Many Ways To Subset Data
- Describe the use cases for different ways to select subsets of data
Announcements:
Midterm in two parts. In class part of midterm through Canvas quizzes on Friday, October 30th. Multiple choice / true false, etc. Low stakes, low pressure. Take home part of midterm due Sunday, November 1st. Let me know ASAP if you have technical / internet / scheduling issues. Watch the review videos, the questions will be on those concepts.
Plan today:
- Review mid quarter check in.
- Showcase some of the unique plots from Q3 in the Roulette homework.
- Group activity to review all the ways to subset a data frame.
Goal today is to step back and review / compare and contrast all the ways we know to select subsets of a data frame.
Group activity
Come up with all the ways you know how to select subsets of a vector or data frame.
We’ll do examples with the mtcars data set, because cars are on my mind right now. 😁
We’ll take what you come up with and try to make a somewhat comprehensive list of the ways to subset a data frame.
Vectors
Here’s a vector for us to play with.
x = 1:10 / 10
logical
x[x < 0.5]
index
x[c(5, 9)]
And negative index:
x[-c(5, 9)]
names
If the vector has names we can use them.
x = seq_along(letters)
names(x) = letters
x[c("a", "b")]
Data Frames
Recall the general form is
DATA[ROWS, COLUMNS]
logical
Selection through logical critieria is the most generally useful for data analysis.
For example, “cars that have greater than 30 mpg”
mtcars[30 < mtcars$mpg, ]
Some variants:
subset(mtcars, 30 < mpg)
with(mtcars, mtcars[30 < mpg, ])
index
# First five rows
mtcars[1:5, ]
# First four columns
mtcars[, 1:4]
# columns and rows
mtcars[1:5, 1:4]
Negative index
mtcars[-(1:5), ]
mtcars[, -(1:5)]
names
mtcars[, "mpg"]
mtcars["Honda Civic", ]
mtcars["Honda Civic", "mpg"]
Then there is the $ operator.
mtcars$mpg
Will this partial match work?
mtcars$mp
Yes- it’s useful for interactive situations, say when a column name has 20 characters.
Check out the difference between [ and $ if we try to use a column that doesn’t exist.
mtcars$stat128
mtcars[, "stat128"]