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<h1 class="title toc-ignore">Introduction to <code>map()</code>: extract elements</h1>
</div>
<pre class="r"><code>search()
#> [1] ".GlobalEnv" "package:stats" "package:graphics"
#> [4] "package:grDevices" "package:utils" "package:datasets"
#> [7] "package:methods" "Autoloads" "tools:callr"
#> [10] "package:base"</code></pre>
<div id="load-packages" class="section level2">
<h2>Load packages</h2>
<p>Load purrr and repurrrsive, which contains recursive list examples.</p>
<pre class="r"><code>library(purrr)
library(repurrrsive)</code></pre>
</div>
<div id="vectorized-and-list-ized-operations" class="section level2">
<h2>Vectorized and “list-ized” operations</h2>
<p>This lesson picks up where <a href="bk00_vectors-and-lists.html#vectorized_operations">the primer on vectors and lists</a> left off. Recall that many operations “just work” in a vectorized fashion in R:</p>
<pre class="r"><code>(3:5) ^ 2
#> [1] 9 16 25
sqrt(c(9, 16, 25))
#> [1] 3 4 5</code></pre>
<p>Through the magic of R, the operations “raise to the power of 2” and “take the square root” were applied to each individual element of the numeric vector input. Someone – but not you! – has written a <code>for()</code> loop:</p>
<pre class="r"><code>for (i in 1:n) {
output[[i]] <- f(input[[i]])
}</code></pre>
<p>Automatic vectorization is possible because our input is an atomic vector: the individual atoms are always of length one, always of uniform type.</p>
<p>What if the input is a list? You have to be more intentional to apply a function <code>f()</code> to each element of a list, i.e. to “list-ize” computation. This makes sense because the data structure itself does not guarantee that it makes any sense at all to apply a common function <code>f()</code> to each element of the list. You must guarantee that.</p>
<p><code>purrr::map()</code> is a function for applying a function to each element of a list. The <a href="bk01_base-functions.html">closest base R function</a> is <code>lapply()</code>. Here’s how the square root example of the above would look if the input was in a list.</p>
<pre class="r"><code>map(c(9, 16, 25), sqrt)
#> [[1]]
#> [1] 3
#>
#> [[2]]
#> [1] 4
#>
#> [[3]]
#> [1] 5</code></pre>
<p>A template for basic <code>map()</code> usage:</p>
<pre class="r"><code>map(YOUR_LIST, YOUR_FUNCTION)</code></pre>
<p>Below we explore these useful features of <code>purrr::map()</code> and friends:</p>
<ul>
<li>Shortcuts for <code>YOUR_FUNCTION</code> when you want to extract list elements by name or position</li>
<li>Simplify and specify the type of output via <code>map_chr()</code>, <code>map_lgl()</code>, etc.</li>
</ul>
<p>This is where you begin to see the differences between <code>purrr::map()</code> and <code>base::lapply()</code>.</p>
<div id="name-and-position-shortcuts" class="section level3">
<h3>Name and position shortcuts</h3>
<p>Who are these Game of Thrones characters?</p>
<p>We want the elements with name “name”, so we do this (we restrict to the first few elements purely to conserve space):</p>
<pre class="r"><code>map(got_chars[1:4], "name")
#> [[1]]
#> [1] "Theon Greyjoy"
#>
#> [[2]]
#> [1] "Tyrion Lannister"
#>
#> [[3]]
#> [1] "Victarion Greyjoy"
#>
#> [[4]]
#> [1] "Will"</code></pre>
<p>We are exploiting one of purrr’s most useful features: a shortcut to create a function that extracts an element based on its name.</p>
<p>A companion shortcut is used if you provide a positive integer to <code>map()</code>. This creates a function that extracts an element based on position.</p>
<p>The 3rd element of each character’s list is his or her name and we get them like so:</p>
<pre class="r"><code>map(got_chars[5:8], 3)
#> [[1]]
#> [1] "Areo Hotah"
#>
#> [[2]]
#> [1] "Chett"
#>
#> [[3]]
#> [1] "Cressen"
#>
#> [[4]]
#> [1] "Arianne Martell"</code></pre>
<p>To recap, here are two shortcuts for making the <code>.f</code> function that <code>map()</code> will apply:</p>
<ul>
<li>provide “TEXT” to extract the element named “TEXT”
<ul>
<li>equivalent to <code>function(x) x[["TEXT"]]</code></li>
</ul></li>
<li>provide <code>i</code> to extract the <code>i</code>-th element
<ul>
<li>equivalent to <code>function(x) x[[i]]</code></li>
</ul></li>
</ul>
<p>You will frequently see <code>map()</code> used together with <a href="http://r4ds.had.co.nz/pipes.html">the pipe <code>%>%</code></a>. These calls produce the same result as the above.</p>
<pre class="r"><code>got_chars %>%
map("name")
got_chars %>%
map(3)</code></pre>
<div id="exercises" class="section level4">
<h4>Exercises</h4>
<ol style="list-style-type: decimal">
<li>Use <code>names()</code> to inspect the names of the list elements associated with a single character. What is the index or position of the <code>playedBy</code> element? Use the character and position shortcuts to extract the <code>playedBy</code> elements for all characters.</li>
<li>What happens if you use the character shortcut with a string that does not appear in the lists’ names?</li>
<li>What happens if you use the position shortcut with a number greater than the length of the lists?</li>
<li>What if these shortcuts did not exist? Write a function that takes a list and a string as input and returns the list element that bears the name in the string. Apply this to <code>got_chars</code> via <code>map()</code>. Do you get the same result as with the shortcut? Reflect on code length and readability.</li>
<li>Write another function that takes a list and an integer as input and returns the list element at that position. Apply this to <code>got_chars</code> via <code>map()</code>. How does this result and process compare with the shortcut?</li>
</ol>
</div>
</div>
<div id="type-specific-map" class="section level3">
<h3>Type-specific map</h3>
<p><code>map()</code> always returns a list, even if all the elements have the same flavor and are of length one. But in that case, you might prefer a simpler object: <strong>an atomic vector</strong>.</p>
<p>If you expect <code>map()</code> to return output that can be turned into an atomic vector, it is best to use a type-specific variant of <code>map()</code>. This is more efficient than using <code>map()</code> to get a list and then simplifying the result in a second step. Also purrr will alert you to any problems, i.e. if one or more inputs has the wrong type or length. This is the <a href="bk00_vectors-and-lists.html#coercion">increased rigor about type alluded to in the section about coercion</a>.</p>
<p>Our current examples are suitable for demonstrating <code>map_chr()</code>, since the requested elements are always character.</p>
<pre class="r"><code>map_chr(got_chars[9:12], "name")
#> [1] "Daenerys Targaryen" "Davos Seaworth" "Arya Stark"
#> [4] "Arys Oakheart"
map_chr(got_chars[13:16], 3)
#> [1] "Asha Greyjoy" "Barristan Selmy" "Varamyr" "Brandon Stark"</code></pre>
<p>Besides <code>map_chr()</code>, there are other variants of <code>map()</code>, with the target type conveyed by the name:</p>
<ul>
<li><code>map_lgl()</code>, <code>map_int()</code>, <code>map_dbl()</code></li>
</ul>
<div id="exercises-1" class="section level4">
<h4>Exercises</h4>
<ol style="list-style-type: decimal">
<li>For each character, the second element is named “id”. This is the character’s id in the <a href="https://anapioficeandfire.com">API Of Ice And Fire</a>. Use a type-specific form of <code>map()</code> and an extraction shortcut to extract these ids into an integer vector.</li>
<li>Use your list inspection strategies to find the list element that is logical. There is one! Use a type-specific form of <code>map()</code> and an extraction shortcut to extract these values for all characters into a logical vector.</li>
</ol>
</div>
</div>
<div id="extract-multiple-values" class="section level3">
<h3>Extract multiple values</h3>
<p>What if you want to retrieve multiple elements? Such as the character’s name and culture? First, recall how we do this with the list for a single user:</p>
<pre class="r"><code>got_chars[[3]][c("name", "culture", "gender", "born")]
#> $name
#> [1] "Victarion Greyjoy"
#>
#> $culture
#> [1] "Ironborn"
#>
#> $gender
#> [1] "Male"
#>
#> $born
#> [1] "In 268 AC or before, at Pyke"</code></pre>
<p>We use single square bracket indexing and a character vector to index by name. How will we ram this into the <code>map()</code> framework? To paraphrase Chambers, <a href="http://adv-r.had.co.nz/Functions.html#all-calls">“everything that happens in R is a function call”</a> and indexing with <code>[</code> is no exception.</p>
<p>It feels (and maybe looks) weird, but we can map <code>[</code> just like any other function. Recall <code>map()</code> usage:</p>
<pre class="r"><code>map(.x, .f, ...)</code></pre>
<p>The function <code>.f</code> will be <code>[</code>. And we finally get to use <code>...</code>! This is where we pass the character vector of the names of our desired elements. We inspect the result for two characters.</p>
<pre class="r"><code>x <- map(got_chars, `[`, c("name", "culture", "gender", "born"))
str(x[16:17])
#> List of 2
#> $ :List of 4
#> ..$ name : chr "Brandon Stark"
#> ..$ culture: chr "Northmen"
#> ..$ gender : chr "Male"
#> ..$ born : chr "In 290 AC, at Winterfell"
#> $ :List of 4
#> ..$ name : chr "Brienne of Tarth"
#> ..$ culture: chr ""
#> ..$ gender : chr "Female"
#> ..$ born : chr "In 280 AC"</code></pre>
<p>Some people find this ugly and might prefer the <code>extract()</code> function from magrittr.</p>
<pre class="r"><code>library(magrittr)
#>
#> Attaching package: 'magrittr'
#> The following object is masked from 'package:purrr':
#>
#> set_names
x <- map(got_chars, extract, c("name", "culture", "gender", "born"))
str(x[18:19])
#> List of 2
#> $ :List of 4
#> ..$ name : chr "Catelyn Stark"
#> ..$ culture: chr "Rivermen"
#> ..$ gender : chr "Female"
#> ..$ born : chr "In 264 AC, at Riverrun"
#> $ :List of 4
#> ..$ name : chr "Cersei Lannister"
#> ..$ culture: chr "Westerman"
#> ..$ gender : chr "Female"
#> ..$ born : chr "In 266 AC, at Casterly Rock"</code></pre>
<div id="exercises-2" class="section level4">
<h4>Exercises</h4>
<ol style="list-style-type: decimal">
<li>Use your list inspection skills to determine the position of the elements named “name”, “gender”, “culture”, “born”, and “died”. Map <code>[</code> or <code>magrittr::extract()</code> over users, requesting these elements by position instead of name.</li>
</ol>
</div>
</div>
<div id="data-frame-output" class="section level3">
<h3>Data frame output</h3>
<p>We just learned how to extract multiple elements per user by mapping <code>[</code>. But, since <code>[</code> is non-simplifying, each user’s elements are returned in a list. And, as it must, <code>map()</code> itself returns list. We’ve traded one recursive list for another recursive list, albeit a slightly less complicated one.</p>
<p>How can we “stack up” these results row-wise, i.e. one row per user and variables for “name”, “gender”, etc.? A data frame would be the perfect data structure for this information.</p>
<p>This is what <code>map_dfr()</code> is for.</p>
<pre class="r"><code>map_dfr(got_chars, extract, c("name", "culture", "gender", "id", "born", "alive"))
#> # A tibble: 30 x 6
#> name culture gender id born alive
#> <chr> <chr> <chr> <int> <chr> <lgl>
#> 1 Theon Greyjoy Ironborn Male 1022 In 278 AC or 279 AC, at Pyke TRUE
#> 2 Tyrion Lannist… "" Male 1052 In 273 AC, at Casterly Rock TRUE
#> 3 Victarion Grey… Ironborn Male 1074 In 268 AC or before, at Pyke TRUE
#> 4 Will "" Male 1109 "" FALSE
#> 5 Areo Hotah Norvoshi Male 1166 In 257 AC or before, at Nor… TRUE
#> 6 Chett "" Male 1267 At Hag's Mire FALSE
#> 7 Cressen "" Male 1295 In 219 AC or 220 AC FALSE
#> 8 Arianne Martell Dornish Female 130 In 276 AC, at Sunspear TRUE
#> 9 Daenerys Targa… Valyrian Female 1303 In 284 AC, at Dragonstone TRUE
#> 10 Davos Seaworth Westeros Male 1319 In 260 AC or before, at Kin… TRUE
#> # … with 20 more rows</code></pre>
<p>Finally! A data frame! Hallelujah!</p>
<p>Notice how the variables have been automatically type converted. It’s a beautiful thing. Until it’s not. When programming, it is safer, but more cumbersome, to explicitly specify type and build your data frame the usual way.</p>
<pre class="r"><code>library(tibble)
got_chars %>% {
tibble(
name = map_chr(., "name"),
culture = map_chr(., "culture"),
gender = map_chr(., "gender"),
id = map_int(., "id"),
born = map_chr(., "born"),
alive = map_lgl(., "alive")
)
}
#> # A tibble: 30 x 6
#> name culture gender id born alive
#> <chr> <chr> <chr> <int> <chr> <lgl>
#> 1 Theon Greyjoy Ironborn Male 1022 In 278 AC or 279 AC, at Pyke TRUE
#> 2 Tyrion Lannist… "" Male 1052 In 273 AC, at Casterly Rock TRUE
#> 3 Victarion Grey… Ironborn Male 1074 In 268 AC or before, at Pyke TRUE
#> 4 Will "" Male 1109 "" FALSE
#> 5 Areo Hotah Norvoshi Male 1166 In 257 AC or before, at Nor… TRUE
#> 6 Chett "" Male 1267 At Hag's Mire FALSE
#> 7 Cressen "" Male 1295 In 219 AC or 220 AC FALSE
#> 8 Arianne Martell Dornish Female 130 In 276 AC, at Sunspear TRUE
#> 9 Daenerys Targa… Valyrian Female 1303 In 284 AC, at Dragonstone TRUE
#> 10 Davos Seaworth Westeros Male 1319 In 260 AC or before, at Kin… TRUE
#> # … with 20 more rows</code></pre>
<p><em>Syntax notes: The dot <code>.</code> above is the placeholder for the primary input: <code>got_chars</code> in this case. The curly braces <code>{}</code> surrounding the <code>tibble()</code> call prevent <code>got_chars</code> from being passed in as the first argument of <code>tibble()</code>.</em></p>
<div id="exercises-3" class="section level4">
<h4>Exercises</h4>
<ol style="list-style-type: decimal">
<li>Use <code>map_dfr()</code> to create the same data frame as above, but indexing with a vector of positive integers instead of names.</li>
</ol>
</div>
</div>
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