Control structures in R allow you to control the flow of execution of a series of R expressions.

Commonly used control structures are

1.  if and else: testing a condition and acting on it
2.  for: execute a loop a fixed number of times
3. while: execute a loop while a condition is true

In R, the if/else syntax is:

if (…) {

R code

} else {

R code

}

if-else in R

 

 

#Take input from user. Then run the next line.
x <- readline(prompt="Enter a number: ") 
y <- if(x > 10){
print("Value is greater than 10")
} else {
print("Value is less than or equal to 10")
}

Output:

> x <- readline(prompt="Enter a number: ")
Enter a number: 11

[1] "Value is more than 10"

x = 4
y = 6
if (x > y) {
print("x is larger than y")
} else {
print("x is less than or equal to y")
}

Output:

[1] “x is less than or equal to y”

 

You can use if-else like this also

ifelse(4 > 3, "greater","less")

Output:

[1] "greater"

 

This is how we use ifelse() to a vector.

x<- 1:20
ifelse(x> 10, "greater", "less")

Output:

[1] "less" "less" "less" "less" "less" "less" "less" "less" "less" "less" "greater" "greater"
[13] "greater" "greater" "greater" "greater" "greater" "greater" "greater" "greater"

for loop in R

Here is a for loop example in R.

for(i in 1:10) {
print(i)
}

x <- c("a", "b", "c", "d")
for(i in 1:4) {
print(x[i])
}

Output:

[1] 1

[1] 2

[1] 3

[1] 4

[1] 5

[1] 6

[1] 7

[1] 8

[1] 9

[1] 10

> 

> x <- c("a", "b", "c", "d")

> for(i in 1:4) {

+ print(x[i])

+ }

[1] "a"

[1] "b"

[1] "c"

[1] "d"

Print out each element of ‘x’.

for(letter in x) {
print(letter)
}

Output:

[1] "a"

[1] "b"

[1] "c"

[1] "d"

For one line loops, the curly braces are not strictly necessary.

for(i in 1:4) print(x[i])

Output:

[1] "a"

[1] "b"

[1] "c"

[1] "d"

for loops can be nested inside of each other like the below example.

 

x <- matrix(1:10, 5, 2)
for(i in seq_len(nrow(x))) {
for(j in seq_len(ncol(x))) {
print(x[i, j])
}
}

Output:

[1] 1
[1] 6
[1] 2
[1] 7
[1] 3
[1] 8
[1] 4
[1] 9
[1] 5
[1] 10

while Loops in R

 

count <- 0
while(count < 10) {
print(count)
count <- count + 1
}

Output:

[1] 0
[1] 1
[1] 2
[1] 3
[1] 4
[1] 5
[1] 6
[1] 7
[1] 8
[1] 9