Say I have this dataset
test <- data.table(X = rep(1, 3), Y = rep("a", 3))
which gives us
test
# X Y
#1: 1 a
#2: 1 a
#3: 1 a
I'm wondering why
test[, X[Y == "a"], keyby = .(X)]
gives
# X V1
#1: 1 1
#2: 1 NA
#3: 1 NA
Thank you in advance for your answers!
If you run X and Y=="a" separately
> test[, X, keyby = .(X)]
X X
1: 1 1
> test[, Y == "a", keyby = .(X)]
X V1
1: 1 TRUE
2: 1 TRUE
3: 1 TRUE
you will see that, the first one gives numeric value 1 of length 1, and the second one gives logical values TRUE of length 3.
Since you don't have matched lengths for subsetting, you will obtain NAs to fill in the corresponding places, e.g.,
> 1[rep(TRUE,3)]
[1] 1 NA NA
It returns 2 in uniqueN because there are two values - 1) the 'X' grouping value 1 and the NA filled up. We could use na.rm = TRUE in uniqueN
test[, uniqueN(X[Y == "a"], na.rm = TRUE), keyby = .(X)]
# X V1
#1: 1 1
As mentioned in @ThomasIsCoding post, it the mismatch in length between the logical vector and the length of grouping variable (which returns length 1) cause the filling of additional TRUE positions with NA. An option would be to replicate
test[, rep(X, .N)[Y == "a"], keyby = .(X)]
# X V1
#1: 1 1
#2: 1 1
#3: 1 1
Well, its complicated, in a way.
It has to do with what X is inside a grouping.
Consider these variations:
| description | expression |
|---|---|
| Yours | test[, X[Y == "a"], keyby=.(X) ] |
| X only | test[, X, keyby=.(X) ] |
| Y=="a" only | test[, Y == "a", keyby=.(X) ] |
X only gives:
> test[, X, keyby=.(X) ]
X X
1: 1 1
This is what 'X' is inside your grouping. Only that one value.
The third expression:
> test[, Y == "a", keyby=.(X) ]
X V1
1: 1 TRUE
2: 1 TRUE
3: 1 TRUE
There you see what Y == "a" looks like inside your grouping.
If you combine these, to do: X[ Y == "a" ] inside your grouping, you effectively do:
X <- 1
X[ c(TRUE,TRUE,TRUE) ]
X having only one value, but are asked to return the first, second and third values, will give you its one value and 2 NA's, which is what you see.