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

Question

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>
NA.1           NA          NA           NA          NA    <NA>

Thanks

Answer 1

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

Answer 2

With dplyr:

library(dplyr)
set.seed(12)
filter(iris, Species != "setosa") %>% sample_n(., 2) 

Output:

   Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
7           6.3         3.3          4.7         1.6 versicolor
81          7.4         2.8          6.1         1.9  virginica