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Data Analytics Coding Fundamentals: UVic BIDA302: Course Book

18 Strings

18.1 Objectives

  • Understand the basics of regular expressions (regexps)

  • Understand basic functions in {stringr} for working with strings

  • Demonstrate ability to use those functions

18.3 Strings: introduction

For this exercise, we will be using the tidyverse package {stringr}.

18.4 Setup

This chunk of R code loads the packages that we will be using.

Strings are characters, numbers, etc. that are enclosed inside quotation marks. A character vector is made up of multiple strings.

string1 <- "This is my 1st string!"

string1
## [1] "This is my 1st string!"

18.5 Regular expressions

regular expressions becomes shortened to “regex”

regexps are a concise language for describing patterns in strings

They are powerful ways to filter and manipulate strings based on those patterns. But they have a certain reputation:

Some people, when confronted with a problem, think “I know, I’ll use regular expressions.” Now they have two problems. – Jamie Zawinski

18.5.1 regex matching functions

Here are some basic matching functions:

Note: the square brackets enclose the regex to identify a single character. In the examples below, with three sets of square brackets, “[a][b][c]” contains three separate character positions, whereas “[abc]” is identifying a single character.

character what it does
“abc” matches “abc”
“[a][b][c]” matches “abc”
“[abc]” matches “a”, “b”, or “c”
“[^abc]” matches anything except “a”, “b”, or “c”
“^” match start of string
“$” match end of string

The {stringr} function str_view() shows where in our string the pattern appears. First we create an object “fruit_list” with three strings in it, then find two different patterns.

fruit_list <- c("apple", "banana", "pear")

# which words start with "a"?
str_view(fruit_list, "^a")
## [1] │ <a>pple
# which words have an "ea" combination?
str_view(fruit_list, "ea")
## [3] │ p<ea>r

18.5.1.1 Your turn

Which words end with “a”?

Use str_view() to find out.

Solution 
# solution
str_view(fruit_list, "a$")
## [2] │ banan<a>

frequency of match

character what it does
“?” 0 or 1
“+” 1 or more
“*” 0 or more

The question mark is useful for words with various spellings—the British and American variations of words like “colour” and “neighbour”.

character what it does
“{n}” exactly n times
“{n,}” n or more
“{n,m}” between n and m times 
# which words have a double "p"?

str_view(fruit_list, "p{2}")
## [1] │ a<pp>le

18.6 Special characters

quotes

To find single and double quotes in our string, they need to be “escaped” with a backslash \.

double_quote <- "\"" # or '"'
single_quote <- '\'' # or "'"

double_quote
## [1] "\""
single_quote
## [1] "'"
  • to see a representation of the string as it will print, use the function writeLines() 
writeLines(double_quote)
## "
string2 <- 'The 2nd string has a "quote" so it is inside single quotes'

string2
## [1] "The 2nd string has a \"quote\" so it is inside single quotes"
writeLines(string2)
## The 2nd string has a "quote" so it is inside single quotes

other special characters

These also need to be escaped:

character what it is
“\” backslash
“ any digit
“” newline (line break)
“” any whitespace (space, tab, newline)
“ tab
“*” asterisk
“..” unicode characters*

See the help function for ?'"'

interrobang <- "\u2048"
interrobang
## [1] "⁈"

To make a shruggie, you need to escape the backslash.

shruggie <- "¯\\_(ツ)_/¯"
shruggie
## [1] "¯\\_(ツ)_/¯"
shruggie <- glue::glue("¯\\", "_(ツ)_", "/¯")
shruggie
## ¯\_(ツ)_/¯

18.7 Basic functions

stringr hex
stringr hex

Some {stringr} functions

function purpose
str_length(x) the number of characters in x
str_c() concatenates a list of strings
str_sub(x, start = , end = ) returns characters of x
str_detect(string, pattern) TRUE/FALSE if there is a pattern match
str_replace(string, pattern, newtext) replace
str_remove(string, pattern) removes the first instance of pattern
str_remove_all (string, pattern) removes all instances of pattern
str_trim (string) removes whitespace at the beginning and end

The full list of {stringr} functions can be found at https://stringr.tidyverse.org/reference/index.html

musician_first <- c("Sly", "Billie", "Thelonious", "Maroon", "Willie", "Led")
musician_first
## [1] "Sly"        "Billie"     "Thelonious" "Maroon"     "Willie"     "Led"
str_length(musician_first) 
## [1]  3  6 10  6  6  3

use str_c to collapse list into one string

str_c(musician_first, collapse = ", ")
## [1] "Sly, Billie, Thelonious, Maroon, Willie, Led"

18.8 Combining strings

Use str_c to join two character vectors, separated by a space.

musician_last <- c("Stone", "Ellish", "Monk", "5", "Nelson", "Zeppelin")

str_c(musician_first, musician_last, sep = " ")
## [1] "Sly Stone"       "Billie Ellish"   "Thelonious Monk" "Maroon 5"        "Willie Nelson"  
## [6] "Led Zeppelin"

18.8.0.1 Your turn

Now join musician_first to musician_last (the other way around!), separated by an apostrophe and a space.

Solution 
#solution
str_c(musician_last, musician_first, sep = ", ")
## [1] "Stone, Sly"       "Ellish, Billie"   "Monk, Thelonious" "5, Maroon"        "Nelson, Willie"  
## [6] "Zeppelin, Led"

18.9 Pattern matching

18.9.0.1 Your turn

Are there any vowels in the string musician_first?

Use str_detect() to find them, and count them with str_count().

Solution 
# solution
str_detect(musician_first, "[aeiou]")
## [1] FALSE  TRUE  TRUE  TRUE  TRUE  TRUE
str_count(musician_first, "[aeiou]")
## [1] 0 3 5 3 3 1

18.9.0.2 Your turn

Are there any digits in musician_last?

Solution

Don’t forget that you need to escape any backslashes you use!

# solution
str_detect(musician_last, "\\d")
## [1] FALSE FALSE FALSE  TRUE FALSE FALSE

We can extract chunks of text by their location using the following function: str_sub(musician_first, start, end).

# solution
str_sub(musician_first, 1, 2)  
## [1] "Sl" "Bi" "Th" "Ma" "Wi" "Le"

str_locate() finds the first position of the pattern:

# look for pairs of vowels
str_locate(musician_first, "[aeiou][aeiou]")
##      start end
## [1,]    NA  NA
## [2,]     5   6
## [3,]     7   8
## [4,]     4   5
## [5,]     5   6
## [6,]    NA  NA
# look for a specific match ...
str_locate(musician_first, "oo")
##      start end
## [1,]    NA  NA
## [2,]    NA  NA
## [3,]    NA  NA
## [4,]     4   5
## [5,]    NA  NA
## [6,]    NA  NA

Extract the first case of a vowel:

str_extract(musician_first, "[aeiou]")
## [1] NA  "i" "e" "a" "i" "e"

Extract the pairs of vowels:

# solution
str_extract(musician_first, "[aeiou][aeiou]")
## [1] NA   "ie" "io" "oo" "ie" NA
# alternate solution
str_extract(musician_first, "[aeiou]{2}")
## [1] NA   "ie" "io" "oo" "ie" NA

18.9.1 Filtering on patterns

Filter by country that has “land” in the name

gapminder <- gapminder::gapminder 

gapminder |> 
  filter(str_detect(country, "land")) |> 
  distinct(country)
## # A tibble: 9 × 1
##   country    
##   <fct>      
## 1 Finland    
## 2 Iceland    
## 3 Ireland    
## 4 Netherlands
## 5 New Zealand
## 6 Poland     
## 7 Swaziland  
## 8 Switzerland
## 9 Thailand

What about those countries where “land” is at the end of the name?

gapminder |> 
  filter(str_detect(country, "land$")) |> 
  distinct(country)
## # A tibble: 8 × 1
##   country    
##   <fct>      
## 1 Finland    
## 2 Iceland    
## 3 Ireland    
## 4 New Zealand
## 5 Poland     
## 6 Swaziland  
## 7 Switzerland
## 8 Thailand

Or those where “land” is at the end and is preceded by only three letters?

pattern_string <- "^\\w{3}land$"

gapminder |> 
  filter(str_detect(country, pattern_string)) |> 
  distinct(country)
## # A tibble: 3 × 1
##   country
##   <fct>  
## 1 Finland
## 2 Iceland
## 3 Ireland

18.9.2 Selecting on patterns

Earlier, we covered the {dplyr} function to select() variables (columns) in a data frame.

Here’s some tools to make that selection process a bit more streamlined:

function tool
starts_with() Starts with a prefix
ends_with() Ends with a suffix
contains() Contains a literal string
matches() Matches a regular expression

(Note: these functions are some of the “select_helpers” imported into {dplyr} from the {tidyselect} package. )

Examples using the {palmerpenguins} data:

## # A tibble: 6 × 8
##   species island    bill_length_mm bill_depth_mm flipper_length_mm body_mass_g sex     year
##   <fct>   <fct>              <dbl>         <dbl>             <int>       <int> <fct>  <int>
## 1 Adelie  Torgersen           39.1          18.7               181        3750 male    2007
## 2 Adelie  Torgersen           39.5          17.4               186        3800 female  2007
## 3 Adelie  Torgersen           40.3          18                 195        3250 female  2007
## 4 Adelie  Torgersen           NA            NA                  NA          NA <NA>    2007
## 5 Adelie  Torgersen           36.7          19.3               193        3450 female  2007
## 6 Adelie  Torgersen           39.3          20.6               190        3650 male    2007

Here is a way to select the variables that have the length measurements:

penguins |> 
  select(contains("length")) |>
  head()
## # A tibble: 6 × 2
##   bill_length_mm flipper_length_mm
##            <dbl>             <int>
## 1           39.1               181
## 2           39.5               186
## 3           40.3               195
## 4           NA                  NA
## 5           36.7               193
## 6           39.3               190

And two ways to select those with the bill measurements:

penguins |> 
  select(starts_with("bill_")) |>
  head()
## # A tibble: 6 × 2
##   bill_length_mm bill_depth_mm
##            <dbl>         <dbl>
## 1           39.1          18.7
## 2           39.5          17.4
## 3           40.3          18  
## 4           NA            NA  
## 5           36.7          19.3
## 6           39.3          20.6

This second case incorporates regex into the selection code, with the “^” for the start of the string:

penguins |> 
  select(matches("^b")) |>
  head()
## # A tibble: 6 × 3
##   bill_length_mm bill_depth_mm body_mass_g
##            <dbl>         <dbl>       <int>
## 1           39.1          18.7        3750
## 2           39.5          17.4        3800
## 3           40.3          18          3250
## 4           NA            NA            NA
## 5           36.7          19.3        3450
## 6           39.3          20.6        3650

18.10 Cleaning strings

18.10.1 Removing characters

In the Week 3 assignment, you probably noticed that some of the community names have a pair of asterisks at the end, indicating that the community includes more than one municipality. For example, “North Vancouver” includes both the City of North Vancouver and the District of North Vancouver (two distinct towns, with the same name…)

Here’s a long form of the “Purpose Built Rental” sheet (“pbr” for short) from that Excel file:

housing1_long <- read_csv("data/bc-stats_2018-new-homes-data_PBR.csv")
housing1_long
## # A tibble: 480 × 3
##    community       year units
##    <chr>          <dbl> <dbl>
##  1 100 Mile House  2016    NA
##  2 100 Mile House  2017    NA
##  3 100 Mile House  2018    NA
##  4 Abbotsford      2016   327
##  5 Abbotsford      2017    NA
##  6 Abbotsford      2018   428
##  7 Alert Bay       2016    NA
##  8 Alert Bay       2017    NA
##  9 Alert Bay       2018    NA
## 10 Anmore          2016    NA
## # ℹ 470 more rows

If we plot the top 10 municipalities (remember the slice() function!), we see that North Vancouver shows up…complete with the two asterisks.

ggplot(housing1_long |> 
  group_by(community) |> 
  tally(units, sort = TRUE, name = "3 Year Total") |> 
  slice(1:10), aes(x = reorder(community, `3 Year Total`), y = `3 Year Total`))+
  geom_col() +
  coord_flip()

We can use our knowledge of regular expressions and the functions in {stringr} to remove these. The first thing we need to recall is that an asterisk is one of those Special characters{strings-special}, and we need to “escape” it with a backslash, and we need to escape the first backslash too. Here’s an example:

city_name <- "North Vancouver**"

# detect the presence of an asterisk
str_detect(city_name, "\\*")
## [1] TRUE
# detect the presence of an asterisk
str_count(city_name, "\\*")
## [1] 2

{stringr} provides us functions for this very task:

In a pipe with our dataframe, it would look like this:

housing1_long <- housing1_long |> 
  mutate(community = str_remove_all(community, "\\*"))

Let’s filter for North Vancouver to check the success of our efforts:

housing1_long |> 
  filter(community == "North Vancouver")
## # A tibble: 0 × 3
## # ℹ 3 variables: community <chr>, year <dbl>, units <dbl>

What? Where are those rows? If we do a visual inspection, they are there. It turns out that there is a blank space between the community name and the asterisks.

There’s another {stringr} function for this: str_trim().

housing1_long <- housing1_long |> 
  mutate(community = str_trim(community))

housing1_long |> 
  filter(community == "North Vancouver")
## # A tibble: 3 × 3
##   community        year units
##   <chr>           <dbl> <dbl>
## 1 North Vancouver  2016   140
## 2 North Vancouver  2017   981
## 3 North Vancouver  2018    NA

  • str_trim() has an argument side = if you need to specify left, right, or (the default) both

  • there is also a function str_squish() that also takes out excess whitespace (it still does leading and trailing, like str_trim())

  • and if you need to add whitespace, there’s str_pad()

Now our plot will have clean names:

ggplot(housing1_long |> 
  group_by(community) |> 
  tally(units, sort = TRUE, name = "3 Year Total") |> 
  slice(1:10), aes(x = reorder(community, `3 Year Total`), y = `3 Year Total`))+
  geom_col() +
  coord_flip()

18.10.2 Splitting strings

This file has a single column, with both the name of every electoral district in British Columbia, preceded by a three-letter abbreviation (TLA).

bced <- read_csv("data/bc_electoral_districts_2015.txt", col_names = FALSE)
bced
## # A tibble: 87 × 1
##    X1                         
##    <chr>                      
##  1 "ABM\tAbbotsford-Mission"  
##  2 "ABS\tAbbotsford South"    
##  3 "ABW\tAbbotsford West"     
##  4 "BDS\tBoundary-Similkameen"
##  5 "BND\tBurnaby-Deer Lake"   
##  6 "BNE\tBurnaby-Edmonds"     
##  7 "BNL\tBurnaby-Lougheed"    
##  8 "BNN\tBurnaby North"       
##  9 "CBC\tCariboo-Chilcotin"   
## 10 "CBN\tCariboo North"       
## # ℹ 77 more rows

You’ll notice that in this file the three letter abbreviation is separated by “\t”. This is the regular expression for a tab.

From {stringr}, we can use the str_split() function:

head(str_split(bced$X1, "\t"))
## [[1]]
## [1] "ABM"                "Abbotsford-Mission"
## 
## [[2]]
## [1] "ABS"              "Abbotsford South"
## 
## [[3]]
## [1] "ABW"             "Abbotsford West"
## 
## [[4]]
## [1] "BDS"                  "Boundary-Similkameen"
## 
## [[5]]
## [1] "BND"               "Burnaby-Deer Lake"
## 
## [[6]]
## [1] "BNE"             "Burnaby-Edmonds"

But there’s a solution designed for tabular data, in the {tidyr} package: separate()

bced |> 
  separate(X1, "\t", into = c("ed_tla", "ed_name"))
## # A tibble: 87 × 2
##    ed_tla ed_name             
##    <chr>  <chr>               
##  1 ABM    Abbotsford-Mission  
##  2 ABS    Abbotsford South    
##  3 ABW    Abbotsford West     
##  4 BDS    Boundary-Similkameen
##  5 BND    Burnaby-Deer Lake   
##  6 BNE    Burnaby-Edmonds     
##  7 BNL    Burnaby-Lougheed    
##  8 BNN    Burnaby North       
##  9 CBC    Cariboo-Chilcotin   
## 10 CBN    Cariboo North       
## # ℹ 77 more rows

If we add the remove = argument set to FALSE, the original variable remains:

bced |> 
  separate(X1, "\t", into = c("ed_tla", "ed_name"), remove = FALSE)
## # A tibble: 87 × 3
##    X1                          ed_tla ed_name             
##    <chr>                       <chr>  <chr>               
##  1 "ABM\tAbbotsford-Mission"   ABM    Abbotsford-Mission  
##  2 "ABS\tAbbotsford South"     ABS    Abbotsford South    
##  3 "ABW\tAbbotsford West"      ABW    Abbotsford West     
##  4 "BDS\tBoundary-Similkameen" BDS    Boundary-Similkameen
##  5 "BND\tBurnaby-Deer Lake"    BND    Burnaby-Deer Lake   
##  6 "BNE\tBurnaby-Edmonds"      BNE    Burnaby-Edmonds     
##  7 "BNL\tBurnaby-Lougheed"     BNL    Burnaby-Lougheed    
##  8 "BNN\tBurnaby North"        BNN    Burnaby North       
##  9 "CBC\tCariboo-Chilcotin"    CBC    Cariboo-Chilcotin   
## 10 "CBN\tCariboo North"        CBN    Cariboo North       
## # ℹ 77 more rows

In the above example, the tab symbol is the separator, and there’s only one per line. But what if the abbreviation and the name were separated by spaces? In that case, we would want to separate on only the first space.

bced_space <- read_csv("data/bc_electoral_districts_2015_space.txt", col_names = FALSE)
bced_space
## # A tibble: 87 × 1
##    X1                      
##    <chr>                   
##  1 ABM Abbotsford-Mission  
##  2 ABS Abbotsford South    
##  3 ABW Abbotsford West     
##  4 BDS Boundary-Similkameen
##  5 BND Burnaby-Deer Lake   
##  6 BNE Burnaby-Edmonds     
##  7 BNL Burnaby-Lougheed    
##  8 BNN Burnaby North       
##  9 CBC Cariboo-Chilcotin   
## 10 CBN Cariboo North       
## # ℹ 77 more rows
bced_space |> 
  separate(X1, " ", into = c("ed_tla", "ed_name"))
## # A tibble: 87 × 2
##    ed_tla ed_name             
##    <chr>  <chr>               
##  1 ABM    Abbotsford-Mission  
##  2 ABS    Abbotsford          
##  3 ABW    Abbotsford          
##  4 BDS    Boundary-Similkameen
##  5 BND    Burnaby-Deer        
##  6 BNE    Burnaby-Edmonds     
##  7 BNL    Burnaby-Lougheed    
##  8 BNN    Burnaby             
##  9 CBC    Cariboo-Chilcotin   
## 10 CBN    Cariboo             
## # ℹ 77 more rows

For the electoral districts with only a single word or a hyphen, it works fine. But notice that the two Abbotsford ridings are now just “Abbotsford”, since everything after the second space has been dropped.

The solution is to use the extra = "merge" argument. (Note that the default is extra = "drop".)

bced_space |> 
  separate(X1, " ", into = c("ed_tla", "ed_name"), extra = "merge")
## # A tibble: 87 × 2
##    ed_tla ed_name             
##    <chr>  <chr>               
##  1 ABM    Abbotsford-Mission  
##  2 ABS    Abbotsford South    
##  3 ABW    Abbotsford West     
##  4 BDS    Boundary-Similkameen
##  5 BND    Burnaby-Deer Lake   
##  6 BNE    Burnaby-Edmonds     
##  7 BNL    Burnaby-Lougheed    
##  8 BNN    Burnaby North       
##  9 CBC    Cariboo-Chilcotin   
## 10 CBN    Cariboo North       
## # ℹ 77 more rows

Nice!

-30-