Iterating over each value in column go get 5 previous values in same column-CodePudding

Lets say i have dataframe:

index	x
0	5
1	20
2	25
3	50
4	10
5	11
6	18
7	28
8	22
9	55
10	40

And I want to get previous 5 values for every value/index in column x so expected output would be:

index	x	expected_output
0	5	[null null null null null ]
1	20	null null null null 5
2	25	null null null 5 20
3	50	null null 5 20 25
4	10	null 5 20 25 50
5	11	5 20 25 50 10
6	18	20 25 50 10 11
7	28	25 50 10 11 18
8	22	50 10 11 18 28
9	55	10 11 18 28 22
10	40	11 18 28 22 55

I have used this iloc

for index, row in df.iterrows(): 
    print(index,row['x'],df.iloc[-6:-1,2].values)

Is there any similar iloc that can solve this method?

CodePudding user response：

Use strides with append missing values:

n = 5
x = np.concatenate([[np.nan] * (n), df['x']])

def rolling_window(a, window):
    shape = a.shape[:-1]   (a.shape[-1] - window   1, window)
    strides = a.strides   (a.strides[-1],)
    return np.lib.stride_tricks.as_strided(a, shape=shape, strides=strides)

df['expected_output'] = rolling_window(x, n)[:-1].tolist()
print (df)
        x                 expected_output
index                                    
0       5       [nan, nan, nan, nan, nan]
1      20       [nan, nan, nan, nan, 5.0]
2      25      [nan, nan, nan, 5.0, 20.0]
3      50     [nan, nan, 5.0, 20.0, 25.0]
4      10    [nan, 5.0, 20.0, 25.0, 50.0]
5      11   [5.0, 20.0, 25.0, 50.0, 10.0]
6      18  [20.0, 25.0, 50.0, 10.0, 11.0]
7      28  [25.0, 50.0, 10.0, 11.0, 18.0]
8      22  [50.0, 10.0, 11.0, 18.0, 28.0]
9      55  [10.0, 11.0, 18.0, 28.0, 22.0]
10     40  [11.0, 18.0, 28.0, 22.0, 55.0]