basic python: how do i create the next item on the list based on previous item?

Question

Is there a simpler way to write the following code? Currently the code is taking forever to run, which is strange. Something is wrong with it...seems way too complicated for the objective. I also get a SettingWithCopyWarning notice that says:

a value is trying to be set on a copy of a slice from a DataFrame

The objective is: I have four time series/columns - a, b, c and d , where d is the output column that needs to be populated.

1. if a is larger than b , then return 1 in column d ,
2. if a is below c , then return 0 in column d ,
3. and if a is between b and c , then return the previous item in column d .

Notice that this last if statement references the previous item in column d.

data['d']=1
data['previous_d']=1

for i in range (len(data.a)):
    data.previous_d.iloc[i]=data.d.iloc[i-1]
    data.stance.iloc[i] = np.where((data.a.iloc[i]> data.b.iloc[i]),1,np.where((data.a.iloc[i]< data.c.iloc[i]),0,data.previous_d.iloc[i]))

Answer 1

Just some Boolean indexing will work

data.loc[data.a> data.b,'d'] = 1
data.loc[data.a < data.c,'d'] = 0
data.loc[(data.a < data.b) & (data.a > data.c),'d'] = data.d.shift(1)

Or if the third conditions means that every thing between unfilled numbers is filled from the last populated value, use

data.d.fillna(method='ffill',inplace=True)

instead of the last line