Introduction to for-loops

What is a loop?

Loops are a control structure in programming to repeat statements (i.e. lines of code) multiple times. This can save a lot of tedious work by reducing code redundancy. The syntax of for-loops is quite simple. In the following scheme, statement 1 will be executed once before the loop. Then the green loop block starts. statement 2 is executed multiple times in this case 10 times, once for each run through of the loop. After this, statement 3 is executed once again, as it is outside the loop block.

Translated into R code, this would look something like this:

print("This text is printed once before the loop.") # statement  1

for(i in 1:10){
    print("statement 2") # statement 2
}

print("This text is printed once after the loop.") # statement 3

[1] "This text is printed once before the loop."
[1] "statement 2"
[1] "statement 2"
[1] "statement 2"
[1] "statement 2"
[1] "statement 2"
[1] "statement 2"
[1] "statement 2"
[1] "statement 2"
[1] "statement 2"
[1] "statement 2"
[1] "This text is printed once after the loop."

What is the iterator?

Intuitively, when we write a for-loop, we want to define, how many time the loop should run through the code. However, this is not the right approach to think about loops. Instead, we define a so called iterator, in the example above the variable i. In each run through of the loop, i takes on a different value, here the numbers 1 to 10. The loop stops, once i reached the last defined value.

See in the example below, we define a vector with 4 elements and use it in the for() function for the different states of iterator (here called xx). Hence, the loop goes through the print statement 4 times.

numbers <- c(4, 3, 1, 10)

for(xx in numbers){
    print("Hi")
}

[1] "Hi"
[1] "Hi"
[1] "Hi"
[1] "Hi"

Loops unlock their full potential once you understand that you can use the iterator (here again xx) inside the loop’s statements. On each run of the loop, you do the same steps, but with slightly different input:

numbers <- c(4, 25, 1, 49)

for(xx in numbers){
  print(paste0("The iterator xx is now ", xx, " and it's square root is:"))
  print(sqrt(xx))
}

[1] "The iterator xx is now 4 and it's square root is:"
[1] 2
[1] "The iterator xx is now 25 and it's square root is:"
[1] 5
[1] "The iterator xx is now 1 and it's square root is:"
[1] 1
[1] "The iterator xx is now 49 and it's square root is:"
[1] 7

Storing results of a for loop

x <- 1:10
for(i in x){
  result <- i^2
}

result

[1] 100

result

Why is the result 100? What did you expect?

If you want to save the result of the loop, you have to assign the output to e.g. element of a vector.

x <- 1:10
result <- c()

for(i in x){
  result[i] <- x[i]^2
}

result

 [1]   1   4   9  16  25  36  49  64  81 100

Task

Create a vector with 10 random numbers.
Create a loop that does the following:
- multiply the number with 2 and print the result
- divide the number by 3 and save the result