Control Structures Loops in R

February 27, 2016
By

(This article was first published on Data Perspective, and kindly contributed to R-bloggers)

As part of Data Science tutorial Series in my previous post I posted on basic data types in R. I have kept the tutorial very simple so that beginners of R programming  may takeoff immediately.
Please find the online R editor at the end of the post so that you can execute the code on the page itself.
In this section we learn about control structures loops used in R. Control strcutures in R contains conditionals, loop statements like any other programming languages.

Loops are very important and forms backbone to any programming languages.Before we get into the control structures in R, just type as below in Rstudio:

 ?control 


If else statement:

#See the code syntax below for if else statement 
if(x>1){
print("x is greater than 1")
}else{
print("x is less than 1")
}

#See the code below for nested if else statement

x=10
x=10
if(x>1 & x<7){
print("x is between 1 and 7")}else if(x>8 & x< 15){
print("x is between 8 and 15")
}

[1] "x is between 8 and 15"

For loops:
As we know for loops are used for iterating items

 #Below code shows for  loop implementation
x = c(1,2,3,4,5)
for(i in 1:5){
print(x[i])
}
[1] 1
[1] 2
[1] 3
[1] 4
[1] 5

While loop :

 #Below code shows while loop in R
x = 2.987
while(x <= 4.987) {
x = x + 0.987
print(c(x,x-2,x-1))
}
[1] 3.974 1.974 2.974
[1] 4.961 2.961 3.961
[1] 5.948 3.948 4.948

Repeat Loop:
The repeat loop is an infinite loop and used in association with a break statement.

 #Below code shows repeat loop:
a = 1
repeat { print(a) a = a+1 if(a > 4) break }
[1] 1
[1] 2
[1] 3
[1] 4

Break statement:
A break statement is used in a loop to stop the iterations and flow the control outside of the loop.

 #Below code shows break statement:
x = 1:10
for (i in x){
if (i == 2){
break
}
print(i)
}
[1] 1

Next statement:
Next statement enables to skip the current iteration of a loop without terminating it.

 #Below code shows next statement 
x = 1: 4
for (i in x) {
if (i == 2){
next}
print(i)
}
[1] 1
[1] 3
[1] 4

Creating a function in R:
function() is a built-in R function whose job is to create functions. In the below example function() takes one parameter x, executes a for loop logic.
The function object thus created using function() is assigned to a variable (‘words.names’). Now this created function will be called using the variable ‘word.names’

 #Below code shows us, how a function is created in R:

Syntax:
function_name = function(parameters,..){ code}

words = c("R", "datascience", "machinelearning","algorithms","AI")
words.names = function(x) {
for(name in x){
print(name)
}
}
#Calling the function
words.names(words)
[1] "R"
[1] "datascience"
[1] "machinelearning"
[1] "algorithms"
[1] "AI"

Hands on exercise of what we have learnt so far


We create a data frame DF, run for loop, ifelse in a function and call the function
#create 3 vectors name,age,salary
name = c("David","John","Mathew")
age = c(30,40,50)
salary = c(30000,120000,55000)
#create a data frame DF by combining the 3 vectors using cbind() function
DF = data.frame(cbind(name,Age,salary))
#display DF
DF
name Age salary
1 David 30 30000
2 John 40 120000
3 Mathew 50 55000
#dimensions of DF
dim(DF)
[1] 3 3

#write a function which displays the salaried person name
findHighSalary = function(df){
Maxsal = 0
empname = ""
for(i in 1:nrow(DF)){
tmpsal = as.numeric(DF[i,3] )
if(tmpsal > Maxsal){
Maxsal = tmpsal
empname = DF[i,1]
}
}
return(as.character(empname))
}
#calling the function
findHighSalary(DF)
[1] "Mathew"

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