In the exercises below we cover the basics of functional programming in R( part 1 of a two series exercises on functional programming) . We consider recursion with R , apply family of functions , higher order functions such as Map ,Reduce,Filter in R .
Answers to the exercises are available here.
If you obtained a different (correct) answer than those listed on the solutions page, please feel free to post your answer as a comment on that page.
Exercise 1
Create a function which calculates factorial of a number with help of Reduce ,
Exercise 2
Create the same function which calculates factorial but with recursion and memoization. :
Exercise 3
Create a function cum_add which makes cumulative summation for e.g if x <- 1:3 cum_add(x) will result in 1 3 6 .Don’t use cumsum .
Exercise 4
Create a function which takes a dataframe and returns mean , minimum and maximum of all numeric columns . Your function should take a dataframe as input .for e.g your_func(iris).Try to avoid loops ,its possible to make it in one line .swag your functional skills .
Exercise 5
Create a function centerColumnAroundMean which takes a numeric dataframe and manipulates the dataframe such a way that all column’s values are centered with mean value of the column . For example if my data frame is like this
df <- data.frame(x= 1:5,y=6:10,c=11:15)
then centerColumnAroundMean(df) will result in
It is possible to achieve this by single line of R , Please dont use loops .
Exercise 6
I have list of movies ,which have two movie franchieses as the elements ,Starwars and LOTR.You can create the movie list by
my_movie_list<- list(STARWARS= list("A NEW HOPE","The Last Jedi","The Force Awakens"),LOTR=list("THE FELLOWSHIP OF THE RING","THE Two Towers","The RETURN of the KING","Hobbit" = list("An unexpected Journey","The Battle of the FIVE ARMY","The Desolation of Smaug") ))
now the problem I am facing is some of the texts are in caps and some of them are in small ,without any particular order . I would like the list to have a format like
“The Last Jedi” , Help me in writing a function which will do the same . Please keep in mind that the list is a nested list .
Exercise 7
Load the diamonds data set from ggplot 2 package , I want to buy a diamond of each color but don’t want to pay the top price , I believe that the second highest price is good enough for me . Help me in finding the second highest price for each color from the dataset.
Exercise 8
Use the already loaded diamonds dataset from previous exercise , I want to know the average price for each combination of cut and color .Your output should be similar to following.Don’t use table. 
Exercise 9
Load the iris dataset , I want to get the third row for each group of species . Help me to write a short function for me to achieve that .
Exercise 10
Create a new environment with new.env() command and create 3 vector variables under that environment as a=1:10;b=100:500;c=1000:1500
without knowing or manually calling mean for all the vector variables can you print the mean of all variables of the new environment .
while functional, this is a nice, simple approach to centering columns around their mean:
scale(x, scale= False)
http://gastonsanchez.com/visually-enforced/how-to/2014/01/15/Center-data-in-R/