Seasonal Differences at Lag m

Problem

Compute the seasonal difference of a series at lag m, given by x[t] - x[t-m] for every t from m to n-1. Differencing at the seasonal lag removes a repeating pattern of period m.

Return an empty list when m is not positive or m is at least the length of the series.

Hint

With m = 1 this reduces to ordinary first differences.

Test cases

case 1

in: [[1,2,3,10,20,30],3]

expected: [9,18,27]

case 2

in: [[5,7,9],1]

expected: [2,2]

case 3

in: [[1,2],2]

expected: []

case 4

in: [[1,2,3],0]

expected: []

Starter codePython

def seasonal_differences(series, m):
    # Your code here
    pass

4 test cases · run your code against them in the editor