1. Different versions of software
2. Data type problems (is that a string, number, or factor?)
3. Working directory problems: trying to read files that R "can't find"
   • RStudio can help, and so do RStudio Projects
   • discuss in Data Input/Output lecture
4. Typos (R is case sensitive, x and X are different)
   • RStudio helps with "tab completion"
   • discussed throughout

• You should have the latest version of R installed!
• Open RStudio
• Files –> New –> R Script
• Save the blank R script in a directory of your choosing
• Add a comment header

Add a comment header to day1.R :# is the comment symbol

#################
# Title: Demo R Script
# Author: Steve Bellan
# Purpose: Demonstrate comments in R
###################
 
# nothing to the right of the symbol is evaluated by R

# this # is still a comment
### you can use many #'s as you want

# sometimes you have a really long comment,
#    like explaining what you are doing 
#    for a step in analysis. 
# Take it to another line

In slides, a command (we'll also call them code or a code chunk) will look like this

print("I'm code")

[1] "I'm code"

And then directly after it, will be the output of the code.
So print("I'm code") is the code chunk and [1] "I'm code" is the output.

2 + 2

[1] 4

2 * 4

[1] 8

2 ^ 3

[1] 8

Note, when you type your command, R inherently thinks you want to print the result.

For now, let's ignore the [1]. We'll return back to why R prints this in front of each object it prints later on.

• The R console is a full calculator
• Try to play around with it:
  • +, -, /, * are add, subtract, divide and multiply
  • ^ or ** is power
  • parentheses – ( and ) – work with order of operations

2 + (2 * 3)^2

[1] 38

(1 + 3) / 2 + 45

[1] 47

Try evaluating the following:

• 2 + 2 * 3 / 4 -3
• 2 * 3 / 4 * 2
• 2^4 - 1

• You can create variables from within the R environment and from files on your computer
• R uses "=" or "<-" to assign values to a variable name
• Variable names are case-sensitive, i.e. X and x are different

x = 2 # Same as: x <- 2
x

[1] 2

x * 4

[1] 8

x + 2

[1] 4

• The most comfortable and familiar class/data type for many of you will be data.frame
• You can think of these as essentially Excel spreadsheets with rows (usually subjects or observations) and columns (usually variables)

• data.frames are somewhat advanced objects in R; we will start with simpler objects;
• Here we introduce "1 dimensional" classes; often referred to as 'vectors'
• Vectors can have multiple sets of observations, but each observation has to be the same class.

class(x)

[1] "numeric"

y = "hello world!"
print(y)

[1] "hello world!"

class(y)

[1] "character"

Try assigning your full name to an R variable called name

Try assigning your full name to an R variable called name

name = "Steve Bellan"
name

[1] "Steve Bellan"

The function c() collects/combines/joins single R objects into a vector of R objects. It is mostly used for creating vectors of numbers, character strings, and other data types.

x <- c(1, 4, 6, 8, 1, 1, 3, 4)
x

[1] 1 4 6 8 1 1 3 4

class(x)

[1] "numeric"

Try assigning your first and last name as 2 separate character strings into a single vector called name2

Try assigning your first and last name as 2 separate character strings into a length-2 vector called name2

name2 = c("Steve","Bellan")
name2

[1] "Steve"  "Bellan"

length(): Get or set the length of vectors (including lists) and factors, and of any other R object for which a method has been defined.

length(x)

[1] 8

y

[1] "hello world!"

length(y)

[1] 1

What do you expect for the length of the name variable? What about the name2 variable?

What are the lengths of each?

What do you expect for the length of the name variable? What about the name2 variable?

What are the lengths of each?

length(name)

[1] 1

length(name2)

[1] 2

You can perform functions to entire vectors of numbers very easily.

x + 2

[1]  3  6  8 10  3  3  5  6

x * 3

[1]  3 12 18 24  3  3  9 12

x + c(1, 2, 3, 4)

[1]  2  6  9 12  2  3  6  8

But things like algebra can only be performed on numbers.

Learn to read R's Errors and Warnings. They will help you debug code.

> name2 + 4
[1] Error in name2 + 4 : non-numeric argument
 to binary operator

And save these modified vectors as a new vector.

y = x + c(1, 2, 3, 4)
y 

[1]  2  6  9 12  2  3  6  8

Note that the R object y is no longer "Hello World!" - It has effectively been overwritten by assigning new data to the variable

• You can get more attributes than just class. The function str gives you the structure of the object.

str(x)

 num [1:8] 1 4 6 8 1 1 3 4

str(y)

 num [1:8] 2 6 9 12 2 3 6 8

This tells you that x is a numeric vector and tells you the length.

• Creating a new script
• Using R as a calculator
• Assigning values to variables
• Performing algebra on numeric variables

EPID 7500 Course Website

John Muschelli's Course at Johns Hopkins & github repository