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<h1 class="title toc-ignore">Specifying the function in map() + parallel mapping</h1>
</div>
<div id="load-packages" class="section level2">
<h2>Load packages</h2>
<p>Load purrr and repurrrsive, which contains recursive list examples. If you’re just jumping here, the example datasets are introduced <a href="https://jennybc.github.io/purrr-tutorial/ls00_inspect-explore.html">elsewhere</a>, including via interactive listviewer widgets.</p>
<pre class="r"><code>library(purrr)
library(repurrrsive)</code></pre>
</div>
<div id="map-overview" class="section level2">
<h2><code>map()</code> overview</h2>
<p>Recall the usage of purrr’s core <code>map()</code> function:</p>
<pre class="r"><code>map(.x, .f, ...)
map(VECTOR_OR_LIST_INPUT, FUNCTION_TO_APPLY, OPTIONAL_OTHER_STUFF)</code></pre>
<p>You can provide further arguments via <code>...</code>, but you don’t have to. The above expands to something like this:</p>
<pre class="r"><code>res <- vector(mode = "list", length = length(.x))
res[[1]] <- .f(.x[[1]], ...)
res[[2]] <- .f(.x[[2]], ...)
## and so on, until the end of .x
res</code></pre>
<p>Note that any additional arguments provided via <code>...</code> are used “as is” in each call to <code>.f</code>. In other words, <code>map()</code> is not vectorized over these arguments. If you need that, check out <code>map2()</code>, <code>pmap()</code>, and friends.</p>
</div>
<div id="map-function-specification" class="section level2">
<h2><code>map()</code> function specification</h2>
<p>One of the main reasons to use purrr is the flexible and concise syntax for specifying <code>.f</code>, the function to apply.</p>
<p>The <a href="ls01_map-name-position-shortcuts.html">shortcuts for extracting by name and position</a> are covered thoroughly elsewhere and won’t be repeated here.</p>
<p>We demonstrate three more ways to specify general <code>.f</code>:</p>
<ul>
<li>an existing function</li>
<li>an anonymous function, defined on-the-fly, as usual</li>
<li>a formula: this is unique to purrr and provides a very concise way to define an anonymous function</li>
</ul>
<p>We work with the Game of Thrones character list, <code>got_chars</code>. Each character can have aliases, which are stored in a vector in each character’s component. We pull out the aliases for three characters to use as our demo.</p>
<pre class="r"><code>aliases <- set_names(map(got_chars, "aliases"), map_chr(got_chars, "name"))
(aliases <- aliases[c("Theon Greyjoy", "Asha Greyjoy", "Brienne of Tarth")])
#> $`Theon Greyjoy`
#> [1] "Prince of Fools" "Theon Turncloak" "Reek" "Theon Kinslayer"
#>
#> $`Asha Greyjoy`
#> [1] "Esgred" "The Kraken's Daughter"
#>
#> $`Brienne of Tarth`
#> [1] "The Maid of Tarth" "Brienne the Beauty" "Brienne the Blue"</code></pre>
<div id="existing-function" class="section level3">
<h3>Existing function</h3>
<p>Use a pre-existing function. Or, as here, define one ourselves, which gives a nice way to build-in our specification for the <code>collapse</code> argument.</p>
<pre class="r"><code>my_fun <- function(x) paste(x, collapse = " | ")
map(aliases, my_fun)
#> $`Theon Greyjoy`
#> [1] "Prince of Fools | Theon Turncloak | Reek | Theon Kinslayer"
#>
#> $`Asha Greyjoy`
#> [1] "Esgred | The Kraken's Daughter"
#>
#> $`Brienne of Tarth`
#> [1] "The Maid of Tarth | Brienne the Beauty | Brienne the Blue"</code></pre>
</div>
<div id="anonymous-function-conventional" class="section level3">
<h3>Anonymous function, conventional</h3>
<p>Define an anonymous function on-the-fly, in the conventional way. Here we put our desired value for the <code>collapse</code> argument into the function defintion itself.</p>
<pre class="r"><code>map(aliases, function(x) paste(x, collapse = " | "))
#> $`Theon Greyjoy`
#> [1] "Prince of Fools | Theon Turncloak | Reek | Theon Kinslayer"
#>
#> $`Asha Greyjoy`
#> [1] "Esgred | The Kraken's Daughter"
#>
#> $`Brienne of Tarth`
#> [1] "The Maid of Tarth | Brienne the Beauty | Brienne the Blue"</code></pre>
<p>Alternatively you can simply name the function and provide <code>collapse</code> via <code>...</code>.</p>
<pre class="r"><code>map(aliases, paste, collapse = " | ")
#> $`Theon Greyjoy`
#> [1] "Prince of Fools | Theon Turncloak | Reek | Theon Kinslayer"
#>
#> $`Asha Greyjoy`
#> [1] "Esgred | The Kraken's Daughter"
#>
#> $`Brienne of Tarth`
#> [1] "The Maid of Tarth | Brienne the Beauty | Brienne the Blue"</code></pre>
</div>
<div id="anonymous-function-formula" class="section level3">
<h3>Anonymous function, formula</h3>
<p>We saved possibly the best for last.</p>
<p>purrr provides a very concise way to define an anonymous function: as a formula. This should start with the <code>~</code> symbol and then look like a typical top-level expression, as you might write in a script. Use <code>.x</code> to refer to the input, i.e. an individual element of the primary vector or list.</p>
<pre class="r"><code>map(aliases, ~ paste(.x, collapse = " | "))
#> $`Theon Greyjoy`
#> [1] "Prince of Fools | Theon Turncloak | Reek | Theon Kinslayer"
#>
#> $`Asha Greyjoy`
#> [1] "Esgred | The Kraken's Daughter"
#>
#> $`Brienne of Tarth`
#> [1] "The Maid of Tarth | Brienne the Beauty | Brienne the Blue"</code></pre>
</div>
<div id="workflow-advice" class="section level3">
<h3>Workflow advice</h3>
<p>It’s rare to write these calls perfect and whole the first time. You should probably pilot your idea on a single element. Then drop your <strong>proven, working logic</strong> into one of the above templates. When things aren’t working as expected, consider: have you tried to skip too many steps? Pull out an example, get everything to work there, check it on another example, then scale back up again.</p>
<p>A development process for the above might look like this:</p>
<pre class="r"><code>(a <- map(got_chars, "aliases")[[19]]) ## OOPS! NULL --> a useless example
#> NULL
(a <- map(got_chars, "aliases")[[16]]) ## ok good
#> [1] "Bran" "Bran the Broken" "The Winged Wolf"
paste(a, sep = " | ") ## OOPS! not what I want
#> [1] "Bran" "Bran the Broken" "The Winged Wolf"
paste(a, collapse = " | ") ## ok good
#> [1] "Bran | Bran the Broken | The Winged Wolf"
got_chars[15:17] %>% ## I am a programming god
map("aliases") %>%
map_chr(paste, collapse = " | ")
#> [1] "Varamyr Sixskins | Haggon | Lump"
#> [2] "Bran | Bran the Broken | The Winged Wolf"
#> [3] "The Maid of Tarth | Brienne the Beauty | Brienne the Blue"</code></pre>
</div>
</div>
<div id="list-to-data-frame" class="section level2">
<h2>List to data frame</h2>
<p>Since we’ve simplifed the aliases to a single string for each character, we can hold them as an atomic character vector instead of as list. Wouldn’t it be nice to put that in a data frame, with another variable holding the names? The <code>enframe()</code> function from tibble takes a named vector and promotes the names to a proper variable.</p>
<p>From the top, using four characters to conserve space:</p>
<pre class="r"><code>aliases <- set_names(map(got_chars, "aliases"), map_chr(got_chars, "name"))
map_chr(aliases[c(3, 10, 20, 24)], ~ paste(.x, collapse = " | ")) %>%
tibble::enframe(value = "aliases")
#> # A tibble: 4 x 2
#> name aliases
#> <chr> <chr>
#> 1 Victarion Grey… The Iron Captain
#> 2 Davos Seaworth Onion Knight | Davos Shorthand | Ser Onions | Onion Lord…
#> 3 Eddard Stark Ned | The Ned | The Quiet Wolf
#> 4 Aeron Greyjoy The Damphair | Aeron Damphair</code></pre>
<p>Alternative way to get same data frame</p>
<pre class="r"><code>tibble::tibble(
name = map_chr(got_chars, "name"),
aliases = got_chars %>%
map("aliases") %>%
map_chr(~ paste(.x, collapse = " | "))
) %>%
dplyr::slice(c(3, 10, 20, 24))
#> # A tibble: 4 x 2
#> name aliases
#> <chr> <chr>
#> 1 Victarion Grey… The Iron Captain
#> 2 Davos Seaworth Onion Knight | Davos Shorthand | Ser Onions | Onion Lord…
#> 3 Eddard Stark Ned | The Ned | The Quiet Wolf
#> 4 Aeron Greyjoy The Damphair | Aeron Damphair</code></pre>
<p>This is a very typical workflow: take an unwieldy nested list and, via extraction and/or simplification, produce a more approachable data frame.</p>
</div>
<div id="recap" class="section level2">
<h2>Recap</h2>
<p>These are the different ways to specify the function <code>.f</code> in the <code>map()</code>-type functions in purrr.</p>
<pre class="r"><code>map(aliases, function(x) paste(x, collapse = "|"))
map(aliases, paste, collapse = "|")
map(aliases, ~ paste(.x, collapse = " | "))</code></pre>
<div id="exercises" class="section level3">
<h3>Exercises</h3>
<p>Each character can be allied with one of the <a href="https://anapioficeandfire.com/Documentation#houses">houses</a> (or with several or with zero). These allegiances are held as a vector in each character’s component.</p>
<ol style="list-style-type: decimal">
<li>Create a list <code>allegiances</code> that holds the characters’ house affiliations.</li>
<li>Create a character vector <code>nms</code> that holds the characters’ names.</li>
<li>Apply the names in <code>nms</code> to the <code>allegiances</code> list via <code>set_names</code>.</li>
<li>Re-use the code from above to collapse each character’s vector of allegiances down to a string.</li>
<li>We said that any elements passed via <code>...</code> would be used “as is”. Specifically they are not used in a vectorized fashion. What happens if you pass <code>collapse = c(" | ", " * ")</code>? Why is that?</li>
</ol>
</div>
</div>
<div id="parallel-map" class="section level2">
<h2>Parallel map</h2>
<div id="map2" class="section level3">
<h3><code>map2()</code></h3>
<p>What if you need to map a function over two vectors or lists in parallel?</p>
<p>You can use <code>map2()</code> for that. Here is the usage:</p>
<pre class="r"><code>map2(.x, .y, .f, ...)
map(INPUT_ONE, INPUT_TWO, FUNCTION_TO_APPLY, OPTIONAL_OTHER_STUFF)</code></pre>
<p><code>map2()</code> has all the type-specific friends you would expect: <code>map2_chr()</code>, <code>map2_lgl()</code>, etc.</p>
<p>How will we specify the function to apply? All the usual options are open.</p>
<p>What shall our example be? Each character has a free text field, giving the date and possibly location of his or her birth. Let’s paste that together with the character’s name to get a sentence.</p>
<p>First, obtain the two inputs.</p>
<pre class="r"><code>nms <- got_chars %>%
map_chr("name")
birth <- got_chars %>%
map_chr("born")</code></pre>
<p>Now map over both with an existing function, defined by us.</p>
<pre class="r"><code>my_fun <- function(x, y) paste(x, "was born", y)
map2_chr(nms, birth, my_fun) %>% head()
#> [1] "Theon Greyjoy was born In 278 AC or 279 AC, at Pyke"
#> [2] "Tyrion Lannister was born In 273 AC, at Casterly Rock"
#> [3] "Victarion Greyjoy was born In 268 AC or before, at Pyke"
#> [4] "Will was born "
#> [5] "Areo Hotah was born In 257 AC or before, at Norvos"
#> [6] "Chett was born At Hag's Mire"</code></pre>
<p>Anonymous function, conventional form.</p>
<pre class="r"><code>map2_chr(nms, birth, function(x, y) paste(x, "was born", y)) %>% head()
#> [1] "Theon Greyjoy was born In 278 AC or 279 AC, at Pyke"
#> [2] "Tyrion Lannister was born In 273 AC, at Casterly Rock"
#> [3] "Victarion Greyjoy was born In 268 AC or before, at Pyke"
#> [4] "Will was born "
#> [5] "Areo Hotah was born In 257 AC or before, at Norvos"
#> [6] "Chett was born At Hag's Mire"</code></pre>
<p>Anonymous function via formula. Use <code>.x</code> and <code>.y</code> to refer to the individual elements of the two primary inputs.</p>
<pre class="r"><code>map2_chr(nms[16:18], birth[16:18], ~ paste(.x, "was born", .y)) %>% tail()
#> [1] "Brandon Stark was born In 290 AC, at Winterfell"
#> [2] "Brienne of Tarth was born In 280 AC"
#> [3] "Catelyn Stark was born In 264 AC, at Riverrun"</code></pre>
</div>
<div id="pmap" class="section level3">
<h3><code>pmap()</code></h3>
<p>What if you need to map a function over <strong>two or more</strong> vectors or lists in parallel?</p>
<p>You can use <code>pmap()</code> for that. Here is the usage:</p>
<pre class="r"><code>pmap(.l, .f, ...)
map(LIST_OF_INPUT_LISTS, FUNCTION_TO_APPLY, OPTIONAL_OTHER_STUFF)</code></pre>
<p><em>words</em></p>
<pre class="r"><code>df <- got_chars %>% {
tibble::tibble(
name = map_chr(., "name"),
aliases = map(., "aliases"),
allegiances = map(., "allegiances")
)
}
my_fun <- function(name, aliases, allegiances) {
paste(name, "has", length(aliases), "aliases and",
length(allegiances), "allegiances")
}
df %>%
pmap_chr(my_fun) %>%
tail()
#> [1] "Kevan Lannister has 1 aliases and 1 allegiances"
#> [2] "Melisandre has 5 aliases and 0 allegiances"
#> [3] "Merrett Frey has 1 aliases and 1 allegiances"
#> [4] "Quentyn Martell has 4 aliases and 1 allegiances"
#> [5] "Samwell Tarly has 7 aliases and 1 allegiances"
#> [6] "Sansa Stark has 3 aliases and 2 allegiances"</code></pre>
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