Beginning R

sounds <- c(
    "cat"="meow", 
    "dog"="woof", 
    "horse"="neigh"
)

dog_sound <- sounds["dog"]
horse_sound <- sounds["horse"]

print(paste("Dog goes", dog_sound))
print(paste("Horse goes", horse_sound))

Rscript dict.R

[1] "Dog goes woof"
[1] "Horse goes neigh"

If we edit our script so that it asks for a key that doesn’t exist, we will see that is returns NA.

sounds <- c(
    "cat"="meow",
    "dog"="woof", 
    "horse"="neigh"
)

fish_sound <- sounds["fish"]

print(paste("Fish goes", fish_sound))

Rscript dict.R

[1] "Fish goes NA"

As with lists, returning NA for a non-existant value can be a major source of bugs. You can test if a name (key) exists in a dictionary by using the %in% keyword, to see if the name is in the names() of the dictionary;

sounds <- c(
    "cat"="meow",
    "dog"="woof",
    "horse"="neigh"
)

if ("fish" %in% names(sounds)){
    fish_sound <- sounds["fish"]
    print(paste("Fish goes", fish_sound))
} else {
    print("What sounds does a fish make?")
}

Rscript dict.R

[1] "What sounds does a fish make?"