1. Data: Lists and Dataframes
Beau Benjamin Bruce, MD, PhD
Analytics Lead, Enteric Diseases Epidemiology Branch
DFWED/NCEZID/CDC

2. New food: list
Lists allow you to mix different types of atomic data together
list(“A”, 2, TRUE)
[[1]]
[1] "A"
[[2]]
[1] 2
[[3]]
[1] TRUE

3. New food: list Can even include other composite data types
list(“A”, c(1,2,3,4), TRUE)
[[1]]
[1] "A"
[[2]]
[1]
[[3]]
[1] TRUE

4. New food: list How to access the elements of a list
mylist <- list(a=“A”, b=c(1,2,3,4), c=TRUE)
mylist[[1]] => “A”
mylist[[2]] =>
What if wanted to get the 3 out alone??

5. New food: list How to access the elements of a list
mylist <- list(a=“A”, b=c(1,2,3,4), c=TRUE)
mylist[[2]] =>
mylist[[2]][3] => 3

6. List accessors for list ‘train’
A list (train) made only of the first car
train[c(1,2)]
A list made of the first and second car
train[3]
A list made of the third car

7. List accessors for list ‘train’
A list made of the second car
train[[2]]
The contents of the second car: a vector of cargo containers (CCs)
train[[2]][2]
The second CC in vector of CCs in the second car

8. How about a list of lists: trainb?
A list made of the first element of trainb (which contains another list)
A list made of the second car
trainb[[1]]
The contents of the first element of trainb: a list with two cars
train[[2]][2]
trainb[[1]][1] …
The list made from the first element of the list extracted just above

9. How about a list of lists: trainb?
Same list we extracted from first care of trainb on previous slide
A list made of the second car
trainb[[1]][[1]] …
Now extract the contents of its first element: a vector of CCs
trainb[[1]][[1]][2]
train[[2]][2] …
And take the second element of that vector of CCs: a single CC

10. New food: list Can name the elements list(a=“A”, b=c(1,2,3,4), c=TRUE)
[1] $c
[1] TRUE

11. New food: list
Can use $ to access by name (another very shy creature!)
mylist <- list(a=“A”, b=c(1,2,3,4), c=TRUE)
mylist$b =>
mylist$b[2] => 2

12. New food: list Like a vector can change a part
mylist <- list(a=“A”, b=c(1,2,3,4), c=TRUE)
mylist$b[2] <- 4
mylist$b => ???
is.list(mylist) => ???

13. New food: data.frame
A special list whose elements (typically vectors) all have same length (i.e., the same number of observations)
data.frame(age=c(3,2,3,3,1,2,4,4),
sorethroat=factor(c(“y”,”y”,”y”, “y”,”n”,”n”,”n”,”n”)))
two variables, 8 observations

14. New food: data.frame
Usually will create data.frames by importing them from other files (e.g., CSV, Excel, SAS data files)
Show the variable names:
names(iih_data)
Access variable by name with $ like a list:
iih_data$sex

15. New food: data.frame Access row by number: Access column by number:
iih_data[3, ]
Access column by number:
iih_data[, 3]
Access rows by test:
iih_data[iih_data$sex == “M”, ]
Change a value
iih_data$sex[3] <- “F”

16. class function Can be used instead of the is.xxxx functions
Returns what type of object something is, but may not tell you the whole story:
class(iih_data) => “data.frame”
is.data.frame(iih_data) => TRUE
is.list(iih_data) => TRUE