r4ds/strings.qmd

# Strings {#sec-strings}

```{r}
#| results: "asis"
#| echo: false
source("_common.R")
status("polishing")
```

## Introduction

So far, you've used a bunch of strings without learning much about the details.
Now it's time to dive into them, learning what makes strings tick, and mastering some of the powerful string manipulation tool you have at your disposal.

We'll begin with the details of creating strings and character vectors.
You'll then dive into creating strings from data, then the opposite; extracting strings from data.
The chapter finishes up with functions that work with individual letters and a brief discussion of where your expectations from English might steer you wrong when working with other languages.

We'll keep working with strings in the next chapter, where you'll learn more about the power of regular expressions.

### Prerequisites

::: callout-important
This chapter relies on features only found in stringr 1.5.0 and tidyr 1.3.0 which are still in development.
If you want to live life on the edge you can get the dev versions with `devtools::install_github(c("tidyverse/stringr", "tidyverse/tidyr"))`.
:::

In this chapter, we'll use functions from the stringr package which is part of the core tidyverse.
We'll also use the babynames data since it provides some fun strings to manipulate.

```{r}
#| label: setup
#| message: false

library(tidyverse)
library(babynames)
```

You can easily tell when you're using a stringr function because all stringr functions start with `str_`.
This is particularly useful if you use RStudio, because typing `str_` will trigger autocomplete, allowing you jog your memory of which functions are available.

```{r}
#| echo: false

knitr::include_graphics("screenshots/stringr-autocomplete.png")
```

## Creating a string

We've created strings in passing earlier in the book, but didn't discuss the details.
Firstly, you can create a string using either single quotes (`'`) or double quotes (`"`).
There's no difference in behavior between the two so in the interests of consistency the [tidyverse style guide](https://style.tidyverse.org/syntax.html#character-vectors) recommends using `"`, unless the string contains multiple `"`.

```{r}
string1 <- "This is a string"
string2 <- 'If I want to include a "quote" inside a string, I use single quotes'
```

If you forget to close a quote, you'll see `+`, the continuation character:

    > "This is a string without a closing quote
    + 
    + 
    + HELP I'M STUCK IN A STRING

If this happens to you and you can't figure out which quote you need to close, press Escape to cancel, and try again.

### Escapes

To include a literal single or double quote in a string you can use `\` to "escape" it:

```{r}
double_quote <- "\"" # or '"'
single_quote <- '\'' # or "'"
```

So if you want to include a literal backslash in your string, you'll need to escape it: `"\\"`:

```{r}
backslash <- "\\"
```

Beware that the printed representation of a string is not the same as string itself, because the printed representation shows the escapes (in other words, when you print a string, you can copy and paste the output to recreate that string).
To see the raw contents of the string, use `str_view()`[^strings-1]:

[^strings-1]: Or use the base R function `writeLines()`.

```{r}
x <- c(single_quote, double_quote, backslash)
x

str_view(x)
```

### Raw strings {#sec-raw-strings}

Creating a string with multiple quotes or backslashes gets confusing quickly.
To illustrate the problem, lets create a string that contains the contents of the code block where we define the `double_quote` and `single_quote` variables:

```{r}
tricky <- "double_quote <- \"\\\"\" # or '\"'
single_quote <- '\\'' # or \"'\""
str_view(tricky)
```

That's a lot of backslashes!
(This is sometimes called [leaning toothpick syndrome](https://en.wikipedia.org/wiki/Leaning_toothpick_syndrome).) To eliminate the escaping you can instead use a **raw string**[^strings-2]:

[^strings-2]: Available in R 4.0.0 and above.

```{r}
tricky <- r"(double_quote <- "\"" # or '"'
single_quote <- '\'' # or "'")"
str_view(tricky)
```

A raw string usually starts with `r"(` and finishes with `)"`.
But if your string contains `)"` you can instead use `r"[]"` or `r"{}"`, and if that's still not enough, you can insert any number of dashes to make the opening and closing pairs unique, e.g. `` `r"--()--" ``, `` `r"---()---" ``, etc. Raw strings are flexible enough to handle any text.

### Other special characters

As well as `\"`, `\'`, and `\\` there are a handful of other special characters that may come in handy. The most common are `\n`, newline, and `\t`, tab. You'll also sometimes see strings containing Unicode escapes that start with `\u` or `\U`. This is a way of writing non-English characters that works on all systems. You can see the complete list of other special characters in `?'"'`.

```{r}
x <- c("one\ntwo", "one\ttwo", "\u00b5", "\U0001f604")
x
str_view(x)
```

Note that `str_view()` uses a blue background for tabs to make them easier to spot.
One of the challenges of working with text is that there's a variety of ways that white space can end up in text, so this background helps you recognize that something strange is going on.

### Exercises

1.  Create strings that contain the following values:

    1.  `He said "That's amazing!"`

    2.  `\a\b\c\d`

    3.  `\\\\\\`

2.  Create the string in your R session and print it.
    What happens to the special "\\u00a0"?
    How does `str_view()` display it?
    Can you do a little googling to figure out what this special character is?

    ```{r}
    x <- "This\u00a0is\u00a0tricky"
    ```

## Creating many strings from data

Now that you've learned the basics of creating a string or two by "hand", we'll go into the details of creating strings from other strings.
This will help you solve the common problem where you have some text that you wrote that you want to combine with strings from a data frame.
For example, to create a greeting you might combine "Hello" with a `name` variable.
We'll show you how to do this with `str_c()` and `str_glue()` and how you can you use them with `mutate()`.
That naturally raises the question of what string functions you might use with `summarise()`, so we'll finish this section with a discussion of `str_flatten()` which is a summary function for strings.

### `str_c()`

`str_c()`[^strings-3] takes any number of vectors as arguments and returns a character vector:

[^strings-3]: `str_c()` is very similar to the base `paste0()`.
    There are two main reasons we recommend it: it propagates `NA`s (rather than converting them to `"NA"`) and it uses the tidyverse recycling rules.

```{r}
str_c("x", "y")
str_c("x", "y", "z")
str_c("Hello ", c("John", "Susan"))
```

`str_c()` is designed to be used with `mutate()` so it obeys the usual rules for recycling and missing values:

```{r}
set.seed(1410)
df <- tibble(name = c(wakefield::name(3), NA))
df |> mutate(greeting = str_c("Hi ", name, "!"))
```

If you want missing values to display in some other way, use `coalesce()`.
Depending on what you want, you might use it either inside or outside of `str_c()`:

```{r}
df |> 
  mutate(
    greeting1 = str_c("Hi ", coalesce(name, "you"), "!"),
    greeting2 = coalesce(str_c("Hi ", name, "!"), "Hi!")
  )
```

### `str_glue()` {#sec-glue}

If you are mixing many fixed and variable strings with `str_c()`, you'll notice that you type a lot of `"`s, making it hard to see the overall goal of the code. An alternative approach is provided by the [glue package](https://glue.tidyverse.org) via `str_glue()`[^strings-4]. You give it a single string that has a special feature: anything inside `{}` will be evaluated like it's outside of the quotes:

[^strings-4]: If you're not using stringr, you can also access it directly with `glue::glue()`.

```{r}
df |> mutate(greeting = str_glue("Hi {name}!"))
```

As you can see, `str_glue()` currently converts missing values to the string `"NA"` unfortunately making it inconsistent with `str_c()`.

You also might wonder what happens if you need to include a regular `{` or `}` in your string.
If you guess that you'll need to somehow escape it, you're on the right track.
The trick is that glue uses a slightly different escaping technique; instead of prefixing with special character like `\`, you double up the special characters:

```{r}
df |> mutate(greeting = str_glue("{{Hi {name}!}}"))
```

### `str_flatten()`

`str_c()` and `glue()` work well with `mutate()` because their output is the same length as their inputs.
What if you want a function that works well with `summarise()`, i.e. something that always returns a single string?
That's the job of `str_flatten()`[^strings-5]: it takes a character vector and combines each element of the vector into a single string:

[^strings-5]: The base R equivalent is `paste()` used with the `collapse` argument.

```{r}
str_flatten(c("x", "y", "z"))
str_flatten(c("x", "y", "z"), ", ")
str_flatten(c("x", "y", "z"), ", ", last = ", and ")
```

This makes it work well with `summarise()`:

```{r}
df <- tribble(
  ~ name, ~ fruit,
  "Carmen", "banana",
  "Carmen", "apple",
  "Marvin", "nectarine",
  "Terence", "cantaloupe",
  "Terence", "papaya",
  "Terence", "madarine"
)
df |>
  group_by(name) |> 
  summarise(fruits = str_flatten(fruit, ", "))
```

### Exercises

1.  Compare and contrast the results of `paste0()` with `str_c()` for the following inputs:

    ```{r}
    #| eval: false

    str_c("hi ", NA)
    str_c(letters[1:2], letters[1:3])
    ```

2.  Convert the following expressions from `str_c()` to `str_glue()` or vice versa:

    a.  `str_c("The price of ", food, " is ", price)`

    b.  `str_glue("I'm {age} years old and live in {country}")`

    c.  `str_c("\\section{", title, "}")`

## Extracting data from strings

It's very common for multiple variables to be crammed together into a single string.
In this section you'll learn how to use four tidyr functions to extract them:

-   `df |> separate_longer_delim(col, delim)`
-   `df |> separate_longer_position(col, width)`
-   `df |> separate_wider_delim(col, delim, names)`
-   `df |> separate_wider_position(col, widths)`

If you look closely you can see there's a common pattern here: `separate_`, then `longer` or `wider`, then `_`, then by `delim` or `position`.
That's because these four functions are composed from two simpler primitives:

-   `longer` makes input data frame longer, creating new rows; `wider` makes the input data frame wider, generating new columns.
-   `delim` splits up a string with a delimiter like `", "` or `" "`; `position` splits at specified widths, like `c(3, 5, 2)`.

We'll come back the last member of this family, `separate_regex_wider()`, in @sec-regular-expressions.
It's the most flexible of the `wider` functions but you need to know something about regular expression before you can use it.

The next two sections will give you the basic idea behind these separate functions, first separating into rows (which is a little simpler) and then separating in to columns.
We'll finish off my discussing the tools that the `wider` functions give you to diagnose problems.

### Separating into rows

Separating a string into rows tends to be most useful when the number of components varies from row to row.
The most common case is requiring `separate_longer_delim()` to split based on a delimiter:

```{r}
df1 <- tibble(x = c("a,b,c", "d,e", "f"))
df1 |> 
  separate_longer_delim(x, delim = ",")
```

It's rarer to see `separate_longer_position()` in the wild, but some older datasets do use very compact format where each character is used to record a value:

```{r}
df2 <- tibble(x = c("1211", "131", "21"))
df2 |> 
  separate_longer_position(x, width = 1)
```

### Separating into columns {#sec-string-columns}

Separating a string into columns tends to be most useful when there are a fixed number of components in each string, and you want to spread them into columns.
They are slightly more complicated than their `longer` equivalents because you need to name the columns.
For example, in this following dataset `x` is made up of a code, an edition number, and a year, separated by `"."`.
To use `separate_wider_delim()` we supply the delimiter and the names in two arguments:

```{r}
df3 <- tibble(x = c("a10.1.2022", "b10.2.2011", "e15.1.2015"))
df3 |> 
  separate_wider_delim(
    x,
    delim = ".",
    names = c("code", "edition", "year")
  )
```

If a specific piece is not useful you can use an `NA` name to omit it from the results:

```{r}
df3 |> 
  separate_wider_delim(
    x,
    delim = ".",
    names = c("code", NA, "year")
  )
```

`separate_wider_position()` works a little differently, because you typically want to specify the width of each column.
So you give it a named integer vector, where the name gives the name of the new column and the value is the number of characters it occupies.
You can omit values from the output by not naming them:

```{r}
df4 <- tibble(x = c("202215TX", "202122LA", "202325CA")) 
df4 |> 
  separate_wider_position(
    x,
    widths = c(year = 4, age = 2, state = 2)
  )
```

### Diagnosing widening problems

`separate_wider_delim()`[^strings-6] requires a fixed and known set of columns.
What happens if some of the rows don't have the expected number of pieces?
There are two possible problems, too few or too many pieces, so `separate_wider_delim()` provides two arguments to help: `too_few` and `too_many`. Let's first look at the `too_few` case with the following sample dataset:

[^strings-6]: The same principles apply to `separate_wider_position()` and `separate_wider_regex()`.

```{r}
#| error: true
df <- tibble(x = c("1-1-1", "1-1-2", "1-3", "1-3-2", "1"))

df |> 
  separate_wider_delim(
    x,
    delim = "-",
    names = c("x", "y", "z")
  )
```

You'll notice that we get an error, but the error gives us some suggestions as to how you might proceed.
Let's start by debugging the problem:

```{r}
debug <- df |> 
  separate_wider_delim(
    x,
    delim = "-",
    names = c("x", "y", "z"),
    too_few = "debug"
  )
debug
```

When you use the debug mode you get three extra columns add to the output: `x_ok`, `x_pieces`, and `x_remainder` (if you separate variable with a different name, you'll get a different prefix).
Here, `x_ok` lets you quickly find the inputs that failed:

```{r}
debug |> filter(!x_ok)
```

`x_pieces` tells us how many pieces were found, compared to the expected 3 (the length of `names`).
`x_remainder` isn't useful when there are too few pieces, but we'll see it again shortly.

Sometimes looking at this debugging information will reveal a problem with your delimiter strategy or suggest that you need to do more preprocessing before separating.
In that case, fix the problem upstream and make sure to remove `too_few = "debug"` to ensure that new problem become errors.

In other cases you may just want to fill in the missing pieces with `NA`s and move on.
That's the job of `too_few = "align_start"` and `too_few = "align_end"` which allow you to control where the `NA`s should go:

```{r}
df |> 
  separate_wider_delim(
    x,
    delim = "-",
    names = c("x", "y", "z"),
    too_few = "align_start"
  )
```

The same principles apply if you have too many pieces:

```{r}
#| error: true
df <- tibble(x = c("1-1-1", "1-1-2", "1-3-5-6", "1-3-2", "1-3-5-7-9"))

df |> 
  separate_wider_delim(
    x,
    delim = "-",
    names = c("x", "y", "z")
  )
```

But now when we debug the result, you can see the purpose of `x_remainder`:

```{r}
debug <- df |> 
  separate_wider_delim(
    x,
    delim = "-",
    names = c("x", "y", "z"),
    too_many = "debug"
  )
debug |> filter(!x_ok)
```

You have a slightly different set of options for handling too many pieces: you can either silently "drop" any additional pieces or "merge" them all into the final column:

```{r}
df |> 
  separate_wider_delim(
    x,
    delim = "-",
    names = c("x", "y", "z"),
    too_many = "drop"
  )


df |> 
  separate_wider_delim(
    x,
    delim = "-",
    names = c("x", "y", "z"),
    too_many = "merge"
  )
```

## Letters

This section discusses stringr functions that work with individual letters.
This is straightforward for English because it uses an alphabet with 26 letters, but things rapidly get complicated when you move beyond English.
Even languages that use the same alphabet but add additional accents (e.g. å, é, ï, ô, ū) are non-trivial because those letters might be represented as an individual character or by combining an unaccented letter (e.g. e) with a diacritic mark (e.g. ´).
And other languages "letters" look quite different: in Japanese each "letter" is a syllable, in Chinese each "letter" is a complex logogram, and in Arabic letters look radically different depending on their location in the word.

In this section, we'll assume that you're working with English text as we introduce to functions for finding the length of a string, extracting substrings, and handling long strings in plots and tables.

### Length

`str_length()` tells you the number of letters in the string:

```{r}
str_length(c("a", "R for data science", NA))
```

You could use this with `count()` to find the distribution of lengths of US babynames, and then with `filter()` to look at the longest names[^strings-7]:

[^strings-7]: Looking at these entries, we'd guess that the babynames data drops spaces or hyphens and truncates after 15 letters.

```{r}
babynames |>
  count(length = str_length(name), wt = n)

babynames |> 
  filter(str_length(name) == 15) |> 
  count(name, wt = n, sort = TRUE)
```

### Subsetting

You can extract parts of a string using `str_sub(string, start, end)`, where `start` and `end` are the letters where the substring should start and end.
The `start` and `end` arguments are inclusive, so the length of the returned string will be `end - start + 1`:

```{r}
x <- c("Apple", "Banana", "Pear")
str_sub(x, 1, 3)
```

You can use negative values to count back from the end of the string: -1 is the last character, -2 is the second to last character, etc.

```{r}
str_sub(x, -3, -1)
```

Note that `str_sub()` won't fail if the string is too short: it will just return as much as possible:

```{r}
str_sub("a", 1, 5)
```

We could use `str_sub()` with `mutate()` to find the first and last letter of each name:

```{r}
babynames |> 
  mutate(
    first = str_sub(name, 1, 1),
    last = str_sub(name, -1, -1)
  )
```

### Long strings

Sometimes the reason you care about the length of a string is because you're trying to fit it into a label on a plot or in a table.
stringr provides two useful tools for cases where your string is too long:

-   `str_trunc(x, 30)` ensures that no string is longer than 30 characters, replacing any letters after 30 with `…`.

-   `str_wrap(x, 30)` wraps a string introducing new lines so that each line is at most 30 characters (it doesn't hyphenate, however, so any word longer than 30 characters will make a longer line)

The following code shows these functions in action with a made up string:

```{r}
x <- "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat."

str_view(str_trunc(x, 30))
str_view(str_wrap(x, 30))
```

### Exercises

1.  Use `str_length()` and `str_sub()` to extract the middle letter from each baby name. What will you do if the string has an even number of characters?
2.  Are there any major trends in the length of babynames over time? What about the popularity of first and last letters?

## Locale dependent {#sec-other-languages}

There are a handful of stringr functions whose behavior depends on your **locale**.
Locale is similar to language, but includes an optional region specifier to handle the fact that (e.g.) many countries speak Spanish, but with regional variations.
A locale is specified by lower-case language abbreviation, optionally followed by a `_` and a upper-case region identifier.
For example, "en" is English, "en_GB" is British English, and "en_US" is American English.
If you don't already know the code for your language, [Wikipedia](https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes) has a good list, and you can see which are supported in stringr by looking at `stringi::stri_locale_list()`.

Base R string functions automatically use the locale set by your operating system.
This means that base R string functions usually use the rules associated with your native language, but such might work differently when you share it with someone who lives in different country.
To avoid this problem, stringr defaults to the "en" locale, and requires you to specify the `locale` argument to override it.
This also makes it easy to tell if a function might behave differently in different locales.

Fortunately there are two sets of functions where the locale matters:

-   **Changing case**: the rules for changing case are not the same in every language.
    For example, Turkish has two i's: with and without a dot, and it has a different rule to English for capitalizing them:

    ```{r}
    str_to_upper(c("i", "ı"))
    str_to_upper(c("i", "ı"), locale = "tr")
    ```

    This also effects `str_equal()` which can optionally ignore:

    ```{r}
    str_equal("i", "I", ignore_case = TRUE)
    str_equal("i", "I", ignore_case = TRUE, locale = "tr")
    ```

-   **Sorting strings**: `str_sort()` and `str_order()` sort vectors alphabetically, but the alphabet is not the same in every language[^strings-8]!
    Here's an example: in Czech, "ch" is a compound letter that appears after `h` in the alphabet.

    ```{r}
    str_sort(c("a", "c", "ch", "h", "z"))
    str_sort(c("a", "c", "ch", "h", "z"), locale = "cs")
    ```

    A similar situation arises in Danish.
    Normally, characters with diacritics (e.g. à, á, â) sort after the plain character (e.g. a).
    But in Danish ø and å are their own letters that come at the end of the alphabet:

    ```{r}
    str_sort(c("a", "å", "o", "ø", "z"))
    str_sort(c("a", "å", "o", "ø", "z"), locale = "da")
    ```

    This also comes up when sorting strings with `dplyr::arrange()` which is why it also has a `locale` argument.

[^strings-8]: Sorting in languages that don't have an alphabet (like Chinese) is more complicated still.

## Summary

In this chapter you've learned a wide of tools for working with strings, but you haven't learned one of the most important and powerful tools: regular expressions.
Regular expressions are very concise, but very expressive, language for describing patterns within strings, and are the topic of the next chapter.