sum of a subset of rows in a data table

Asked Nov 14 '18 at 14:33

Active Nov 14 '18 at 14:33

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I have a little problem...let's say I have a data.table with one numerical column like:

I want to create a new column which is computed in this way: in row j I want the sum of NR in the rows j, j+1, j+2. So I want this result:

  NR    NEW_NR
  1     6
  2     10
  3     15
  5     22
  7     18
  10    11
  1     1

Could anyone help me pls?

asked Nov 14 '18 at 14:33

Bmb58

1

have a look at `shift` or rolling sum functions from other packages. – mt1022 Nov 14 '18 at 14:36
@mt1022 thank you, maybe this is what I need! – Bmb58 Nov 14 '18 at 14:38
`dt[, S := Reduce(\`+\`, shift(NR, 0:2, type = "lead"))]` – Martin Schmelzer Nov 14 '18 at 14:39
@Jimbou you are right, I'll delete asap the question! – Bmb58 Nov 14 '18 at 14:44

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