Multiple pairwise differences based on column name patterns

Question

I have a data.table, dt:

dt

Id  v1 v2 v3 x1 x2 x3
1   7  1  3  5  6  8
2   1  3  5  6  8  5
3   3  5  6  8  5  1

v1, v2, v3 an

Answer 1

You can use set in the loop.

library(data.table)

DT <- fread('Id  v1 v2 v3 x1 x2 x3
1   7  1  3  5  6  8
2   1  3  5  6  8  5
3   3  5  6  8  5  1')

for (i in 1:3) {
  set(DT,j=paste0("Diff_",i),value = DT[[paste0("v",i)]]-DT[[paste0("x",i)]])
}

DT
#>    Id v1 v2 v3 x1 x2 x3 Diff_1 Diff_2 Diff_3
#> 1:  1  7  1  3  5  6  8      2     -5     -5
#> 2:  2  1  3  5  6  8  5     -5     -5      0
#> 3:  3  3  5  6  8  5  1     -5      0      5

^{Created on 2020-05-27 by the reprex package (v0.3.0)}