Control Structures in R

Control structures in R allow you to control the flow of execution of the program, depending onruntime conditions.

Most control structures are not used in interactive sessions, but rather when writing functions or longer expresisons.

If

if(){
do something
}else{
do something else
}

For

for loops take an interator variable and assign it successive values from a sequence or vector. For loops are most commonly used for iterating over the elements of an object (list, vector, etc.)

This loop takes the i variable and in each iteration of the loop gives it values 1, 2, 3, ..., 10, and then exits.

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

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

Other examples

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

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

## [1] "a"
## [1] "b"
## [1] "c"
## [1] "d"

for(i in seq_along(x)){
  print(x[i])
}

## [1] "a"
## [1] "b"
## [1] "c"
## [1] "d"

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

## [1] "a"
## [1] "b"
## [1] "c"
## [1] "d"

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

## [1] "a"
## [1] "b"
## [1] "c"
## [1] "d"

Nested For Loops

x <- matrix(1:6, 2, 3)
x

##      [,1] [,2] [,3]
## [1,]    1    3    5
## [2,]    2    4    6

for(i in seq_len(nrow(x))){
  for(j in seq_len(ncol(x))){
    print(x[i,j])
  }
}

## [1] 1
## [1] 3
## [1] 5
## [1] 2
## [1] 4
## [1] 6

While Loops

While loops begin by testing a condition. If it is true, then they execute the loop body. Once the loop body is executed, the condition is tested again, and so forth.

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

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

##Mutiple conditions
z <- 5
while(z >= 3 && z <= 10) {
 print(z)
 coin <- rbinom(1, 1, 0.5)
 if(coin == 1) { ## random walk
 z <- z + 1
 } else {
 z <- z - 1
 }
}

## [1] 5
## [1] 6
## [1] 7
## [1] 6
## [1] 5
## [1] 4
## [1] 5
## [1] 4
## [1] 3

Repeat

Repeat initiates an infinite loop; these are not commonly used in statistical applications but they do have their uses. The only way to exit a repeat loop is to call break

Next, Return

next is used to skip an iteration of a loop

for(i in 1:100) {
 if(i <= 20) {
 ## Skip the first 20 iterations
 next
 }
 ## Do something here
}

return signals that a function should exit and return a given value.

Use Cases: Control Structures

Control structures mentiond here are primarily useful for writing programs; for command-line interactive work, the apply functions are more useful.