Question: I have dataframe like below :-

```
x<-c(3,2,1,8,7,11,10,9,7,5,4)
y<-c("a","a","a", "b","b","c","c","c","c","c","c")
z<-c(2,2,2,1,1,3,3,3,3,3,3)
df<-data.frame(x,y,z)

df
    x y z
1   3 a 2
2   2 a 2
3   1 a 2
4   8 b 1
5   7 b 1
6  11 c 3
7  10 c 3
8   9 c 3
9   7 c 3
10  5 c 3
11  4 c 3

```

I want to select top n row for each group by column y where n is provided in column z. 
So the output should be like :

```
output:
       x   y  z
     1 3   a  2
     2 2   a  2
     3 8   b  1
     4 11  c  3
     5 10  c  3
     6 9   c  3

```

Answer: 
One approach with `data.table`:

```
library(data.table)
setDT(df)
df[,.(inc=seq_len(.N)<=z,x,z),by=.(y)][inc==T ,-2]
#   y  x z
#1: a  3 2
#2: a  2 2
#3: b  8 1
#4: c 11 3
#5: c 10 3
#6: c  9 3

```
I'm posting the solution I was looking for using dplyr. It is based on @HNSKD:

```
library(dplyr)
x<-c(3,2,1,8,7,11,10,9,7,5,4)
y<-c("a","a","a", "b","b","c","c","c","c","c","c")
z<-c(2,2,2,1,1,3,3,3,3,3,3)

df<-data.frame(x,y,z)

df %>% group_by(y) %>% slice(1:2)

```

Which returns the first two elements for each `y`:

```
# A tibble: 6 x 3
# Groups:   y [3]
      x y         z
  <dbl> <fct> <dbl>
1     3 a         2
2     2 a         2
3     8 b         1
4     7 b         1
5    11 c         3
6    10 c         3

```