CODEWITHSUNDEEP
r
Pronsh
Pronsh

Reputation: 11

Formulating a function in R

I am trying to create this function in R:

get_mpg(): given the name of a car, the type of speed, and the data frame of cars, this function returns the corresponding value of fuel-consumption (i.e. miles-per-gallon).

and this is all the data i have:

car_names <- c("mazda3", "civic", "focus", "prius", 
    "a6quattro", "tacoma", "camaro", "challenger")
speed <- c("city", "hwy")
mpg <- c(30, 41, 31, 41, 29, 40, 53, 46, 18, 
        28, 17, 21, 16, 24, 14, 23)
cars <- data.frame(car = car_names, speed = speed, mpg = mpg)

The function I've written is:

    get_mpg <- function(car_names, speed, frame)
    {
          subset_mpg <- subset(frame, 
                  cars == car_names, speed == speed)
          return(as.numeric(subset_mpg[, 3]))
    }

however when i am doing 

get_mpg("a6quattro", "hwy", cars)

I get 29 16 whereas i should be getting just 28.

Can someone please help me out and correct the code?

Upvotes: 1

Views: 83

Answers (2)

thelatemail
thelatemail

Reputation: 93938

A couple of issues here:

1.) You don't have a row with "a6quattro" and "hwy" at all, so you should be expecting no data returned.

2.) Using subset is causing dramas as speed==speed is comparing frame$speed==frame$speed rather than frame$speed==speed - it is not recommended to use subset for non-interactive coding purposes for this very reason.

3.) You need to combine your selections with & instead of separating them with a comma in subset anyway.

4.) Instead, use something like:

get_mpg2 <- function(car_names, speed, frame) {
  frame[frame$car %in% car_names & frame$speed == speed, "mpg"]
}

get_mpg2("a6quattro", "city", cars)
#[1] 29 16

Upvotes: 4

Chaos
Chaos

Reputation: 486

The problem is probably that your data frame is not exactly what you are expecting it to be. Here's what your data frame looks like:

> car_names <- c("mazda3", "civic", "focus", "prius", "a6quattro", "tacoma", "camaro", "challenger")
> speed <- c("city", "hwy")
> mpg <- c(30, 41, 31, 41, 29, 40, 53, 46, 18, 28, 17, 21, 16, 24, 14, 23)
> carsx <- data.frame(car = car_names, speed = speed, mpg = mpg)
> carsx
          car speed mpg
1      mazda3  city  30
2       civic   hwy  41
3       focus  city  31
4       prius   hwy  41
5   a6quattro  city  29
6      tacoma   hwy  40
7      camaro  city  53
8  challenger   hwy  46
9      mazda3  city  18
10      civic   hwy  28
11      focus  city  17
12      prius   hwy  21
13  a6quattro  city  16
14     tacoma   hwy  24
15     camaro  city  14
16 challenger   hwy  23

As you can see, every car does not get a corresponding value for "city" and "hwy". For instance, mazda3 gets two instances of "city"; civic gets two instances of "hwy". The car in question, a6quattro, has two instances of "city" and no "hwy", which is probably what's causing subset to misbehave.

If you subset using "[" as shown below, you get the right answer (which is to get nothing since the data is incorrect).

#You can replace "a6quattro" and "hwy" with arguments passed to a function
carsx[carsx$car == "a6quattro" & carsx$speed == "hwy", "mpg"]
numeric(0)

Once you correct your data frame, this problem should get resolved.

Upvotes: 2

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