KEY VERSION

This homework assignment will test your overall R know-how, and allow you to practice using RMarkdown.

Complete the following tasks in a new R markdown script in R studio. For each question, first give a brief description (e.g., “Question 1: calculate the lowest miles/gallon in mtcars”), and then provide R code using "```" and the echo = TRUE argument. When finished, Knit your script together into an html report.

Write code that completes the following tasks:

(1) What is the the lowest miles/gallon value listed in the the built in mtcars dataset? Print this value to the screen.
min(mtcars$mpg)
## [1] 10.4
# or
min(mtcars[,1])
## [1] 10.4
(2) Using the mtcars dataset, make a new dataframe called cars.new that has only two columns: the miles per gallon and the number of cylinders.
cars.new <- data.frame(mtcars$mpg,mtcars$cyl)
# or
cars.new <-mtcars[,1:2]
(3) Write some code to sum the number of datapoints with ≥20mpg? (It’s kind of fun, because they can use a sum of booleans)
sum(cars.new$mpg>=20)
## [1] 14
(4) In your new cars.new object, coerce the cyl column into a factor.
cars.new$cyl <-as.factor(cars.new$cyl)
(5) Make a histogram (use = hist()) for mpg in cars.new.
hist(cars.new$mpg)

(6) Write a new function called myFunc that calculates the min, max, and mean of a vector, and returns those values in a new vector.
myfunc <- function(xvec){
  xmin <- min(xvec)
  xmax <- max(xvec)
  xmean <- mean(xvec)
  return(c(xmin,xmax,xmean))
}
(7) Use your function to calculate the min, max, and mean of the first column in the mtcars dataset (mpg).
myfunc(mtcars[,1])
## [1] 10.40000 33.90000 20.09062
(8) Write a for loop that uses your new function to caclulate the min, max, and mean for each column in the mtcars dataset, printing the answer to the screen as it goes.
for (i in 1:ncol(mtcars)){
  print(myfunc(mtcars[,i]))
}
## [1] 10.40000 33.90000 20.09062
## [1] 4.0000 8.0000 6.1875
## [1]  71.1000 472.0000 230.7219
## [1]  52.0000 335.0000 146.6875
## [1] 2.760000 4.930000 3.596563
## [1] 1.51300 5.42400 3.21725
## [1] 14.50000 22.90000 17.84875
## [1] 0.0000 1.0000 0.4375
## [1] 0.00000 1.00000 0.40625
## [1] 3.0000 5.0000 3.6875
## [1] 1.0000 8.0000 2.8125
(9) Use the function apply to calculate the min, max, and mean of each column in mtcars using your new function myfunc.
apply(mtcars,2,myfunc)
##           mpg    cyl     disp       hp     drat      wt     qsec     vs      am
## [1,] 10.40000 4.0000  71.1000  52.0000 2.760000 1.51300 14.50000 0.0000 0.00000
## [2,] 33.90000 8.0000 472.0000 335.0000 4.930000 5.42400 22.90000 1.0000 1.00000
## [3,] 20.09062 6.1875 230.7219 146.6875 3.596563 3.21725 17.84875 0.4375 0.40625
##        gear   carb
## [1,] 3.0000 1.0000
## [2,] 5.0000 8.0000
## [3,] 3.6875 2.8125