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A for loop is used to iterate over a vector in R programming.

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Syntax of for loop

for (val in sequence)
{
statement
}

Here, sequence is a vector and val takes on each of its value during the loop. In each iteration, statement is evaluated.

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Flowchart of for loop

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Example: for loop

Below is an example to count the number of even numbers in a vector.

x <- c(2,5,3,9,8,11,6)
count <- 0
for (val in x) {
if(val %% 2 == 0)  count = count+1
}
print(count)

Output

[1] 3

In the above example, the loop iterates 7 times as the vector x has 7 elements.

In each iteration, val takes on the value of corresponding element of x.

We have used a counter to count the number of even numbers in x. We can see that x contains 3 even numbers.

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Useful example of for loop

Code

# Nice example

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

print(1:10)
fruits <- c("apple", "Banana", "Pomegranate")
fruitLength <- rep(NA, length(fruits))
fruitLength
names(fruitLength) <- fruits
fruitLength

for(a in fruits){
  fruitLength[a] <- nchar(a)
}

fruitLength

# another way to do the above thing

fruitLength2 <- nchar(fruits)
fruitLength2
names(fruitLength2) <- fruits
fruitLength2

identical(fruitLength, fruitLength2)

Output

> 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
> print(1:10)
 [1]  1  2  3  4  5  6  7  8  9 10
> fruits <- c("apple", "Banana", "Pomegranate")
> fruitLength <- rep(NA, length(fruits))
> fruitLength
[1] NA NA NA
> names(fruitLength) <- fruits
> fruitLength
      apple      Banana Pomegranate 
         NA          NA          NA 
> for(a in fruits){
+   fruitLength[a] <- nchar(a)
+ }
> fruitLength
      apple      Banana Pomegranate 
          5           6          11 
> fruitLength2 <- nchar(fruits)
> fruitLength2
[1]  5  6 11
> names(fruitLength2) <- fruits
> fruitLength2
      apple      Banana Pomegranate 
          5           6          11 
> identical(fruitLength, fruitLength2)
[1] TRUE

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