Add exercises. Update adverbs

lists.Rmd

@@ -228,6 +228,24 @@ compute_summary(x, mean)
 
 Instead of hardcoding the summary function, we allow it to vary, by adding an addition argument that is a function. It can take a while to wrap your head around this, but it's very powerful technique. This is one of the reasons that R is known as a "functional" programming language.
 
+### Exercises
+
+1.  Read the documentation for `apply()`. In the 2d case, what two for loops
+    does it generalise?
+    
+1.  It's common to see for loops that don't preallocate the output and instead
+    increase the length of a vector at each step:
+    
+    ```{r}
+    results <- vector("integer", 0)
+    for (i in seq_along(x)) {
+      results <- results(c, results)
+    }
+    results
+    ```
+    
+    How does this impact performance? 
+
 ## The map functions
 
 This pattern of looping over a list and doing something to each element is so common that the purrr package provides a family of functions to do it for you. Each function always returns the same type of output so there are six variations based on what sort of result you want:
@@ -304,14 +322,25 @@ If you're familiar with the apply family of functions in base R, you might have
     `map_lgl(df, is.numeric)`. One advantage to `vapply()` over the map 
     functions is that it can also produce matrices.
 
+### Exercises
+
+1.  How can you determine which columns in a data frame are factors? 
+    (Hint: data frames are lists.)
+
+1.  What happens when you use the map functions on vectors that aren't lists?
+    What does `map(1:5, runif)` do? Why?
+    
+1.  What does `map(-2:2, rnorm, n = 5)` do. Why?
+
 ## Pipelines
 
-`map()` is particularly useful when constructing more complex transformations because it both inputs and outputs a list. That makes it well suited for solving a problem a piece at a time. For example, imagine you want to fit a linear model to each individual in a dataset.
+`map()` is particularly useful when constructing more complex transformations because it both inputs and outputs a list. That makes it well suited for solving a problem a piece at a time. 
 
-Let's start by working through the whole process on the complete dataset. It's always a good idea to start simple (with a single object), and figure out the basic workflow. Then you can generalise up to the harder problem of applying the same steps to multiple models. 
 
 TODO: find interesting dataset
 
+For example, imagine you want to fit a linear model to each individual in a dataset. Let's start by working through the whole process on the complete dataset. It's always a good idea to start simple (with a single object), and figure out the basic workflow. Then you can generalise up to the harder problem of applying the same steps to multiple models. 
+
 You could start by creating a list where each element is a data frame for a different person:
 
 ```{r}
@@ -407,12 +436,12 @@ Other predicate functionals: `head_while()`, `tail_while()`, `some()`, `every()`
 
 When you start doing many operations with purrr, you'll soon discover that not everything always succeeds. For example, you might be fitting a bunch of more complicated models, and not every model will converge. How do you ensure that one bad apple doesn't ruin the whole barrel?
 
-Dealing with errors is fundamentally painful because errors are sort of a side-channel to the way that functions usually return values. The best way to handle them is to turn them into a regular output with the `safe()` function.  This function is similar to the `try()` function in base R, but instead of sometimes returning the original output and sometimes returning a error, `safe()` always returns the same type of object: a list with elements `result` and `error`. For any given run, one will always be `NULL`, but because the structure is always the same its easier to deal with.
+Dealing with errors is fundamentally painful because errors are sort of a side-channel to the way that functions usually return values. The best way to handle them is to turn them into a regular output with the `safely()` function.  This function is similar to the `try()` function in base R, but instead of sometimes returning the original output and sometimes returning a error, `safe()` always returns the same type of object: a list with elements `result` and `error`. For any given run, one will always be `NULL`, but because the structure is always the same its easier to deal with.
 
 Let's illustrate this with a simple example: `log()`:
 
 ```{r}
-safe_log <- safe(log)
+safe_log <- safely(log)
 str(safe_log(10))
 str(safe_log("a"))
 ```
@@ -459,10 +488,10 @@ dplyr::filter(all, is_ok)
 
 Other related functions:
 
-* `maybe()`: if you don't care about the error message, and instead
+* `possibly()`: if you don't care about the error message, and instead
   just want a default value on failure.
   
-* `outputs()`: does a similar job but for other outputs like printed
+* `quietly()`: does a similar job but for other outputs like printed
   ouput, messages, and warnings.
 
 Challenge: read all the csv files in this directory. Which ones failed