Remove rows where all values of a column are identical, based on another column [duplicate]

Answer 1

Another data.table solution using .SD magic variable.

df <- data.table(id = c(1,1,2,2,3,3), info=c(0,0,0,10,20,20))

df[,.SD[uniqueN(.SD)>1],id]

    id info
1:  2    0
2:  2   10

Answer 2

A base R solution,

df[!with(df, ave(info, id, FUN = function(i)var(i) == 0)),]
#slightly different syntax (as per @lmo)
#df[ave(df$info, df$id, FUN=var) > 0,]

which gives,

  id info
3  2    0
4  2   10

Answer 3

Here is a solution with data.table:

library("data.table")
DT <- fread(
"id    info
1     0
1     0
2     0
2     10
3     20
3     20")
DT[, .N, .(id, info)][N==1, .(id, info)]
# > DT[, .N, .(id, info)][N==1, .(id, info)]
#    id info
# 1:  2    0
# 2:  2   10

a variant:

DT[, if (.N==1) TRUE, .(id, info)][, .(id, info)]

Here is a solution using an anti-join:

DT[!DT[duplicated(DT)], on=names(DT)]