Introduction To Data Frames
- describe high level idea of data frames
- load CSV file on local file system
- select columns by name
- interpret interactive results of
class, dim, head, tail, unique, table
123GO - On a scale of 1 (easy) to 10 (hard), how difficult did you find the last homework?
Announcements:
- I expected to see more chatter on the Canvas discussions and Discord. Ask more questions, because we can and should help each other. Don’t sit there stuck for hours.
- Remember to note in the assignment if you work with someone else. That’s what professors do, and people in open source software do. Explaining ideas to another is a wonderful way to learn.
- To get the benefit from this course, you do need to do your own work, and completely understand it.
data frames
Let’s see what R has to say about data frames.
?data.frame
According to the official documentation, data frames are:
tightly coupled collections of variables … used as the fundamental data structure by most of R’s modeling software.
… a matrix-like structure whose columns may be of differing types (numeric, logical, factor and character and so on).
Sounds important.
Think of a data frame as a spreadsheet or table, where each row is an observation.
Let’s check out airquality, a data frame included with R, just like precip.
Air quality should be on your mind if you’re local.
Here are the functions I reach for first when loading a new data set.
class
dim
head
summary
select columns
Type airq and then tab for “tab completion”.
Q: What happened? The name of the object matching the first couple letters showed up.
Q: Why use it? Faster and more accurate.
airquality has a column for “Month”.
Let’s extract this individual column from the data frame and work with it.
One way that works well for interactive use is $.
m = airquality$Month
class(m)
The form is container$element to extract the element from the container.
We can also use tab completion for Month, as in airquality$M then press tab.
Q: Which months are represented in the data? If only we knew the unique months…
unique(m)
How many from each month?
table(m)
load external files
Here is the easiest way to load the data:
air = read.csv("http://webpages.csus.edu/fitzgerald/files/stat128/fall20/ad_viz_plotval_data.csv")
Q: What happened? Was the file local or remote?
R visited a public URL, downloaded a data set, and loaded it into my R session. Cool!
your turn
Everyone: Download the data file for this week’s homework. It consists of air quality measurements around Sacramento for the last year.
Look at the output of the following functions:
class
dim
head
summary
I’d like to know which sites are represented in this data set. Try to figure it out. I’ll be standing by to help with voice or chat.
table(air$Site.Name)