What does this warning message"recycled with remainder" mean In R?

This warning arises when R is given some form of list input to produce some form of list output but the length of the input and output lengths do not match. In these cases, R repeats and reuses the shorter list as appropriate.

See the documentation here: "If a list is supplied, each element is converted to a column in the data.table with shorter elements recycled automatically."

However, if the longer list is not a multiple of the length of the shorter list then the difference in length lists is unlikely to be deliberate, hence this warning.

Some examples:

> library(data.table)
> example1 = list(x = c(1,2,3,4,5,6), y = 3)
> as.data.table(example1)
   x y
1: 1 3
2: 2 3
3: 3 3
4: 4 3
5: 5 3
6: 6 3

> example2 = list(x = c(1,2,3,4,5,6), y = c('a','b'))
> as.data.table(example2)
   x y
1: 1 a
2: 2 b
3: 3 a
4: 4 b
5: 5 a
6: 6 b

In both of these examples, the x and y lists are of different lengths, so the y list has been repeated. This produces no warning as y is length 1 or 2 which is a multiple of x which is length 6.

> example3 = list(x = c(1,2,3,4,5,6), y = c('a','b','c','d'))
> as.data.table(example3)
   x y
1: 1 a
2: 2 b
3: 3 c
4: 4 d
5: 5 a
6: 6 b
Warning message:
In as.data.table.list(example3) :
  Item 2 has 4 rows but longest item has 6; recycled with remainder.

But this example x is length 6 and y is length 4. So only the first two terms of y have been repeated when creating the data table. R gives a warning as not every term in y has been reused the same number of times.

In your example instead of lengths 6 and 4, you have columns of length 1650197 and 1667524. As it is unlikely (but possible) that you wanted to repeat the first 17327 entries (1667524 - 1650197) of the shorter column, R gives you a warning to prompt to you check your input columns.