r - Extracting a random sample of rows in a data.frame with a nested conditional

This question builds from the SO post found here and uses code that was modified from a post on the R-help mailing list which can be seen here

I am trying to extract a random sample of rows in a data frame but with a conditional. Using the R iris data which looks like:

> head(iris)
  Sepal.Length Sepal.Width Petal.Length Petal.Width Species
1          5.1         3.5          1.4         0.2  setosa

2          4.9         3.0          1.4         0.2  setosa
3          4.7         3.2          1.3         0.2  setosa
4          4.6         3.1          1.5         0.2  setosa
5          5.0         3.6          1.4         0.2  setosa
6          5.4         3.9          1.7         0.4  setosa 

To take a simple random sample, the code below works fine to take a sample of 2 rows.

iris[sample(nrow(iris), 2), ]

However I am unsure how to condition the Species field. For example how to take the random sample as indicated above but only when Species != “setosa”

There are three categories of iris$Species

> summary(iris$Species)
    setosa versicolor  virginica 
        50         50         50

I am unsure how to correctly nest conditionals. One of my earlier attempts is below with the obviously incorrect results included….

> iris[sample(nrow(iris)[iris$Species != "setosa"], 2), ]
     Sepal.Length Sepal.Width Petal.Length Petal.Width Species
NA             NA          NA           NA          NA    
NA.1           NA          NA           NA          NA    

Thanks

Answer

I'd use which to get the vector of rows numbers from which you can sample given your condition....

iris[ sample( which( iris$Species != "setosa" ) , 2 ) , ]
#    Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
#59           6.6         2.9          4.6         1.3 versicolor
#133          6.4         2.8          5.6         2.2  virginica