Write section on with/within

content/markup/software/longform/r-the-beautiful-parts.markdown

@@ -161,7 +161,84 @@ I think this serves as an example that if you follow the R way of doing things
 then your code is compact and simple. The alternative is to end up fighting 
 against R to get the results you need.
 
-### with/within
+### Prefer with/within over attach
+
+Accessing each each column vector from a data fame using the `$` name prefix is 
+cumbersome. The `attach` method can be used to get around this by directly 
+adding the column vectors into the R search path. Using this method does not 
+however add any changes made to these variable back into the parent data frame. 
+For example:
+
+    my.data <- data.frame(x = 1:3,
+                          y = (1:3)*2 + 2)
+    my.data
+    #=>    x y
+    #    1 1 4
+    #    2 2 6
+    #    3 3 8
+
+    # Calculate the x*y product
+    # Using the `$` verbose notation
+    my.data$z <- my.data$x * my.data$y
+    my.data
+    #=>    x y  z
+    #    1 1 4  4
+    #    2 2 6 12
+    #    3 3 8 24
+
+    # Calculate using attach
+    # The x and y variables can be directly accessed
+    attach(my.data)
+    z <- x * y
+
+    # The new variable `z` is however not part of my.data
+    my.data
+    #=>    x y
+    #    1 1 4
+    #    2 2 6
+    #    3 3 8
+
+The `with` and `within` functions allow you to do many of the functions you 
+might need to do with a data without verbose syntax whilst still maintaining 
+the data as part of the data frame. For example the following highlights using 
+`within` to perform the same operations:
+
+    # Using `within` returns the updated to the data frame
+    my.data <- within(my.data, z <- x * y)
+    my.data
+    #=>    x y  z
+    #    1 1 4  4
+    #    2 2 6 12
+    #    3 3 8 24
+
+    # Multiple, multi-line operations using curly brace expressions
+    my.data <- within(my.data,{
+      z1 <- x * y
+      z2 <- x + y
+    })
+    my.data
+    #=>    x y z1 z2
+    #    1 1 4  4  5
+    #    2 2 6 12  8
+    #    3 3 8 24 11
+
+The with function is similar to the within function where instead the result of 
+the expression is returned rather than updating the passed data frame. This 
+useful for creating models or plots from data frames, where the last call is 
+returned. This can be illustrated as follows:
+
+    # The result of the expression is returned rather
+    # than updating the data
+    with(my.data, z <- x * y)
+    #=> 4 12 24
+
+    # Create a linear model
+    with(my.data, {
+      z <- x^2 + 1
+      lm(y ~ z)
+    })
+    #=> The linear model is returned
+
 ### apply
 ### anonymous functions
 ### conditionals if/switch