Fix typos (#156)

functions.Rmd

@@ -52,7 +52,7 @@ To write a function you need to first analyse the code. How many inputs does it
   (max(df$a, na.rm = TRUE) - min(df$a, na.rm = TRUE))
 ```
 
-This code only has one input: `df$a`. (It's a little suprisingly that `TRUE` is not an input: you can explore why in the exercise below). To make the single input more clear, it's a good idea to rewrite the code using temporary variables with a general name. Here this function only takes one vector of input, so I'll call it `x`:
+This code only has one input: `df$a`. (It's a little surprisingly that `TRUE` is not an input: you can explore why in the exercise below.) To make the single input more clear, it's a good idea to rewrite the code using temporary variables with a general name. Here this function only takes one vector of input, so I'll call it `x`:
 
 ```{r}
 x <- 1:10
@@ -175,7 +175,7 @@ This is an important part of the "do not repeat yourself" (or DRY) principle. Th
 
 It's important to remember that functions are not just for the computer, but are also for humans. R doesn't care what your function is called, or what comments it contains, but these are important for human readers. This section discusses some things that you should bear in mind when writing functions that humans can understand.
 
-The name of a function is important. Ideally the name of your function will be short, but clearly evoke what the function does. However, it's hard to come up with concise names, and autocomplete makes it easy to type long names, so it's better to err on the side of clear descriptions, rather than short names. 
+The name of a function is important. Ideally, the name of your function will be short, but clearly evoke what the function does. However, it's hard to come up with concise names, and autocomplete makes it easy to type long names, so it's better to err on the side of clear descriptions, rather than short names. 
 
 Generally, function names should be verbs, and arguments should be nouns. There are some exceptions: nouns are ok if the function computes a very well known noun (i.e. `mean()` is better than `compute_mean()`), or accessing some property of an object (i.e. `coef()` is better than `get_coefficients()`). A good sign that a noun might be a better choice is if you're using a very broad verb like "get", "compute", "calculate", or "determine". Use your best judgement and don't be afraid to rename a function if you figure out a better name later.