Loop in R

We can categories the R loops in mainly three types, implicit loops, explicit loops and iterators.

Implicit Loops

          R supports implicit loops, which is called vectorization. This is built-in to many functions and standard operators few of them are as below :-

§  apply()-When we have some structured blob of data that we wish to perform operations

§  lapply()-traversing over a set of data like a list or vector- it will return a list rather than a vector

§  sapply()-traversing over a set of data like a list or vector

§  tapply()

§  aggregate()

§  mapply()

applydemo <- matrix(data=cbind(rnorm(30, 0), rnorm(30, 2), rnorm(30, 5)), nrow=30, ncol=3)

 apply(applydemo,1,mean)

output

 [1] 1.940783 3.034701 1.836892 2.420311 1.569139 2.533843 1.784813 1.366241 2.664395 1.452193 2.896668 1.671177 2.550266 1.358662 2.393071 1.301494

[17] 1.903415 2.060350 2.663350 2.913506 1.987428 2.910139 1.941491 1.950160 2.413743 2.102208 1.444518 1.800371 2.497475 1.740320

 apply(applydemo,2,mean)

output

[1] -0.1035741  1.7916236  4.6222626

sapply(5:8, function(x) x^2)

[1] 25 36 49 64

lapply(5:8, function(x) x^2)

output

[[1]]

[1] 25

[[2]]

[1] 36

[[3]]

[1] 49

[[4]]

[1] 64

Explicit Loops

R provides three ways to write loops:

1.   For

2.   Repeat

3.   While

##For Loops

demodata<- LETTERS[1:6]

 for(item in demodata)

{

    print(item);

 }

##Repeat

rep(1:3,length.out=7)

 repeat

 {

          g <- rnorm(1)

          if (g > 1.0) break

          cat(g,"\n")

 }

##While

while (g < 1)

{

          g <- rnorm(1)

          cat(g,"\n")

}

Loop Control Statement

·         Break

·         Next

#Break

demodata<- LETTERS[1:6]

 for(item in demodata)

{

 if(item=="D")

 {

 break;

 }

 print(item);

 }

 #Next

 demodata<- LETTERS[1:6]

 for(item in demodata)

{

 if(item=="D")

 {

 next;

 }

 print(item);

 }

Iterator

An iterator is a special type of object that generalizes the notion of a looping variable. When passed as an argument to a function that knows what to do with it, the iterator supplies a sequence of values. The iterator also maintains information about its state, in particular its current index

# Simple

i1 <- iter(1:3)

 nextElem(i1)

[1] 1

 nextElem(i1)

[1] 2

 nextElem(i1)

[1] 3

 # vector iterator

i1 <- iter(1:3, checkFunc=function(i) i %% 2 == 0)

 nextElem(i1)

[1] 2

Note : For Iterator install-> library(iterators)