RStudio

2.4.1. A Video Example of Using RStudio

2.5.1. Variables, Loops and Functions

1. Variables
   

   A variable is something that holds a value. A value doesn't have to be a number. It could be name, character, or a list of pupil diameters, of even some larger data structure. As the name implies a variable may vary in most programming languages. You could for instance use a variable to "count" how many times you have done an operation. Repeating an operation or action several times is often referred to as "looping."

   Listing 1: Declaring a Variable in R. Note that this is typeset using a markup language call orgmode that is accessible in emacs, a text editing environment we will learn to use later in this course. If you look at the source code for this file you will see that this code is being executed to produce the final html file that you can view online.

   a = 3
   a = a + 1
   print(a)
   

   4
   

2. Loops
   

   Often we use programs to automate things that need to be done over and over again. In many statistical and experimental procedures this need for repetition is common. Consider finding the mean reaction time for a large number of experimental participants who each completed a large number of experimental trials.

   When you have a task to do that needs to run until some condition is met you will often use a while loop. When you need to do something for a set number of time or through a list of items then you will typically use a for loop. A for loop iterates through a structure and executes the same code, the code in the body of the loop each time through.

   Listing 2: A For Loop in R

   for (i in seq(1,10)) {
     print(i)
   }
   

   [1] 1
   [1] 2
   [1] 3
   [1] 4
   [1] 5
   [1] 6
   [1] 7
   [1] 8
   [1] 9
   [1] 10
   

   Org-babel is a way to include code in an org document and have it properly execute. There is, of course, an Emacs manual : C-h i Search for org mode and in the org mode manual look for the "Library of Babel". A pretty extensive tutorial can be found here. A more recent workshop gives examples here.

   We will want some basic text in our init.el file to make things work. See here as well for some hints on getting started with your init.el file.

   (setq org-confirm-babel-evaluate nil)
   (setq org-babel-python-command "python")
     (org-babel-do-load-languages
     'org-babel-load-languages
     '((emacs-lisp . t) 
     (R . t)
     (python . t)))
   

3. Functions
   

   Functions in programming languages are black boxes for taking input and returning output. You, the author of the function, fill in the details of the black box, but we users only know what we feed in and what we get back. Each programming language has its own way of telling the processor what is and what is not a function. In R we use the keyword function and assign our code to a name that users can type to call our code. It also allows us to say what the input can be and to give names to the input that are used locally in our function.

   For example, let's say we want to know if a number is even. We can use R's built in function to tell us the remainder of a division. This function is an infix function as we put it INside its arguments. It uses the symbols %%. For example ~ 5 %% 2~ equals 1 because if you divide 5 by 2 you have 1 left over.

   To know if a number is odd or even we only need to know if the remainder when divided by two is or is not zero. To do this we can use a predicate. A predicate is a test that comes back true or false depending on whether a condition is met. Putting this altogether we can write a function that evaluates if a number is odd.

   Listing 3: Defining a Function in R

   isOdd <- function(n) {
              return(n %% 2 != 0)
   }
   

   And then we can use our function in a test:

   print(isOdd(5))
   

   [1] TRUE
   

   We could then use this output value to test whether to print the number by using an if statement.

3.1.1. Tasks

1. Comments
   

   We will be using various markdown dialects to combine text, references, code, data, and analyses in making reproducible research reports. The goal of this assignment is to get you started on discovering and using some of the markdown conventions that will enable you to do so. RStudio has a built in cheat sheet for markdown, but you can easily find others on line. There are several different dialects of markdown.

3.2.1. Task

3.2.2. Goal/Purpose

3.2.3. Instructions