Test if character values are all equal by group for data.table-CodePudding

Here's the problem. How does one test if all the values for each group have the same chaarcter values in a column.

For example

DTT<-data.table(id=rep(seq(1:1000), each = 1000), CHAR=rep("A",10000), key="id")

This would be true because all values for the column CHAR are the same for all ids

CodePudding user response：

We could use uniqueN by 'id'

DTT[,uniqueN(CHAR, na.rm = TRUE) == 1, id]

If we need to subset the 'id' having only single 'CHAR'

DTT[DTT[,.I[uniqueN(CHAR, na.rm = TRUE) == 1], id]$V1]

Or may also do

DTT[, if(uniqueN(CHAR, na.rm = TRUE) == 1) .SD, id]

CodePudding user response：

We can also try unique .N like below

> unique(DTT)[, .N == 1, id]
        id   V1
   1:    1 TRUE
   2:    2 TRUE
   3:    3 TRUE
   4:    4 TRUE
   5:    5 TRUE
  ---
 996:  996 TRUE
 997:  997 TRUE
 998:  998 TRUE
 999:  999 TRUE
1000: 1000 TRUE