Chapter 5 Working with Data Objects
How do we work with data objects? Often times, we may want to store results of an R calculation for later access.
<- is called an assignment operator and assigns the value on its right to an object named on its left. The equal sign, =, works the same way (in most cases). However, avoid using the equal sign in the off chance you encounter the rare case where it does not work.
Let us assign 3 to the variable m. Be sure to run the code after each command line. You can click “Run” on the upper right-hand side of the Source panel or you can use keystrokes. Keystrokes are usually easier and faster. For PCs, hit “ctrl + enter” and for macs, hit “command + return”.
##  3
Look at the Console panel for calculations or analyses. If you ever need to see what is stored in a given variable, look at the Environment panel. You should see the variable m and its value.
Items after the # symbol are comments and are ignored by the R interpreter. It is a good practice to comment your code. Each line of a comment should begin with the comment symbol, #, and a single space.
Let us create another vector. Let n be a numeric vector created out of a sequence that starts with 13 and ends 19, incremented by 1. We will be using the function c( ). c stands for concatenate (or coerce or combine).
##  13 14 15 16 17 18 19
We can do arithmetic operations on vectors.
##  18 19 20 21 22 23 24
Let w be a character vector made up of the following: Adam, Bob, Charlie, David, Emma, Fiona and George.
##  "Adam" "Bob" "Charlie" "David" "Emma" "Fiona" "George"
5.2 Retrieving a Value
To retrieve a value in a vector, declare an index inside the square bracket, [ ] operator.
##  "David"
To retrieve several values, use the function c( ) inside the square brackets.
##  "Bob" "Charlie" "David" "Emma"
##  "Adam" "David"
Note that the result of the square bracket operator is another vector.
##  "Adam" "David"
Alternatively, you can look in the Environment panel and look for new_w. You will see the elements of new_w, including its mode.
To determine the length of a particular vector, we use the function length.
##  2
The result shows that there are 2 elements in the vector, new_w.
If the index is negative, the new vector strips the member whose position has the same absolute value as the negative index.
##  "Adam" "Bob" "Charlie" "Emma" "Fiona" "George"
##  "Bob" "Charlie" "Emma" "Fiona" "George"
##  "Adam" "Fiona" "George"
5.3 Error Messages
What happens when the index is out of range? We know that vector w has only 7 entries. Let us extract the 10th entry.
Trying to extract the 10th entry is impossible and hence the message, NA.
##  NA
What happens when we ask for an object that does not yet exist in R’s environment? Let us take a look at the result when we call the object, numeral.
## Error in eval(expr, envir, enclos): object 'numeral' not found
Is R case sensitive? Suppose, instead of typing lowercase, w, we type uppercase, W. Note that the vector, w (in lowercase) exists but not the vector, W.
Yes, R is case sensitive!!
## Error in eval(expr, envir, enclos): object 'W' not found