In the module on iteration, we have presented the flow of execution as something largely unescapable. In practice, there are two important operations we may want to perform: skip an iteration, or stop the iteration process entirely. In this primer, we will see how this can be achieved using the break and next keywords.

Understanding how next and break can be used is extremely useful as you start developing more complex workflows. It can save a lot of time when you know that you want to avoid some operations, and is therefore worth understanding.

Let’s imagine that we have the following loop:

i = 1
while true
  println(i)
  global i += 1
end

You should not run this, because it will run forever. This loop as no termination condition, and therefore will go on forever. Let’s say we want to stop it after i >= 5 – this is something we can do using the break keyword:

i = 1
while true
  println(i)
  i < 5 || break
  global i += 1
end

1
2
3
4
5

The line we added (i < 5 || break) will check that i is less than or equal to 5 – if this is true, then it will break the iteration, i.e. stop. Note that i < 5 || break uses short-circuit evaluation, which is a wonderful bit of notation: simple yet efficient.

The continue keyword has an extremely useful role too – it allows you to skip one iteration. Let’s say that we want to perform an operation only on number smaller than 1/3:

for r in rand(10)
  r ≥ 1/3 || continue
  println(round(r; digits=2))
end

0.86
0.93
0.53
0.91
0.6
0.99
0.82
0.56
0.57

In short, continue will skip ahead to the next element in the iteration. This can be very important to avoid performing operations on objects that are not relevant.