Extracting multiple values
Consider the following example. We have a set of numbers:
numbers <- c(-4, 6, -3, 5, -2, 4, -1, 3)
Imagine that we want a subset to get the positive numbers only. You can do this with defining a vector of the indices that point to the numbers you need.
indices <- c(2, 4, 6, 8)
We can already make this subsetting with a for() loop:
# the subset a container
sub <- NULL
for(i in indices){
# the currently extracted value
current <- numbers[i]
# add to the container
sub <- c(sub, current)
}
sub
[1] 6 5 4 3
This kind of subsetting is extremely useful, and is very frequently used - as a matter of fact, so frequently, that we never use it the way it is written above. We rather used a much simplified form: the entire the construct can be expressed with this single line of code:
numbers[indices]
[1] 6 5 4 3