Conditional rolling sum in Python

Question

I am transitioning from data analysis in Excel to Python and have not been able to find a solution for the equivalent code in Python to use in my data frame. To calculate the Rolling Sum column, I would use the formula IF(C3=FALSE,0,(1+D2)) (For the table posted below). In this example, as long as the Amount value is above 20 in the > 20 column, return the value of 1 and then add it to the amount above it.

My attempt to create the Rolling Sum column in Python:

def f(row):
     if row['> 20'] == False:
          val = 0

     else:
          #getting stuck here as to how to add to the row above, shift(1) is incorrect
          val = 1 + shift(1)
     return val

df['Rolling Sum'] = df.apply(f, axis=1)





Event | Amount | > 20  | Rolling Sum |
+-------+--------+-------+-------------+
|     1 |      7 | FALSE |             |
|     2 |     25 | TRUE  |           1 |
|     3 |     28 | TRUE  |           2 |
|     4 |      3 | FALSE |           0 |
|     5 |     30 | TRUE  |           1 |
|     6 |     35 | TRUE  |           2 |
|     7 |     40 | TRUE  |           3 |
|     8 |      6 | FALSE |           0 |
+-------+--------+-------+-------------+

Answer 1

try this with iterrows:

for index, row in df.iterrows():
    if df.loc[index, '> 20'] == True:
        df.loc[index, 'Rolling Sum'] = df.loc[index-1, 'Rolling Sum']+1
    else:
        df.loc[index, 'Rolling Sum'] = 0