Steffen J. - 4 months ago 22

R Question

I'm grouping a data.table and want to select from each group the first row where x == 1 or, if such a row does not exist, then the first row with any value in x

`d <- data.table(`

a = c(1,1,1, 2,2, 3,3),

x = c(0,1,0, 0,0, 1,1),

y = c(1,2,3, 1,2, 1,2)

)

this attempt

`d[, ifelse(any(.SD[,x] == 1),.SD[x == 1][1], .SD[1]), by = a]`

returns

`a V1`

1: 1 1

2: 2 0

3: 3 1

but i expected

`a x y`

1: 1 1 2

2: 2 0 1

3: 3 1 1

Any ideas how to get it right?

Answer

I think it's a good use case for both `match`

*and* it's `nomatch`

argument

```
d[, .SD[match(1L, x, nomatch = 1L)], by = a]
# a x y
# 1: 1 1 2
# 2: 2 0 1
# 3: 3 1 1
```

This is basically, in case of no-match, returns `1`

, and as a result gives you the first row in the group. If there is a multiple match, then it will return the first one- as per your desire