I want to select the previous row's value only if it meets a certain condition E.g.
df:
Value Marker
10 0
12 0
50 1
42 1
52 0
23 1
I want to select the previous row's value where marker == 0if the current value marker == 1.
Result:
df:
Value Marker Prev_Value
10 0 nan
12 0 nan
50 1 12
42 1 12
52 0 nan
23 1 52
I tried:
df[prev_value] = np.where(df[marker] == 1, df[Value].shift(), np.nan)
but that does not take conditional previous value like i want.
condition = (df.Marker.shift() == 0) & (df.Marker == 1)
df['Prev_Value'] = np.where(condition, df.Value.shift(), np.nan)
Output:
df
Value Marker Prev_Value
0 10 0 NaN
1 12 0 NaN
2 50 1 12.0
3 42 1 NaN
4 52 0 NaN
5 23 1 52.0
You could try this:
df['Prev_Value']=np.where(dataframe['Marker'].diff()==1,dataframe['Value'].shift(1, axis = 0),np.nan)
Output:
df
Value Marker Prev_Value
0 10 0 NaN
1 12 0 NaN
2 50 1 12.0
3 42 1 NaN
4 52 0 NaN
5 23 1 52.0
If you want to get the previous non-1 marker value, if marker==1, you could try this:
prevro=[]
for i in reversed(df.index):
if df.iloc[i,1]==1:
prevro_zero=df.iloc[0:i,0][df.iloc[0:i,1].eq(0)].tolist()
if len(prevro_zero)>0:
prevro.append(prevro_zero[len(prevro_zero)-1])
else:
prevro.append(np.nan)
else:
prevro.append(np.nan)
df['Prev_Value']=list(reversed(prevro))
print(df)
Output:
Value Marker Prev_Value
0 10 0 NaN
1 12 0 NaN
2 50 1 12.0
3 42 1 12.0
4 52 0 NaN
5 23 1 52.0
marker == 0 if the current value marker == 1?. At index[3] you current marker value is 1 and previous row's marker value is 1, not 0.