Looping on the command line

Overview

Teaching: 10 min
Exercises: 0 min

Questions

• How to apply function over the list, matrix?

Objectives

• Learn apply, sapply, lapply function

There are functions in R that make looping easier:

• apply: apply function over margins of array
• lapply: looping over list and evaluate function on element
• sapply: similar to lapply, but simpler
• mapply: multivariate version of lapply
• tapply: apply function over subsets of vector

apply

• Most often used to apply function to row or column of matrix
• Not really faster than loop but simpler coding

str(apply)
m <- matrix(1:12,3,4)
print(m)
apply(m,1,sum)
apply(m,2,sum)

Similar functions for matrix:

rowSums(m)
colSums(m)
rowMeans(m)
colMeans(m)

lapply & sapply

l: long s: short lapply & sapply applies the FUN to each element of a list

list1 <- list(l1 = seq(1,10),l2=20:29,l3=rnorm(4))
lapply(list1,mean)
sapply(list1,mean)

Key Points

• apply function