bikhaab - 8 months ago 18

Python Question

I have a data frame like this that want to apply diff function on:

`test = pd.DataFrame({ 'Observation' : ['0','1','2',`

'3','4','5',

'6','7','8'],

'Value' : [30,60,170,-170,-130,-60,-30,10,20]

})

Observation Value

0 30

1 60

2 170

3 -170

4 -130

5 -60

6 -30

7 10

8 20

The column 'Value' is in degrees. So, the difference between

`-170`

`170`

`20`

`-340`

`d2*d1 < 0`

`d2-d1`

`360-(abs(d1)+abs(d2))`

Here's why I try. But then I don't know how to continue it without using a for loop:

`test['Value_diff_1st_attempt'] = test['Value'].diff(1)`

test['sign_temp'] = test['Value'].shift()

test['Sign'] = np.sign(test['Value']*test['sign_temp'])

Here's what the result should look like:

`Observation Value Delta_Value`

0 30 NAN

1 60 30

2 170 110

3 -170 20

4 -130 40

5 -60 70

6 -30 30

7 10 40

8 20 10

Eventually I'd like to get just the magnitude of differences all in positive values. Thanks.

Update: So, the value results are derived from

`math.atan2`

`0<theta<180`

`-180<theta<0`

`170`

`-170`

`20`

`-30`

`10`

`40`

Answer

I believe this should work (took the definition from @JasonD's answer):

```
test["Value"].rolling(2).apply(lambda x: 180 - abs(abs(x[0] - x[1]) - 180))
Out[45]:
0 NaN
1 30.0
2 110.0
3 20.0
4 40.0
5 70.0
6 30.0
7 40.0
8 10.0
Name: Value, dtype: float64
```

How it works:

Based on your question, the two angles a and b are between `0`

and `+/-180`

. For `0 < d < 180`

I will write `d < 180`

and for `-180 < d < 0`

I will write `d < 0`

. There are four possibilities:

`a < 180`

,`b < 180`

-> the result is simply`|a - b|`

. And since`|a - b| - 180`

cannot be greater than 180, the formula will simplify to`a - b`

if`a > b`

and`b - a`

if`b > a`

.`a < 0`

,`b < 0`

- > The same logic applies here. Both negative and their absolute difference cannot be greater than 180. The result will be`|a - b|`

.`a < 180`

,`b < 0`

- > a - b will be greater than 0 for sure. For the cases where`|a - b| > 180`

, we should look at the other angle and this translates to`360 - |a - b|`

.`a < 0`

,`b < 180`

-> again, similar to the above. If the absolute difference is greater than 180, calculate 360 - absolute difference.

For the pandas part: `rolling(n)`

creates arrays of size n. For 2: (row 0, row1), (row1, row2), ... With `apply`

, you apply that formula to every rolling pair where `x[0]`

is the first element (a) and `x[1]`

is the second element.