Jo chp 9 (#66)

* edits to chapter 9 for Friday's cohort 9 presentation

* a few extra changes post-bookclub

---------

Co-authored-by: Jon Harmon <[email protected]>

09_Functionals.Rmd

@@ -120,9 +120,9 @@ Note that `.x` is the **name** of the first argument in `map()` (`.f` is the nam
 
 ```{r}
 #| error: true
-
 # the task
 map_dbl(mtcars, function(x) length(unique(x)))
+map_dbl(mtcars, function(unicorn) length(unique(unicorn)))
 map_dbl(mtcars, ~length(unique(.x)))
 map_dbl(mtcars, ~length(unique(..1)))
 map_dbl(mtcars, ~length(unique(.)))
@@ -131,16 +131,23 @@ map_dbl(mtcars, ~length(unique(.)))
 map_dbl(mtcars, length)
 map_dbl(mtcars, length(unique))
 map_dbl(mtcars, 1)
+```
 
+```{r}
+#| echo: false
+#| message: false
+#| warning: false
+rm(x)
+```
+
+```{r}
+#| error: true
 #error
 map_dbl(mtcars, length(unique()))
 map_dbl(mtcars, ~length(unique(x)))
-
-
 ```
 
 
-
 ## Modify {-}
 
 Sometimes we might want the output to be the same as the input, then in that case we can use the modify function rather than map
@@ -202,8 +209,6 @@ There are many variants
 ![](images/map_variants.png)
 
 
-
-
 ## `map2_*()` {-}
 
 - raise each value `.x` by 2
@@ -249,6 +254,14 @@ map(1:3, ~cat(.x, "\n"))
 walk(1:3, ~cat(.x, "\n"))
 ```
 
+`cat()` does have a result, it's just usually returned invisibly.
+
+```{r}
+cat("hello")
+
+(cat("hello"))
+```
+
 
 We can use `pwalk()` to save a list of plot to disk.  Note that the "p" in `pwalk()` means that we have more than 1 (or 2) variables to pipe into the function.  Also note that the name of the first argument in all of the "p" functions is now `.l` (instead of `.x`).
 
@@ -340,6 +353,8 @@ The `reduce()` function is a powerful functional that allows you to abstract awa
 
 `reduce()` takes a vector as its first argument, a function as its second argument, and an optional `.init` argument last.  It will then apply the function repeatedly to the vector until there is only a single element left.
 
+(Hint: start at the top of the image and read down.)
+
 ```{r,echo=FALSE,warning=FALSE,message=FALSE}
 knitr::include_graphics(path = 'images/reduce-init.png')
 ```
@@ -422,7 +437,7 @@ accumulate2(
 ```
 
 
-## Not covered: `map_df*()` variants {-}
+## `map_df*()` variants {-}
 
 - `map_dfr()` = row bind the results
 
@@ -447,7 +462,7 @@ map((1:2) * 10, col_stats) |> list_rbind()
 
 ---
 
-## Not covered: `pluck()` {-}
+## `pluck()` {-}
 
 - `pluck()` will pull a single element from a list
 
@@ -472,7 +487,7 @@ map(my_list, pluck, 1)
 map_dbl(my_list, pluck, 1)
 ```
 
-The `map()` functions also have shortcuts for extracting elements from vectors (powered by `purrr::pluck()`).
+The `map()` functions also have shortcuts for extracting elements from vectors (powered by `purrr::pluck()`).  Note that `map(my_list, 3)` is a shortcut for `map(my_list, pluck, 3)`.
 
 ```{r}
 #| error: true
@@ -501,7 +516,6 @@ map_chr(my_list, "z", .default = NA)
 ```
 
 
-
 ## Not covered: `flatten()` {-}
 
 - `flatten()` will turn a list of lists into a simpler vector.