11.3 Exercise 6.

Create the script “exercise6.R” and save it to the “Rcourse/Module2” directory: you will save all the commands of exercise 6 in that script.
Remember you can comment the code using #.

correction

getwd()
setwd("Rcourse/Module2")
setwd("~/Rcourse/Module2")

11.3.1 Exercise 6a. Input / output

1- Download folder “i_o_files” in your current directory with:

# system invokes the OS command specified by the "command" argument.
system(command="svn export https://github.com/sarahbonnin/CRG_RIntroduction/trunk/i_o_files")

All files that will be used for exercise 6 are found in the i_o_files folder !

2- Read in the content of ex6a_input.txt using the scan command; save in object z

How many elements are in z?

correction

# scan content of the file
z <- scan("i_o_files/ex6a_input.txt")
# number of elements (length of vector)
length(z)

3- Sort z: save sorted vector in object “zsorted”.

correction

zsorted <- sort(z)

4- Write zsorted content into file ex6a_output.txt.

correction

write(zsorted, "ex6a_output.txt")

5- Check the file you produced in the RStudio file browser (click on the file in bottom-right panel “Files” tab). Save the content of zsorted again but this time setting the argument “ncolumns” to 1: how is the file different?

correction

write(zsorted, "ex6a_output.txt", ncolumns=1)

11.3.2 Exercise 6b - I/O on data frame: play with the arguments of read.table

1- field separator

• Read ex6b_IO_commas_noheader.txt in object fs. What are the dimensions of fs?

correction

# read in file with default parameters
fs <- read.table("i_o_files/ex6b_IO_commas_noheader.txt")
dim(fs)

• Fields/columns are separated by commas: change the default value of the “sep” argument and read in the file again. What are now the dimensions of fs?

correction

# change field separator to ","
fs <- read.table("i_o_files/ex6b_IO_commas_noheader.txt", 
    sep=",")
dim(fs)

2- field separator + header

• Read ex6b_IO_commas_header.txt in object fs_c. What are the dimensions of fs_c ?

correction

fs_c <- read.table("i_o_files/ex6b_IO_commas_header.txt")
dim(fs_c)

• Check head(fs_c) and change the default field separator to an appropriate one.

correction

fs_c <- read.table("i_o_files/ex6b_IO_commas_header.txt", 
                   sep=",")

• The first row should to be the header (column names): change the default value of the header parameter and read in the file again. What are now the dimensions of fs_c ?

correction

fs_c <- read.table("i_o_files/ex6b_IO_commas_header.txt", 
                   sep=",", 
                   header=TRUE)

3- skipping lines

• Read ex6b_IO_skip.txt in object sk.

correction

sk <- read.table("i_o_files/ex6b_IO_skip.txt")

Is R complaining ?

Check “manually” the file (in the R Studio file browser).

• The skip argument allows you to ignore one or more line(s) before reading in a file. Introduce this argument with the appropriate number of lines to skip, and read the file again.
  

correction

sk <- read.table("i_o_files/ex6b_IO_skip.txt",
                 skip=2)
dim(sk)

• Is R still complaining? What are now the dimensions of sk ?

• Change the default field separator. What are now the dimensions of sk ?

correction

sk <- read.table("i_o_files/ex6b_IO_skip.txt",
                 skip=2,
                 sep=",",
                 header=T)

4- Comment lines

• Read ex6b_IO_comment.txt in object cl.

correction

cl <- read.table("i_o_files/ex6b_IO_comment.txt")

Is R complaining again ? Check manually the file and try to find out what is wrong…

What os the comment.char argument used for ? Adjust the comment.char argument and read the file again.

correction

cl <- read.table("i_o_files/ex6b_IO_comment.txt",
                 comment.char = "*")

• Adjust also the header and sep arguments to read in the file correctly. What are now the dimensions of cl?

correction

cl <- read.table("i_o_files/ex6b_IO_comment.txt",
                 comment.char = "*",
                 sep=",",
                 header=TRUE)
dim(cl)

4- final

• Read ex6b_IO_final.txt in object fin.

correction

fin <- read.table("i_o_files/ex6b_IO_final.txt")

• Adjust the appropriate parameters according to what you have learnt, in order to obtain the data frame “fin” of dimensions 167 x 4.

correction

fin <- read.table("i_o_files/ex6b_IO_final.txt",
                  sep=",",
                  header=TRUE,
                  skip=3,
                  comment.char="&"
                  )

11.3.3 Exercice 6c - I/O on a data frame

1- Read in file ex6c_input.txt in ex6 object

Warning: the file has a header !
Check the structure of ex6 (remember the str command).

correction

ex6 <- read.table("i_o_files/ex6c_input.txt", 
                  header=TRUE)
str(ex6)

2- Now read in the same file but, this time, set the argument as.is to TRUE.

Check again the structure: what has changed ?

correction

ex6 <- read.table("i_o_files/ex6c_input.txt", 
                  header=TRUE,
                  as.is=TRUE)
str(ex6)

3- What are the column names of ex6 ?

correction

colnames(ex6)

4- Change the name of the first column of ex6 from “State” to “Country”.

correction

# extract all column names of ex6
colnames(ex6)
# extract the name of the first column only
colnames(ex6)[1]
# reassign name of the first column only
colnames(ex6)[1] <- "Country"

5- How many countries are in the Eurozone, according to ex6 ?

Remember the table function.

correction

table(ex6$Eurozone)

6- In the Eurozone column: change “TRUE” with “yes” and “FALSE” with “no”.

correction

# select the Eurozone column
ex6$Eurozone
# elements of the Eurozone column that are exactly TRUE
ex6$Eurozone==TRUE
# extract actual values that are TRUE
ex6$Eurozone[ex6$Eurozone==TRUE]
# reassign all elements that are TRUE with "yes"
ex6$Eurozone[ex6$Eurozone==TRUE] <- "yes"
# same with FALSE
ex6$Eurozone[ex6$Eurozone==FALSE] <- "no"

7- In the column Country: how many country names from the list contain the letter “c” (capital- or lower-case) ?

Remember the grep function. Check the help page.

correction

# country names with "c" (lower-case)
grep("c", ex6$Country)
# country names with "c" or "C" (ignoring case)
grep("c", ex6$Country, ignore.case = TRUE)
# show actual country names
grep("c", ex6$Country, value=TRUE, ignore.case = TRUE)

8- According to that data frame: how many people live: + in the European union (whole table) ? + in the Eurozone ?

correction

# sum the whole population column
sum(ex6$Population)
# select elements of ex6 where Eurozone is "yes"
ex6$Eurozone == "yes"
# select only elements in Population for which the corresponding Eurozone elements are "yes"
ex6$Population[ex6$Eurozone == "yes"]
# sum that selection
sum(ex6$Population[ex6$Eurozone == "yes"])

9- Write ex6 into file ex6c_output.txt

After each of the following steps, check the output file in the RStudio file browser (lower-right panel).

• Try with the default arguments.

correction

write.table(ex6, file="ex6c_output.txt")

• Add the argument “row.names” set to FALSE.

correction

write.table(ex6, file="ex6c_output.txt", 
            row.names = FALSE)

• Add the argument “quote” set to FALSE.

correction

write.table(ex6, file="ex6c_output.txt", 
            row.names = FALSE,
            quote = FALSE)

• Add the argument “sep” set to “ or to”,"

correction

write.table(ex6, file="ex6c_output.txt", 
            row.names = FALSE,
            quote = FALSE,
            sep="\t")

correction

write.table(ex6, file="ex6c_output.txt", 
            row.names = FALSE,
            quote = FALSE,
            sep=",")