Answers of Pet Exercise

Here is a data.table contains four pets’ name, species and weight:

library(data.table)
pets = data.table(name = c("Senor", "Cookie", "Lucky", "Meow Meow"),
                  species = c("dog", "dog", "bird", "cat"),
                  weight = c(26, 13, 0.5, 6))

Please:

1. Create a vector called pet_weight contains all pets’ name and

pet_weight = pets[, .(name, weight)]
pet_weight

##         name weight
## 1:     Senor   26.0
## 2:    Cookie   13.0
## 3:     Lucky    0.5
## 4: Meow Meow    6.0

2. Create a unit vector senor_weight contains Senor’s weight

senor_weight = pets[name == "Senor", "weight"]
senor_weight

## [1] "weight"

3. Create a data.table called dogs_weight that contains name and weight of all dogs in pets

dogs_weight = pets[species == "dog", .(name, weight)]
dogs_weight

##      name weight
## 1:  Senor     26
## 2: Cookie     13

4. Modify Cookie’s weight as 15

pets[name == "Cookie", weight := 15]
pets

##         name species weight
## 1:     Senor     dog   26.0
## 2:    Cookie     dog   15.0
## 3:     Lucky    bird    0.5
## 4: Meow Meow     cat    6.0

5. Create a data.table called pet_stat that contains species, avg_weight and count

pet_stat = pets[, .(avg_weight = mean(weight), count = .N), by = "species"]
pet_stat

##    species avg_weight count
## 1:     dog       20.5     2
## 2:    bird        0.5     1
## 3:     cat        6.0     1