Add small map() example

ex04_map-example.R

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+#' ---
+#' title: "Small demo of purrr::map()"
+#' author: "Jenny Bryan"
+#' date: "`r format(Sys.Date())`"
+#' output: github_document
+#' ---
+
+#+ setup, include = FALSE, cache = FALSE
+knitr::opts_chunk$set(
+  collapse = TRUE,
+  comment = "#>",
+  error = TRUE
+)
+options(tidyverse.quiet = TRUE)
+
+#+ body
+# ----
+#' ## `purrr::map()` can be used to work with functions that aren't vectorized.
+
+df_list <- list(
+  iris = head(iris, 2),
+  mtcars = head(mtcars, 3)
+)
+df_list
+
+#' This does not work. `nrow()` expects a single data frame as input.
+nrow(df_list)
+
+#' `purrr::map()` applies `nrow()` to each element of `df_list`.
+library(purrr)
+
+map(df_list, nrow)
+
+#' Different calling styles make sense in more complicated situations. Hard to
+#' justify in this simple example.
+map(df_list, ~ nrow(.x))
+
+df_list %>%
+  map(nrow)
+
+#' If you know what the return type is (or *should* be), use a type-specific
+#' variant of `map()`.
+
+map_int(df_list, ~ nrow(.x))
+
+#' More on coverage of `map()` and friends: <https://jennybc.github.io/purrr-tutorial/>.

ex04_map-example.md

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+Small demo of purrr::map()
+================
+Jenny Bryan
+2018-04-10
+
+## `purrr::map()` can be used to work with functions that aren’t vectorized.
+
+``` r
+df_list <- list(
+  iris = head(iris, 2),
+  mtcars = head(mtcars, 3)
+)
+df_list
+#> $iris
+#>   Sepal.Length Sepal.Width Petal.Length Petal.Width Species
+#> 1          5.1         3.5          1.4         0.2  setosa
+#> 2          4.9         3.0          1.4         0.2  setosa
+#> 
+#> $mtcars
+#>                mpg cyl disp  hp drat    wt  qsec vs am gear carb
+#> Mazda RX4     21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
+#> Mazda RX4 Wag 21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
+#> Datsun 710    22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
+```
+
+This does not work. `nrow()` expects a single data frame as input.
+
+``` r
+nrow(df_list)
+#> NULL
+```
+
+`purrr::map()` applies `nrow()` to each element of `df_list`.
+
+``` r
+library(purrr)
+
+map(df_list, nrow)
+#> $iris
+#> [1] 2
+#> 
+#> $mtcars
+#> [1] 3
+```
+
+Different calling styles make sense in more complicated situations. Hard
+to justify in this simple example.
+
+``` r
+map(df_list, ~ nrow(.x))
+#> $iris
+#> [1] 2
+#> 
+#> $mtcars
+#> [1] 3
+
+df_list %>%
+  map(nrow)
+#> $iris
+#> [1] 2
+#> 
+#> $mtcars
+#> [1] 3
+```
+
+If you know what the return type is (or *should* be), use a
+type-specific variant of `map()`.
+
+``` r
+map_int(df_list, ~ nrow(.x))
+#>   iris mtcars 
+#>      2      3
+```
+
+More on coverage of `map()` and friends:
+<https://jennybc.github.io/purrr-tutorial/>.