How to find values shared between groups in a data frame? [duplicate]

Answer 1

Using base R you can use table:

as.numeric(colnames(a<-table(df))[colSums(a>0)==nrow(a)])
[1] 10 15

you can also do:

df %>%
   mutate(s = val %in% as.numeric(colnames(a<-table(df))[colSums(a>0)==nrow(a)]))
   exp val     s
1    A  10  TRUE
2    A  20 FALSE
3    A  15  TRUE
4    A  10  TRUE
5    B  10  TRUE
6    B  15  TRUE
7    B  99 FALSE
8    B   2 FALSE
9    C  15  TRUE
10   C  20 FALSE
11   C  10  TRUE
12   C   4 FALSE