This is a brief introduction. The syntax is:
functionName = function(inputs) {
< function body >
return(value)
}
Then you would run the 4 lines of the code, which adds it to your workspace.
January 9, 2016
This is a brief introduction. The syntax is:
functionName = function(inputs) {
< function body >
return(value)
}
Then you would run the 4 lines of the code, which adds it to your workspace.
Here we will write a function that returns the second element of a vector:
> return2 = function(x) {
+ return(x[2])
+ }
> return2(c(1,4,5,76))
[1] 4
Note that your function will automatically return the last line of code run:
> return2a = function(x) {
+ x[2]
+ }
> return2a(c(1,4,5,76))
[1] 4
And if your function is really one line or evaluation, like here, you do not need the curly brackets, and you can put everything on one line:
> return2b = function(x) x[2] > return2b(c(1,4,5,76))
[1] 4
Also note that functions can take multiple inputs. Maybe you want users to select which element to extract
> return2c = function(x,n) x[n] > return2c(c(1,4,5,76), 3)
[1] 5
Let's write a function, sqdif, that:
x and y with default values of 2 and 3.> sqdif <- function(x=2,y=3){
+ (x-y)^2
+ }
>
> sqdif()
[1] 1
> sqdif(x=10,y=5)
[1] 25
> sqdif(10,5)
[1] 25
Try to write a function called top() that takes a matrix or data.frame, and returns the first n rows and columns, with the default value of n=5.
Try to write a function called top() that takes a matrix or data.frame, and returns the first n rows and columns
> top = function(mat,n=5) mat[1:n,1:n] > my.mat = matrix(1:1000,nr=100) > top(my.mat) #note that we are using the default value for n
[,1] [,2] [,3] [,4] [,5] [1,] 1 101 201 301 401 [2,] 2 102 202 302 402 [3,] 3 103 203 303 403 [4,] 4 104 204 304 404 [5,] 5 105 205 305 405
applyYou can use any function you want in apply statements. For example, from our split Circulator data
> circ = read.csv("http://www.aejaffe.com/winterR_2016/data/Charm_City_Circulator_Ridership.csv",
+ header=TRUE,as.is=TRUE)
> dayList = split(circ, circ$day)
> lapply(dayList, top, n = 2)
$Friday
day date
5 Friday 01/15/2010
12 Friday 01/22/2010
$Monday
day date
1 Monday 01/11/2010
8 Monday 01/18/2010
$Saturday
day date
6 Saturday 01/16/2010
13 Saturday 01/23/2010
$Sunday
day date
7 Sunday 01/17/2010
14 Sunday 01/24/2010
$Thursday
day date
4 Thursday 01/14/2010
11 Thursday 01/21/2010
$Tuesday
day date
2 Tuesday 01/12/2010
9 Tuesday 01/19/2010
$Wednesday
day date
3 Wednesday 01/13/2010
10 Wednesday 01/20/2010
applyYou can also designate functions "on the fly"
> lapply(dayList, function(x) x[1:2,1:2])
$Friday
day date
5 Friday 01/15/2010
12 Friday 01/22/2010
$Monday
day date
1 Monday 01/11/2010
8 Monday 01/18/2010
$Saturday
day date
6 Saturday 01/16/2010
13 Saturday 01/23/2010
$Sunday
day date
7 Sunday 01/17/2010
14 Sunday 01/24/2010
$Thursday
day date
4 Thursday 01/14/2010
11 Thursday 01/21/2010
$Tuesday
day date
2 Tuesday 01/12/2010
9 Tuesday 01/19/2010
$Wednesday
day date
3 Wednesday 01/13/2010
10 Wednesday 01/20/2010
sapply() is a user-friendly version and wrapper of lapply by default returning a vector, matrix, or array
> sapply(dayList, dim)
Friday Monday Saturday Sunday Thursday Tuesday Wednesday [1,] 164 164 163 163 164 164 164 [2,] 15 15 15 15 15 15 15
> sapply(circ, class)
day date orangeBoardings orangeAlightings
"character" "character" "integer" "integer"
orangeAverage purpleBoardings purpleAlightings purpleAverage
"numeric" "integer" "integer" "numeric"
greenBoardings greenAlightings greenAverage bannerBoardings
"integer" "integer" "numeric" "integer"
bannerAlightings bannerAverage daily
"integer" "numeric" "numeric"
> myList = list(a=1:10, b=c(2,4,5), c = c("a","b","c"),
+ d = factor(c("boy","girl","girl")))
> tmp = lapply(myList,function(x) x[1])
> tmp
$a [1] 1 $b [1] 2 $c [1] "a" $d [1] boy Levels: boy girl
> sapply(tmp, class)
a b c d "integer" "numeric" "character" "factor"
> sapply(myList,function(x) x[1])
a b c d "1" "2" "a" "1"
> sapply(myList,function(x) as.character(x[1]))
a b c d "1" "2" "a" "boy"