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Introduction to the R Language
Loop Functions
Roger D. Peng, Associate Professor of Biostatistics
Johns Hopkins Bloomberg School of Public Health
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io2012
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selfcontained

Looping on the Command Line

Writing for, while loops is useful when programming but not particularly easy when working interactively on the command line. There are some functions which implement looping to make life easier.

  • lapply: Loop over a list and evaluate a function on each element

  • sapply: Same as lapply but try to simplify the result

  • apply: Apply a function over the margins of an array

  • tapply: Apply a function over subsets of a vector

  • mapply: Multivariate version of lapply

An auxiliary function split is also useful, particularly in conjunction with lapply.

lapply

lapply takes three arguments: (1) a list X; (2) a function (or the name of a function) FUN; (3) other arguments via its ... argument. If X is not a list, it will be coerced to a list using as.list.

lapply
## function (X, FUN, ...) 
## {
##     FUN <- match.fun(FUN)
##     if (!is.vector(X) || is.object(X)) 
##         X <- as.list(X)
##     .Internal(lapply(X, FUN))
## }
## <bytecode: 0x7ff7a1951c00>
## <environment: namespace:base>

The actual looping is done internally in C code.

lapply

lapply always returns a list, regardless of the class of the input.

x <- list(a = 1:5, b = rnorm(10))
lapply(x, mean)
## $a
## [1] 3
## 
## $b
## [1] 0.4671

lapply

x <- list(a = 1:4, b = rnorm(10), c = rnorm(20, 1), d = rnorm(100, 5))
lapply(x, mean)
## $a
## [1] 2.5
## 
## $b
## [1] 0.5261
## 
## $c
## [1] 1.421
## 
## $d
## [1] 4.927

lapply

> x <- 1:4
> lapply(x, runif)
[[1]]
[1] 0.2675082

[[2]]
[1] 0.2186453 0.5167968

[[3]]
[1] 0.2689506 0.1811683 0.5185761

[[4]]
[1] 0.5627829 0.1291569 0.2563676 0.7179353

lapply

> x <- 1:4
> lapply(x, runif, min = 0, max = 10)
[[1]]
[1] 3.302142

[[2]]
[1] 6.848960 7.195282

[[3]]
[1] 3.5031416 0.8465707 9.7421014

[[4]]
[1] 1.195114 3.594027 2.930794 2.766946

lapply

lapply and friends make heavy use of anonymous functions.

> x <- list(a = matrix(1:4, 2, 2), b = matrix(1:6, 3, 2)) 
> x
$a
     [,1] [,2]
[1,]    1    3
[2,]    2    4

$b
     [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6

lapply

An anonymous function for extracting the first column of each matrix.

> lapply(x, function(elt) elt[,1])
$a
[1] 1 2

$b
[1] 1 2 3

sapply

sapply will try to simplify the result of lapply if possible.

  • If the result is a list where every element is length 1, then a vector is returned

  • If the result is a list where every element is a vector of the same length (> 1), a matrix is returned.

  • If it can’t figure things out, a list is returned

sapply

> x <- list(a = 1:4, b = rnorm(10), c = rnorm(20, 1), d = rnorm(100, 5))
> lapply(x, mean)
$a
[1] 2.5

$b
[1] 0.06082667

$c
[1] 1.467083

$d
[1] 5.074749

sapply

> sapply(x, mean) 
         a          b          c          d
2.50000000 0.06082667 1.46708277 5.07474950

> mean(x)
[1] NA
Warning message:
In mean.default(x) : argument is not numeric or logical: returning NA