##################################################.# Indexing with NEGATIVE numbers# e.g. # stuff[-3]# stuff[c(-3,-5)]# stuff[-c(3,5)]# stuff[-seq(1,length(stuff),by=3)]##################################################.#-------------------------------------------------------------------------.# If the index is a negative number or a vector of negative numbers, # all except those positions will be accessed. #-------------------------------------------------------------------------.stuff <-c(10,20,30,40,50,60)stuff
stuff[ seq(-3, -length(stuff), by=-3) ] # same result
[1] 10 20 40 50
##################################################################.# QUESTION - part a## Write a command that displays the entire vector, x, except# for the first and last values of x.## EXAMPLE 1# > x = c(10,20,30,40,50)# > YOUR CODE GOES HERE# [1] 20 30 40## EXAMPLE 2# > x = c(100,30,999)# > YOUR CODE GOES HERE# [1] 30## DO THE ANSWER TWO DIFFERENT WAYS# - ONCE WITH POSITIVE INDEX NUMBERS# - ONCE WITH NEGATIVE INDEX NUMBERS##################################################################.
click here for answer with POSITIVE index numbers
#>>>>>>>>>>>>>>>>>>>>>>>>>>>>># ANSWER : Positive index value#>>>>>>>>>>>>>>>>>>>>>>>>>>>>># Example 1x =c(10,20,30,40,50)x = x[2:(length(x)-1)] # ANSWERx
[1] 20 30 40
# Example 2x =c(100,30,999)x = x[2:(length(x)-1)]x
[1] 30
click here for answer with NEGATIVE index numbers
#>>>>>>>>>>>>>>>>>>>>>>>>>>>>># ANSWER: Negative index value#>>>>>>>>>>>>>>>>>>>>>>>>>>>>># Example 1x =c(10,20,30,40,50)x = x[c(-1,-length(x))]x
[1] 20 30 40
# Example 2x =c(100,30,999)x = x[c(-1,-length(x))]x
[1] 30
##################################################################.# QUESTION - part b## Similar to previous question. However, this time, # define a FUNCTION named removeFirstAndLast that takes a single# argument named vec. The function should return the values in # vec but without the first and last values.## EXAMPLES# > removeFirstAndLast ( c(10,20,30,40,50) )# [1] 20 30 40## > removeFirstAndLast( c(100,30,999) )# [1] 30##################################################################.
##################################################################.# QUESTION - part c## Define a function named removeFirstLastFew that takes the following arguments## ARGUMENTS # vec - a vector that is expected to contain a few values# amount - the number of values to trim from the beginning and end of vec## You may assume that length(vec) is greater than or equal to amount * 2.### EXAMPLE A# > removeFirstLastFew ( vec = c(10,20,30,40,50,60,70), amount = 2)# [1] 30 40 50 ## EXAMPLE B# > removeFirstLastFew ( vec = c(90,100,80,65,92), amount = 1)# [1] 100 80 65 ## EXAMPLE C# > removeFirstLastFew ( vec = c(10,20,30,40,50,60,70), amount = 3)# [1] 40 ## DO THE ANSWER TWO DIFFERENT WAYS# - ONCE WITH POSITIVE INDEX NUMBERS# - ONCE WITH NEGATIVE INDEX NUMBERS##################################################################.
#>>>>>>>>>>>>>>>>>>>>>>>>>#> ANSWER#>>>>>>>>>>>>>>>>>>>>>>>>># Using positive index numbersremoveFirstLastFew =function( vec, amount) { firstFew =1:amount# Pay special attention to the "+1" in the next line of code.# To see why the +1 is needed try a few examples "by hand". lastFew = (length(vec)-amount+1):length(vec) vec[ c(-firstFew, -lastFew) ]}removeFirstLastFew(c(10,20,30,40,50,60,70), 3)
[1] 40
removeFirstLastFew(c(10,20,30,40,50,60,70), 2)
[1] 30 40 50
removeFirstLastFew(c(10,20,30,40,50,60,70), 1)
[1] 20 30 40 50 60
removeFirstLastFew(c(10,20,30,40,50,60,70), 4)
numeric(0)
##################################################################.# QUESTION - part a## Write a command that returns the contents of the vector, x,# in reverse order. Write the command so that it works# no matter how many elements the vector contains.## Your command must work UNCHANGED for both examples# (and all other examples that are not shown below).## EXAMPLE 1# > x = c(10,20,30,40,50)# > YOUR CODE GOES HERE# [1] 50 40 30 20 10## EXAMPLE 2# > x = c(100,30,999)# > YOUR CODE GOES HERE# [1] 999 30 100##################################################################.
click here for answer
# EXAMPLE 1 - Set up some valuesx =c(10,20,30,40,50)x[length(x):1] # **ANSWER**
[1] 50 40 30 20 10
# EXAMPLE 2 - Set up some valuesx =c(100,30,999)x[length(x):1] # **ANSWER**
[1] 999 30 100
##################################################################.# QUESTION - part b# # Same as above but this time make a function named myrev.# The myrev function should take a single argument named x.# The function return the reverse of x.## Note - there is a function in R named rev that does this. # your code should create a function named myrev that does the # same thing as the built in rev function.## HINTS # - your answer should use the length function# - your answer should use the : operator or the seq function## EXAMPLE 1# > myrev ( c(10,20,30,40,50) )# [1] 50 40 30 20 10## EXAMPLE 2# > myrev ( x=c(100,30,999) )# [1] 999 30 100##################################################################.
click here for answer
# ANY OF THE FOLLOWING FUNCITON DEFINTIONS WILL WORK JUST FINE. # **ANSWER**myrev =function( x ){ # **ANSWER** x[length(x):1] # **ANSWER**} # **ANSWER**myrev =function( x ){ # **ANSWER**return(x[length(x):1]) # **ANSWER**} # **ANSWER**myrev =function( x ){ # **ANSWER** answer = x[length(x):1] # **ANSWER**return(answer) # **ANSWER**} # **ANSWER**myrev =function( x ){ # **ANSWER** answer = x[length(x):1] # **ANSWER** answer # **ANSWER**} # **ANSWER**# EXAMPLE 1myrev ( c(10,20,30,40,50) ) # 50 40 30 20 10
[1] 50 40 30 20 10
# EXAMPLE 2myrev ( x=c(100,30,999) ) # 999 30 100
[1] 999 30 100
# DONT DO THIS!!!!# DO NOT RETURN THE VALUE OF AN ASSIGNMENT STATEMENT.# THE VALUE OF AN ASSIGNMENT STATEMENT IS "INVISIBLE"myrev =function( x ){ # **ANSWER** - DON'T DO THIS answer = x[length(x):1] # **ANSWER** - DON'T DO THIS} # **ANSWER** - DON'T DO THISmyrev ( c(10,20,30,40,50) ) # no results are displayed # **ANSWER** - DON'T DO THIS myrev ( x=c(100,30,999) ) # no results are displayed # **ANSWER** - DON'T DO THIS
##########################################################################.# QUESTION - part a# # Write code that displays every other value of x.# The same code should work for all values of x.## HINTS: # - Use the length function to make it work no matter how many values# are in x.## - use the seq function## EXMPLE 1## > x = c(100,200,300,400)# > YOUR CODE GOES HERE (must be same code as for EXAMPLE 2)# [1] 100 300## EXMPLE 2## > x = c(83,99,82,67,85,100,89)# > YOUR CODE GOES HERE (must be same code as for EXAMPLE 1)# [1] 83 82 85 89###########################################################################.
##################################################################.# QUESTION - part b## Write a command that returns two copies of every other element # that is in x, starting with the 1st. Write the command so that it will work# correctly no matter how many elements the vector contains.## HINTS # - Build on your from the previous part of this question## - You answer should use the rep function### EXAMPLE 1# > x = c(10, 20, 30, 40, 50)# > # YOUR CODE GOES HERE# [1] 10 10 30 30 50 50 ## EXAMPLE 2# > x = c(92, 100, 79)# > # YOUR CODE GOES HERE# [1] 92 92 79 79##################################################################.
#-----------------------------------------------------------------.# REMEMBER - 2 ways we learned about (so far) to "index a vector"# (i.e. to identify specific values in the vector)#-----------------------------------------------------------------.# FIRST WAY - use a vector in [brackets] that specifies the positions you wantbigNumbers =c(1000,2000,3000,4000,5000)bigNumbers [ c( 1, 3) ] # 1000 3000
[1] 1000 3000
# SECOND WAY - use negative numbers to indicate the positions you DON'T wantbigNumbers [ c(-1, -4, -5)] # 2000 3000
[1] 2000 3000
# the same thing ...bigNumbers [ -c(1, 4, 5)] # 2000 3000
[1] 2000 3000
13.1 DON’T mix positive and negative index numbers
You may index either - with POSITIVE numbers or - with NEGATIVE numbers
BUT DO NOT mix positive and negative numbers in the index. That will generate an ERROR as the following example shows.
stuff =c(10,20,30,40,50)stuff[c(2,3)]
[1] 20 30
stuff[c(-2,-3)]
[1] 10 40 50
stuff[c(-2,3)] # ERROR - you may not mix positive and negative index numbers
Error in stuff[c(-2, 3)]: only 0's may be mixed with negative subscripts
##############################################################.# QUESTION## Write a function, longestSeq that takes a single argument named nums.# The function should return the sequence counting# by ones, starting from the smallest value in nums to the largest value# in nums.## HINT - you can use the functions min and max in your answer## EXAMPLE 1# > longestSeq( c ( 3, 9, 4, 2, 5, 5, 4, 8 ))# [1] 2 3 4 5 6 7 8 9## EXAMPLE 2# > longestSeq( c(100, 99, 103, 100))# [1] 99 100 101 102 103##############################################################.
###############################################################################.# QUESTION TOPICS: indexing with numbers ##### # Write a function named squish, that takes a single argument named nums.# The function should ## add together the first value with the 2nd value# add together the 2nd value with the 3rd value# etc.## EXAMPLE 1:# > squish( c(10,20,30,40) )# [1] 30 50 70## EXAMPLE 2:# > squish( c(100,50,30,200,150,10) )# [1] 150 80 230 350 160### PART A - write the answer by indexing with POSITIVE numbers# PART B - write the answer by indexing with NEGATIVE numbers#############################################################################.
Part A - using POSITIVE index numbers - click here for answer
#--------------------------------------------------------------.# PART A - write the answer by indexing with POSITIVE numbers#--------------------------------------------------------------.# First think about the problem. It often is useful to start thinking# about the problem with a simple example.## In the example, we are given the vector c(10,20,30,40)# For this example, adding together # the 1st number with the 2nd number, # the 2nd number with the 3rd number, # etc# # Is the same as adding together the following two vectors:# c(10,20,30) + c(20,30,40)## The 1st vector, c(10,20,30) is the original vector without the last number.# The 2nd vector, c(20,30,40) is the original vector without the first number.# # Therefore our function needs to calculate these two vectors and then add them.# We can do this with either POSITIVE index numbers or NEGATIVE index numbers# as shown below.squish =function (vec) { allButLast = vec[1:(length(vec)-1)] allButFirst = vec[2:length(vec)] answer = allButLast + allButFirstreturn(answer)}# Make sure both examples worksquish(c(10,20,30,40))
[1] 30 50 70
squish( c(100,50,30,200,150,10) )
[1] 150 80 230 350 160
Part B - using NEGATIVE index numbers - click here for answer
#--------------------------------------------------------------.# ANSWER - PART B - write the answer by indexing with NEGATIVE numbers#--------------------------------------------------------------.# I recommend you first see the analysis# in the answer to PART A above.squish =function (vec) { allButLast = vec[-length(vec)] allButFirst = vec[-1] answer = allButLast + allButFirstreturn(answer)}# Make sure both examples worksquish(c(10,20,30,40))