] = df['amount'].diff().fillna(0)
print (df)
id amount date amount_diff1 amount_diff2
0 20 -7 2017:12:25 0.0 0.0
1 20 -170 2017:12:26 -163.0 -163.0
2 20 7 2017:12:27 177.0 177.0