Worksheet 3

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xfun::pkg_attach2("teachingtools")

In this worksheet covers the material from the subsetting session.

Subsetting vectors

Problem 3.1

The vector vector1 contains a list of numbers.

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vector1

 [1]  4  2 12 15  5  9  6  2  3 14  5  8

1. Select the 4st the and 7th elements

2. Select the the first 4 elements

Get a hint

• Use [] to take a subset of a vector
• Use c() to concatenate a list of values

Solution 1

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vector1[c(4,7)]

[1] 15  6

Solution 2

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vector1[1:4]

[1]  4  2 12 15

Problem 3.2

The vector vector1 contains a list of numbers.

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vector1

 [1]  4  2 12 15  5  9  6  2  3 14  5  8

1. Check each element of vector1 and determine which are equal to 6.

2. The index of the elements that are equal to 6

3. Subset vector1 to keep only the elements that are equal to 6

Get a hint

• Use == to test for equality.
• The function which() can be used to see find the index of elements that match a condition
• Use [] to take a subset of a vector

Solution 1

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vector1 == 6

 [1] FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE

Solution 2

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which(vector1 == 6)

[1] 7

Solution 3

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vector1[vector1 == 6]

[1] 6

Problem 3.3

The vector vector2 contains a list of numbers, although some values are missing.

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vector2

 [1] 15 13  9 NA 10  6  6 13  6 14 NA  3

1. Find the index elements of vector2 that are missing

2. Take a subset of vector2 that contains only values between 5 and 10 (including 5 and 10). Make sure only to return numbers.

3. Determine how many missing values vector2 contains

Get a hint

• is.na() can be used to find missing values
• which() will tell you which elements are TRUE
• You can use the length() function to determine then length (i.e., how many elements) are in a vector.

Solution 1

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which(is.na(vector2))

[1]  4 11

Solution 2

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vector2[which(vector2 >= 5 & vector2 <= 10)] 

[1]  9 10  6  6  6

Solution 3

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length(which(is.na(vector2)))

[1] 2

Problem 3.4

The vector months1 contains a list of months.

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months1

[1] "February" "October"  "July"     "November" "May"      "May"     

1. Take a subset of the of the vector so that it only contains elements with months in the set {May, June, July}.
2. Take a subset of the vector so that it only contains elements that are NOT part of the set {May, June, July}

Get a hint

• To test whether elements are a member of a set use the %in% operator.
• If you have a vector of TRUE and FALSE values, you can switch the them around by wrapping them in !()

Solution 1

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months1[months1 %in% c("May","June","July")]

[1] "July" "May"  "May" 

Solution 2

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months1[!(months1 %in% c("May","June","July"))]

[1] "February" "October"  "November"

Subsetting parts of data tables

Problem 3.5

The data table starwars contains information about star wars characters.

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starwars

# A tibble: 87 x 14
   name  height  mass hair_color skin_color eye_color birth_year sex   gender
   <chr>  <int> <dbl> <chr>      <chr>      <chr>          <dbl> <chr> <chr> 
 1 Luke…    172    77 blond      fair       blue            19   male  mascu…
 2 C-3PO    167    75 <NA>       gold       yellow         112   none  mascu…
 3 R2-D2     96    32 <NA>       white, bl… red             33   none  mascu…
 4 Dart…    202   136 none       white      yellow          41.9 male  mascu…
 5 Leia…    150    49 brown      light      brown           19   fema… femin…
 6 Owen…    178   120 brown, gr… light      blue            52   male  mascu…
 7 Beru…    165    75 brown      light      blue            47   fema… femin…
 8 R5-D4     97    32 <NA>       white, red red             NA   none  mascu…
 9 Bigg…    183    84 black      light      brown           24   male  mascu…
10 Obi-…    182    77 auburn, w… fair       blue-gray       57   male  mascu…
# … with 77 more rows, and 5 more variables: homeworld <chr>, species <chr>,
#   films <list>, vehicles <list>, starships <list>

1. Select the column that contains the names. The result should still be a data table.

2. Select the values inside the species column. The result should be a vector.

Get a hint

• For one of these problems you’‘ll use [] for one and for the other you’’ll use [[]] / $

Solution 1

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starwars['name']

# A tibble: 87 x 1
   name              
   <chr>             
 1 Luke Skywalker    
 2 C-3PO             
 3 R2-D2             
 4 Darth Vader       
 5 Leia Organa       
 6 Owen Lars         
 7 Beru Whitesun lars
 8 R5-D4             
 9 Biggs Darklighter 
10 Obi-Wan Kenobi    
# … with 77 more rows

Solution 2

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starwars$species # option 1

 [1] "Human"          "Droid"          "Droid"          "Human"         
 [5] "Human"          "Human"          "Human"          "Droid"         
 [9] "Human"          "Human"          "Human"          "Human"         
[13] "Wookiee"        "Human"          "Rodian"         "Hutt"          
[17] "Human"          "Human"          "Yoda's species" "Human"         
[21] "Human"          "Droid"          "Trandoshan"     "Human"         
[25] "Human"          "Mon Calamari"   "Human"          "Human"         
[29] "Ewok"           "Sullustan"      "Human"          "Neimodian"     
[33] "Human"          "Gungan"         "Gungan"         "Gungan"        
[37] NA               "Toydarian"      "Dug"            NA              
[41] "Human"          "Zabrak"         "Twi'lek"        "Twi'lek"       
[45] "Vulptereen"     "Xexto"          "Toong"          "Human"         
[49] "Cerean"         "Nautolan"       "Zabrak"         "Tholothian"    
[53] "Iktotchi"       "Quermian"       "Kel Dor"        "Chagrian"      
[57] "Human"          "Human"          "Human"          "Geonosian"     
[61] "Mirialan"       "Mirialan"       "Human"          "Human"         
[65] "Human"          "Human"          "Clawdite"       "Besalisk"      
[69] "Kaminoan"       "Kaminoan"       "Human"          "Aleena"        
[73] "Droid"          "Skakoan"        "Muun"           "Togruta"       
[77] "Kaleesh"        "Wookiee"        "Human"          NA              
[81] "Pau'an"         "Human"          "Human"          "Human"         
[85] "Droid"          NA               "Human"         

Solution 2 (alternative approach)

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starwars[['species']] # another way of doing it

 [1] "Human"          "Droid"          "Droid"          "Human"         
 [5] "Human"          "Human"          "Human"          "Droid"         
 [9] "Human"          "Human"          "Human"          "Human"         
[13] "Wookiee"        "Human"          "Rodian"         "Hutt"          
[17] "Human"          "Human"          "Yoda's species" "Human"         
[21] "Human"          "Droid"          "Trandoshan"     "Human"         
[25] "Human"          "Mon Calamari"   "Human"          "Human"         
[29] "Ewok"           "Sullustan"      "Human"          "Neimodian"     
[33] "Human"          "Gungan"         "Gungan"         "Gungan"        
[37] NA               "Toydarian"      "Dug"            NA              
[41] "Human"          "Zabrak"         "Twi'lek"        "Twi'lek"       
[45] "Vulptereen"     "Xexto"          "Toong"          "Human"         
[49] "Cerean"         "Nautolan"       "Zabrak"         "Tholothian"    
[53] "Iktotchi"       "Quermian"       "Kel Dor"        "Chagrian"      
[57] "Human"          "Human"          "Human"          "Geonosian"     
[61] "Mirialan"       "Mirialan"       "Human"          "Human"         
[65] "Human"          "Human"          "Clawdite"       "Besalisk"      
[69] "Kaminoan"       "Kaminoan"       "Human"          "Aleena"        
[73] "Droid"          "Skakoan"        "Muun"           "Togruta"       
[77] "Kaleesh"        "Wookiee"        "Human"          NA              
[81] "Pau'an"         "Human"          "Human"          "Human"         
[85] "Droid"          NA               "Human"         

Problem 3.6

1. Again using the starwars data table, take a subset so that it only contains humans (i.e., Human in the species column). Be careful, because some rows have missing values.

Get a hint

• use which(starwars$species == "Human") will tell you which rows contain humans.
  
• When selecting specific rows, you should also indicate that you want all the columns

Solution

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starwars[which(starwars$species == "Human"),]

# A tibble: 35 x 14
   name  height  mass hair_color skin_color eye_color birth_year sex   gender
   <chr>  <int> <dbl> <chr>      <chr>      <chr>          <dbl> <chr> <chr> 
 1 Luke…    172    77 blond      fair       blue            19   male  mascu…
 2 Dart…    202   136 none       white      yellow          41.9 male  mascu…
 3 Leia…    150    49 brown      light      brown           19   fema… femin…
 4 Owen…    178   120 brown, gr… light      blue            52   male  mascu…
 5 Beru…    165    75 brown      light      blue            47   fema… femin…
 6 Bigg…    183    84 black      light      brown           24   male  mascu…
 7 Obi-…    182    77 auburn, w… fair       blue-gray       57   male  mascu…
 8 Anak…    188    84 blond      fair       blue            41.9 male  mascu…
 9 Wilh…    180    NA auburn, g… fair       blue            64   male  mascu…
10 Han …    180    80 brown      fair       brown           29   male  mascu…
# … with 25 more rows, and 5 more variables: homeworld <chr>, species <chr>,
#   films <list>, vehicles <list>, starships <list>

Subsetting parts of lists

Problem 3.7

The list list1 has two elements. animals contains a vector of animal names, and cars contains a vector of car names.

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list1

$animals
 [1] "Indri"           "Chamois"         "Sea Turtle"      "Chamois"        
 [5] "Coral"           "Axolotl"         "Coral"           "Tasmanian Devil"
 [9] "Sea Turtle"      "Labradoodle"    

$cars
 [1] "Merc 450SLC"         "Toyota Corona"       "Lotus Europa"       
 [4] "Pontiac Firebird"    "Toyota Corona"       "Ferrari Dino"       
 [7] "Maserati Bora"       "Datsun 710"          "Porsche 914-2"      
[10] "Camaro Z28"          "Maserati Bora"       "Lincoln Continental"
[13] "Merc 240D"           "Honda Civic"         "Hornet Sportabout"  

1. Take a subset of list1, so that you create a new list with one element that only has car names
2. Take a subset of list1, so that you create a vector of only animal names.
3. Select the 3 item in the cars element of list

Solution 1

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list1['cars']

$cars
 [1] "Merc 450SLC"         "Toyota Corona"       "Lotus Europa"       
 [4] "Pontiac Firebird"    "Toyota Corona"       "Ferrari Dino"       
 [7] "Maserati Bora"       "Datsun 710"          "Porsche 914-2"      
[10] "Camaro Z28"          "Maserati Bora"       "Lincoln Continental"
[13] "Merc 240D"           "Honda Civic"         "Hornet Sportabout"  

Solution 2

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list1$animals

 [1] "Indri"           "Chamois"         "Sea Turtle"      "Chamois"        
 [5] "Coral"           "Axolotl"         "Coral"           "Tasmanian Devil"
 [9] "Sea Turtle"      "Labradoodle"    

Solution 2 (alternative appraoch)

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list1[["animals"]]

 [1] "Indri"           "Chamois"         "Sea Turtle"      "Chamois"        
 [5] "Coral"           "Axolotl"         "Coral"           "Tasmanian Devil"
 [9] "Sea Turtle"      "Labradoodle"    

Solution 3

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list1$cars[3]

[1] "Lotus Europa"

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