Appendix B — B. DO NOT use parentheses when they are clearly not necessary.

#####################################################################.
# In general, DO NOT use parentheses when they are clearly not necessary. ####
# It shows poor knowledge of the R language. 
#####################################################################.

# Don't do this! - you never need parentheses around a single value
seq((1), (3))  

[1] 1 2 3

# Don't do this! - unnecessary parentheses
(seq(1,3))   

[1] 1 2 3

# Don't do this! - you never need parentheses around a single argument
x=2
seq(1,(3+x))  # do not put parentheses around a single argument

[1] 1 2 3 4 5

# This is the way to do it.
seq(1 , 3+x)  # better!

[1] 1 2 3 4 5

#####################################################################.
# It is acceptable to use extra parentheses when they are used to 
# clarify the order of operations. Not everyone knows the order of 
# operations by heart and this can make your code more readable.
#####################################################################.

# For example:

10-(2:10)

[1] 8 7 6 5 4 3 2 1 0

# is very different from 

(10-2):10

[1]  8  9 10

# The following (without the parentheses)
10-2:10

[1] 8 7 6 5 4 3 2 1 0

# is the same as 10-(2:10) as the order of operations lists the
# colon operator (:) before the subtraction operator (-)
# ( see ?Syntax )
#
# Therefore, I'd accept 10-(2:10) even thought strictly speaking the 
# parentheses are not necessary as the parentheses help to clarify the
# order of operations.


############################################################################.
# QUESTION: The vector, x, contains at lest 3 values.
#           Write a command to show the last 3 values in the vector.
#           Your code must work no matter how many values are in the vector.
#
# Example 1: 
#   > x = c("anne","bob","carla","dave","ed","fran","george","hugh","ike","jill")
#   YOUR CODE GOES HERE
#   [1] "hugh" "ike" "jill"
#
# Example 2: 
#   > x = seq(10,50,by=10)
#   YOUR CODE GOES HERE
#   [1] 30 40 50
############################################################################.


#------------.
# Answer: 
#------------.

# Example 1:
x = c("anne","bob","carla","dave","ed","fran","george","hugh","ike","jill")
x[ (length(x)-2):length(x) ]    # ANSWER

[1] "hugh" "ike"  "jill"

# Example 2: 
x = seq(10,50,by=10)
x[ (length(x)-2):length(x) ]    # ANSWER

[1] 30 40 50

#####################################################################.
# Don't use c(...) when it is not necessary  ####
#####################################################################.

# c() is only required to combine multiple vectors into a single vector.
# Do not use c for a single vector.

students = c("joe", "sue")   # c is required
c(students)     # c is not required - since there is only one vector - dont use c here

[1] "joe" "sue"

students

[1] "joe" "sue"

c(seq(1,10,by=2))  # c is not required because seq is already a vector! - don't use c here

[1] 1 3 5 7 9

seq(1,10,by=2)

[1] 1 3 5 7 9

c ( rep(10,3), rep(20, 2))  # c IS required because you're combining two different vectors

[1] 10 10 10 20 20