While loops

Finally, we will take a look at the while loops which use a different way of looping than for loops. In the case of for loops, we know the number of elements in the object we use to iterate, so we know in advance the number of iterations that will be performed. However, there are times where we don't know this number before we start iterating, and instead, we will iterate based on some condition being true after each iteration. That's when while loops are useful.

The way while loops work is that we specify a condition, just as with if…else conditions, and if the condition is met, then we proceed to iterate. When the iteration is finished, we check the condition again, and if it continues to be true, then we iterate again, and so on. Note that in this case if we want to stop at some point, we must modify the elements used in the condition such that it evaluates to FALSE at some point. You can also use break and next inside the while loops.

The following example shows how to print all integers starting at 1 and until 10. Note that if we start at 1 as we do, but instead of adding 1 after each iteration, we subtracted 1 or didn't change x at all, then we would never stop iterating. That's why you need to be very careful when using while loops since the number of iterations can be infinite:

x <- 1
while (x <= 10) {
    print(x)
    x <- x + 1
}
#> [1] 1
#> [1] 2
#> [1] 3
#> [1] 4
#> [1] 5
#> [1] 6
#> [1] 7
#> [1] 8
#> [1] 9
#> [1] 10

In case you do want to execute an infinite loop, you may use the while loop with a TRUE value in the place of the conditional. If you do not include a break command, the code will effectively provide an infinite loop, and it will repeat itself until stopped with the CTRL + C keyboard command or any other stopping mechanism in the IDE you're using. However, in such cases, it's cleaner to use the repeat construct as is shown below. It may seem counter intuitive, but there are times when using infinite loops is useful. We will see one such case in Chapter 8, Object-Oriented System to Track Cryptocurrencies, but in such cases, you have an external mechanism used to stop the program based on a condition external to R.

Executing the following example will crash your R session:

# DO NOTE EXCEUTE THIS, IT's AN INFINITE LOOP

x <- 1
repeat {
    print(x)
    x <- x + 1
}

#> [1] 1
#> [1] 2
#> [1] 3
#> [1] 4
#> [1] 5
#> [1] 5
...