Subsetting and Ordering in R

Question

I have a random created dataframe named a:

  group marks  upd
1     a     2   up
2     b     3   up
3     a     4 down
4     b     5   up
5     a     6 down
6     c     7   up
7     a     8 down

I wanted to subset it with only group='a' and marks in decreasing order of a. I used this query but the marks are not arranged in decreasing order. What is the error in my code?

Here is my code and output:

> a[a$group=="a" & a$marks[order(a$marks,decreasing=T)],]
  group marks  upd
1     a     2   up
3     a     4 down
5     a     6 down
7     a     8 down

Warning message:
In a$group == "a" & order(a$marks, decreasing = TRUE)[1:3] :
  longer object length is not a multiple of shorter object length

Answer 1

a$group=="a" gives

# [1]  TRUE FALSE  TRUE FALSE  TRUE FALSE  TRUE

and a$marks[order(a$marks,decreasing=T)] gives

# [1] 8 7 6 5 4 3 2

Combining those to using a logical and (&) does not make much sense here. TRUE & 8 is TRUE - the subsetting may accidentally work. But you want order to look at your subset only, not at all rows in a.

Try for instance

newa <- a[a$group=="a", ] 
newa[order(newa$marks, decreasing = TRUE), ]
#   group marks  upd
# 7     a     8 down
# 5     a     6 down
# 3     a     4 down
# 1     a     2   up

or, more convenient and readable:

library(dplyr)
a %>% filter(group=="a") %>% arrange(desc(marks))

Answer 2

without Dplyr, you could use

b<- a[a$group=="a",]       
c<- b[b$marks[order(b$marks,decreasing=T),]  

this because you are referring to the order of $b$ inherited from a, and you can not use the ranking of the items of the list a to indicate where the rows of b should go.

If you need the first three items in there, take

head(c,3)