R：按因子细分数据

Question

assume we have the following data frame

foo
  k h=1 h=2 h=3
1 3   3   6   9
2 2   2   5   8
3 1   1   4   7

with

str(check)
'data.frame':   3 obs. of  4 variables:
 $ k  : Factor w/ 3 levels "3","2","1": 1 2 3
 $ h=1: int  3 2 1
 $ h=2: int  6 5 4
 $ h=3: int  9 8 7

How can I subset my dataframe based on the factor of k ? For instance, to get only the row for k=3 or all rows k<3. I tried working with subet(foo, k=3) but it doesn't work. I also tried to convert the column k to numeric, but then my data.frame loses its order. It's important that the data is of descending order with regard to k (so 3, 2, 1)

Answer 1

Bracket notation should be able to subset on factors without any problems:

# Returns all rows of foo where k == '3'
foo[foo$k == '3',]

Two possible problems with what you did before:

1) subset(foo, k=3) should be subset(foo, k==3) , don't confuse the equality operator ( == ) with the assignment operator ( = )

2) Since you're comparing with the actual level of your factor, you should check for equality with the character '3' instead of the numeric 3 . You can see in the output from str() that k's levels are "3","2","1" , with quotes, whereas the integers for the other variables are shown without quotes 3 2 1