In programming, loops are used to repeat the execution of a block of code. Loops help you to save time, avoid repeatable blocks of code, and write cleaner code.

In R, there are three types of loops:

• while loops
• for loops
• repeat loops

R for Loop

A for loop is used to iterate over a list, vector or any other object of elements. The syntax of for loop is:

for (value in sequence) {
  # block of code
}

Here, sequence is an object of elements and value takes in each of those elements. In each iteration, the block of code is executed. For example,

numbers = c(1, 2, 3, 4, 5)

# for loop to print all elements in numbers
for (x in numbers) {
  print(x)
}

Output

[1] 1
[1] 2
[1] 3
[1] 4
[1] 5

In this program, we have used a for loop to iterate through a sequence of numbers called numbers. In each iteration, the variable x stores the element from the sequence and the block of code is executed.

Example 1: Count the Number of Even Numbers

Let’s use a for loop to count the number of even numbers stored inside a vector of numbers.

# vector of numbers
num = c(2, 3, 12, 14, 5, 19, 23, 64)

# variable to store the count of even numbers
count = 0

# for loop to count even numbers
for (i in num) {

  # check if i is even
  if (i %% 2 == 0) {
    count = count + 1
  }
}

print(count)

Output

[1] 4

In this program, we have used a for loop to count the number of even numbers in the num vector. Here is how this program works:

• We first initialized the count variable to 0. We use this variable to store the count of even numbers in the num vector.
• We then use a for loop to iterate through the num vector using the variable i.
  
  

  for (i in num) {
    # code block
  }

• Inside the for loop, we check if each element is divisible by 2 or not. If yes, then we increment count by 1.
  
  

  if (i %% 2 == 0) {
    count = count + 1
  }

Example 2: for Loop With break Statement

You can use the break statement to exit from the for loop in any iteration. For example,

# vector of numbers
numbers = c(2, 3, 12, 14, 5, 19, 23, 64)

# for loop with break
for (i in numbers) {

  # break the loop if number is 5
  if( i == 5) {
    break
  }

  print(i)
}

Output

[1] 2
[1] 3
[1] 12
[1] 14

Here, we have used an if statement inside the for loop. If the current element is equal to 5, we break the loop using the break statement. After this, no iteration will be executed.

Example 3: for Loop With next Statement

Instead of terminating the loop, you can skip an iteration using the next statement. For example,

# vector of numbers
numbers = c(2, 3, 12, 14, 5, 19, 23, 64)

# for loop with next
for (i in numbers) {

  # use next to skip odd numbers
  if( i %% 2 != 0) {
    next
  }

  print(i)
}

Output

[1] 2
[1] 12
[1] 14
[1] 64

Here, we have used an if statement inside the for loop to check for odd numbers. If the number is odd, we skip the iteration using the next statement and print only even numbers.

Nested for Loops

You can include a for loop inside another for loop to create a nested loop.

Consider the example below. Suppose we have two sequences of numbers. We want to print all the combinations where the sum of numbers in both the sequences is even.

# vector of numbers
sequence_1 = c(1, 2, 3)
sequence_2 = c(1, 2, 3)

# nested for loop
for (i in sequence_1) {
  for (j in sequence_2) {
    
    # check if sum is even
    if ( (i+j) %% 2 == 0 ) {
      print(paste(i, j))   
    }

  }
}

Output

[1] "1 1"
[1] "1 3"
[1] "2 2"
[1] "3 1"
[1] "3 3"

In the above program, we have created two sequences: sequence_1 and sequence_2, both containing numbers from 1 to 3.

We then used a nested for loop to iterate through the sequences. The outer loop iterates through sequence_1 and the inner loop iterates through sequence_2.

for (i in sequence_1) {
  for (j in sequence_2) {
    # code block
  }
}

In each iteration,

• i stores the current number of sequence_1
• j stores the current number of sequence_2

The if statement inside the nested loops checks if i + j is even or not. If it is, then we print i and j.

if ( (i+j) %% 2 == 0 ) {
  print(paste(i, j))   
}